Co-reporter:Katherine L. Walker, Laura M. Dornan, Richard N. Zare, Robert M. Waymouth, and Mark J. Muldoon
Journal of the American Chemical Society September 13, 2017 Volume 139(Issue 36) pp:12495-12495
Publication Date(Web):August 29, 2017
DOI:10.1021/jacs.7b05413
Kinetic studies, isotope labeling, and in situ high-resolution mass spectrometry are used to elucidate the mechanism for the catalytic oxidation of styrenes using aqueous hydrogen peroxide (H2O2) and the cationic palladium(II) compound, [(PBO)Pd(NCMe)2][OTf]2 (PBO = 2-(pyridin-2-yl)benzoxazole). Previous studies have shown that this reaction yields acetophenones with high selectivity. We find that H2O2 binds to Pd(II) followed by styrene binding to generate a Pd-alkylperoxide that liberates acetophenone by at least two competitive processes, one of which involves a palladium enolate intermediate that has not been previously observed in olefin oxidation reactions. We suggest that acetophenone is formed from the palladium enolate intermediate by protonation from H2O2. We replaced hydrogen peroxide with t-butyl hydroperoxide and found that, although the palladium enolate intermediate was observed, it was not on the major product-generating pathway, indicating that the form of the oxidant plays a key role in the reaction mechanism.
Co-reporter:Kate M. Waldie, Srinivasan Ramakrishnan, Sung-Kwan Kim, Jana K. Maclaren, Christopher E. D. Chidsey, and Robert M. Waymouth
Journal of the American Chemical Society March 29, 2017 Volume 139(Issue 12) pp:4540-4540
Publication Date(Web):March 6, 2017
DOI:10.1021/jacs.7b01047
The dicationic complex [CpCo(azpy)(CH3CN)](ClO4)2 1 (azpy = phenylazopyridine) exhibits a reversible two-electron reduction at a very mild potential (−0.16 V versus Fc0/+) in acetonitrile. This behavior is not observed with the analogous bipyridine and pyrazolylpyridine complexes (3 and 4), which display an electrochemical signature typical of CoIII systems: two sequential one-electron reductions to CoII at −0.4 V and CoI at −1.0 to −1.3 V versus Fc0/+. The doubly reduced, neutral complex [CpCo(azpy)] 2 is isolated as an air-stable, diamagnetic solid via chemical reduction with cobaltocene. Crystallographic and spectroscopic characterization together with experimentally calibrated density functional theory calculations illuminate the key structural and electronic changes that occur upon reduction of 1 to 2. The electrochemical potential inversion observed with 1 is attributed to effective overlap between the metal d and the low-energy azo π* orbitals in the intermediary redox state and additional stabilization of 2 from structural reorganization, leading to a two-electron reduction. This result serves as a key milestone in the quest for two-electron transformations with mononuclear first-row transition metal complexes at mild potentials.
Co-reporter:Xiangyi Zhang and Robert M. Waymouth
Journal of the American Chemical Society March 15, 2017 Volume 139(Issue 10) pp:3822-3822
Publication Date(Web):February 28, 2017
DOI:10.1021/jacs.7b00039
The use of dithiolane-containing polymers to construct responsive and dynamic networks is an attractive strategy in material design. Here, we provide a detailed mechanistic study on the self-assembly and gelation behavior of a class of ABA triblock copolymers containing a central poly(ethylene oxide) block and terminal polycarbonate blocks with pendant 1,2-dithiolane functionalities. In aqueous solution, these amphiphilic block copolymers self-assemble into bridged flower micelles at high concentrations. The addition of a thiol initiates the reversible ring-opening polymerizations of dithiolanes in the micellar cores to induce the cross-linking and gelation of the micellar network. The properties of the resulting hydrogels depend sensitively on the structures of 1,2-dithiolanes. While the methyl asparagusic acid-derived hydrogels are highly dynamic, adaptable, and self-healing, those derived from lipoic acid are rigid, resilient, and brittle. The thermodynamics and kinetics of ring-opening polymerization of the two dithiolanes were investigated to provide important insights on the dramatically different properties of the hydrogels derived from the two different dithiolanes. The incorporation of both dithiolane monomers into the block copolymers provides a facile way to tailor the properties of these hydrogels.
Co-reporter:Binhong Lin and Robert M. Waymouth
Journal of the American Chemical Society 2017 Volume 139(Issue 4) pp:1645-1652
Publication Date(Web):January 20, 2017
DOI:10.1021/jacs.6b11864
Aliphatic polyesters and polycarbonates are a class of biorenewable, biocompatible, and biodegradable materials. One of the most powerful methods for accessing these materials is the ring-opening polymerization (ROP) of cyclic monomers. Here we report that the deprotonation of ureas generates a class of versatile catalysts that are simultaneously fast and selective for the living ring-opening polymerization of several common monomers, including lactide, δ-valerolactone, ε-caprolactone, a cyclic carbonate, and a cyclic phosphoester. Spanning several orders of magnitude, the reactivities of several diaryl urea anions correlated to the electron-withdrawing substituents on the aryl rings. With the appropriate urea anions, the polymerizations reached high conversions (∼90%) at room temperature within seconds (1–12 s), yielding polymers with narrow molecular weight distributions (Đ = 1.06 to 1.14). These versatile catalysts are simple to prepare, easy to use, and exhibit a range of activities that can be tuned for the optimal performance of a broad range of monomers.
Co-reporter:Dr. Katherine Margulis;Dr. Xiangyi Zhang;Dr. Lydia-Marie Joubert;Dr. Karsten Bruening;Dr. Christopher J. Tassone; Dr. Richard N. Zare; Dr. Robert M. Waymouth
Angewandte Chemie International Edition 2017 Volume 56(Issue 51) pp:16357-16362
Publication Date(Web):2017/12/18
DOI:10.1002/anie.201709564
AbstractTemplate-free fabrication of non-spherical polymeric nanoparticles is desirable for various applications, but has had limited success owing to thermodynamic favorability of sphere formation. Herein we present a simple way to prepare cubic nanoparticles of block copolymers by self-assembly from aqueous solutions at room temperature. Nanocubes with edges of 40–200 nm are formed spontaneously on different surfaces upon water evaporation from micellar solutions of triblock copolymers containing a central poly(ethylene oxide) block and terminal trimethylene carbonate/dithiolane blocks. These polymers self-assemble into 28±5 nm micelles in water. Upon drying, micelle aggregation and a kinetically controlled crystallization of central blocks evidently induce solid cubic particle formation. An approach for preserving the structures of these cubes in water by thiol- or photo-induced crosslinking was developed. The ability to solubilize a model hydrophobic drug, curcumin, was also explored.
Co-reporter:Dr. Katherine Margulis;Dr. Xiangyi Zhang;Dr. Lydia-Marie Joubert;Dr. Karsten Bruening;Dr. Christopher J. Tassone; Dr. Richard N. Zare; Dr. Robert M. Waymouth
Angewandte Chemie 2017 Volume 129(Issue 51) pp:16575-16580
Publication Date(Web):2017/12/18
DOI:10.1002/ange.201709564
AbstractTemplate-free fabrication of non-spherical polymeric nanoparticles is desirable for various applications, but has had limited success owing to thermodynamic favorability of sphere formation. Herein we present a simple way to prepare cubic nanoparticles of block copolymers by self-assembly from aqueous solutions at room temperature. Nanocubes with edges of 40–200 nm are formed spontaneously on different surfaces upon water evaporation from micellar solutions of triblock copolymers containing a central poly(ethylene oxide) block and terminal trimethylene carbonate/dithiolane blocks. These polymers self-assemble into 28±5 nm micelles in water. Upon drying, micelle aggregation and a kinetically controlled crystallization of central blocks evidently induce solid cubic particle formation. An approach for preserving the structures of these cubes in water by thiol- or photo-induced crosslinking was developed. The ability to solubilize a model hydrophobic drug, curcumin, was also explored.
Co-reporter:Colin J. McKinlay;Jessica R. Vargas;Timothy R. Blake;Jonathan W. Hardy;Masamitsu Kanada;Christopher H. Contag;Paul A. Wender
PNAS 2017 Volume 114 (Issue 4 ) pp:E448-E456
Publication Date(Web):2017-01-24
DOI:10.1073/pnas.1614193114
Functional delivery of mRNA to tissues in the body is key to implementing fundamentally new and potentially transformative
strategies for vaccination, protein replacement therapy, and genome editing, collectively affecting approaches for the prevention,
detection, and treatment of disease. Broadly applicable tools for the efficient delivery of mRNA into cultured cells would
advance many areas of research, and effective and safe in vivo mRNA delivery could fundamentally transform clinical practice.
Here we report the step-economical synthesis and evaluation of a tunable and effective class of synthetic biodegradable materials:
charge-altering releasable transporters (CARTs) for mRNA delivery into cells. CARTs are structurally unique and operate through
an unprecedented mechanism, serving initially as oligo(α-amino ester) cations that complex, protect, and deliver mRNA and
then change physical properties through a degradative, charge-neutralizing intramolecular rearrangement, leading to intracellular
release of functional mRNA and highly efficient protein translation. With demonstrated utility in both cultured cells and
animals, this mRNA delivery technology should be broadly applicable to numerous research and therapeutic applications.
Co-reporter:Kate M. Waldie, Kristen R. Flajslik, Elizabeth McLoughlinChristopher E. D. Chidsey, Robert M. Waymouth
Journal of the American Chemical Society 2016 Volume 139(Issue 2) pp:738-748
Publication Date(Web):December 20, 2016
DOI:10.1021/jacs.6b09705
Octahedral ruthenium complexes [RuX(CNN)(dppb)] (1, X = Cl; 2, X = H; CNN = 2-aminomethyl-6-tolylpyridine, dppb = 1,4-bis(diphenylphosphino)butane) are highly active for the transfer hydrogenation of ketones with isopropanol under ambient conditions. Turnover frequencies of 0.88 and 0.89 s–1 are achieved at 25 °C using 0.1 mol % of 1 or 2, respectively, in the presence of 20 equiv of potassium t-butoxide relative to catalyst. Electrochemical studies reveal that the Ru–hydride 2 is oxidized at low potential (−0.80 V versus ferrocene/ferrocenium, Fc0/+) via a chemically irreversible process with concomitant formation of dihydrogen. Complexes 1 and 2 are active for the electrooxidation of isopropanol in the presence of strong base (potassium t-butoxide) with an onset potential near −1 V versus Fc0/+. By cyclic voltammetry, fast turnover frequencies of 3.2 and 4.8 s–1 for isopropanol oxidation are achieved with 1 and 2, respectively. Controlled potential electrolysis studies confirm that the product of isopropanol electrooxidation is acetone, generated with a Faradaic efficiency of 94 ± 5%.
Co-reporter:Dexter C. Davis, Katherine L. Walker, Chunhua Hu, Richard N. Zare, Robert M. Waymouth, and Mingji Dai
Journal of the American Chemical Society 2016 Volume 138(Issue 33) pp:10693-10699
Publication Date(Web):July 26, 2016
DOI:10.1021/jacs.6b06573
A palladium-catalyzed cascade carbonylative spirolactonization of hydroxycyclopropanols has been developed to efficiently synthesize oxaspirolactones common to many complex natural products of important therapeutic value. The mild reaction conditions, high atom economy, broad substrate scope, and scalability of this new method were highlighted in expedient total syntheses of the Turkish tobacco natural products α-levantanolide and α-levantenolide in two and four steps, respectively. The hydroxycyclopropanol substrates are readily available in one step via a Kulinkovich reaction of the corresponding lactones. Mechanistic studies utilizing high-resolution electrospray ionization mass spectrometry (ESI-MS) identified several key intermediates in the catalytic cycle, as well as those related to catalyst decomposition and competitive pathways.
Co-reporter:Colin J. McKinlay; Robert M. Waymouth;Paul A. Wender
Journal of the American Chemical Society 2016 Volume 138(Issue 10) pp:3510-3517
Publication Date(Web):February 22, 2016
DOI:10.1021/jacs.5b13452
The design, synthesis, and biological evaluation of a new family of highly effective cell-penetrating molecular transporters, guanidinium-rich oligophosphoesters, are described. These unique transporters are synthesized in two steps, irrespective of oligomer length, by the organocatalytic ring-opening polymerization (OROP) of 5-membered cyclic phospholane monomers followed by oligomer deprotection. Varying the initiating alcohol results in a wide variety of cargo attachment strategies for releasable or nonreleasable transporter applications. Initiation of oligomerization with a fluorescent probe produces, upon deprotection, a transporter-probe conjugate that is shown to readily enter multiple cell lines in a dose-dependent manner. These new transporters are superior in cell uptake to previously studied guanidinium-rich oligocarbonates and oligoarginines, showing over 2-fold higher uptake than the former and 6-fold higher uptake than the latter. Initiation with a protected thiol gives, upon deprotection, thiol-terminated transporters which can be thiol-click conjugated to a variety of probes, drugs and other cargos as exemplified by the conjugation and delivery of the model probe fluorescein-maleimide and the medicinal agent paclitaxel (PTX) into cells. Of particular significance given that drug resistance is a major cause of chemotherapy failure, the PTX-transporter conjugate, designed to evade Pgp export and release free PTX after cell entry, shows efficacy against PTX-resistant ovarian cancer cells. Collectively this study introduces a new and highly effective class of guanidinium-rich cell-penetrating transporters and methodology for their single-step conjugation to drugs and probes, and demonstrates that the resulting drug/probe-conjugates readily enter cells, outperforming previously reported guanidinium-rich oligocarbonates and peptide transporters.
Co-reporter:Kevin Chung and Robert M. Waymouth
ACS Catalysis 2016 Volume 6(Issue 7) pp:4653
Publication Date(Web):June 6, 2016
DOI:10.1021/acscatal.6b01501
The development of new strategies for the direct catalytic functionalization of unprotected carbohydrates would be an enabling advance for glycoscience. Herein we report that the catalytic oxidation of unprotected carbohydrates can be carried out selectively with [(neocuproine)Pd(OAc)]2(OTf)2 (1) to generate the 3-ketoses. Catalytic aerobic oxidation can be carried out with Pd loadings as low as 1% in the presence of phenolic additives. Catalytic oxidation of a variety of unprotected pyranosides in acetonitrile or acetonitrile/water with Pd catalyst 1 with either oxygen or benzoquinone selectively generates the 3-ketoses. Minor amounts of the 4-ketoses are formed competitively, particularly in the case of pyranosides bearing axial substituents at C4 of the pyranoside. Catalytic oxidations can also be carried out in trifluorethanol, but for pyranosides bearing axial substituents at C2 or C4, selective oxidation to the 3-ketose is accompanied by epimerization to afford the equatorial 3-ketoses. Catalytic oxidation of unprotected hexafuranosides or sialic acid derivatives occurs selectively at the exocyclic diol or triol in trifluoroethanol to generate exocyclic hydroxyketones.Keywords: aerobic; alcohol oxidation; carbohydrates; homogeneous catalysis; palladium; selective oxidation; solvent effects
Co-reporter:Young A. Chang, Andrey E. Rudenko, and Robert M. Waymouth
ACS Macro Letters 2016 Volume 5(Issue 10) pp:1162
Publication Date(Web):September 30, 2016
DOI:10.1021/acsmacrolett.6b00591
The zwitterionic ring-opening polymerization of N-functionalized eight-membered cyclic carbonates with N-heterocyclic carbenes (NHC) in the absence of alcohol initiators generates cyclic polycarbonates of Mn ∼ 30–100 kDa. The polymerization behavior of these eight-membered cyclic azacarbonates depends sensitively on the nature of the nitrogen substituent. The N-benzyl-substituted eight-membered cyclic carbonate (8CCBn) polymerizes readily with 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene to generate cyclic polycarbonates with molecular weights of Mn = 14 000 to 96 000 Da. In contrast, the N-phenyl-substituted cyclic carbonate (8CCPh) catalytically dimerizes in the presence of the NHC to afford the crystalline cyclic dimer. The zwitterionic ring-opening copolymerization of δ-valerolactone (VL) and the cyclic carbonates afford gradient cyclic copolymers. The cyclic topology of both the homopolymers and copolymers was supported by MALDI-TOF MS and intrinsic viscosity measurements. 13C NMR and differential scanning calorimetry of the cyclic copolymers are indicative of a gradient sequence distribution as a consequence of the more rapid enchainment of the cyclic carbonates relative to valerolactone.
Co-reporter:Hsiao-Tieh Hsu, Brian M. Trantow, Robert M. Waymouth, and Paul A. Wender
Bioconjugate Chemistry 2016 Volume 27(Issue 2) pp:376
Publication Date(Web):September 14, 2015
DOI:10.1021/acs.bioconjchem.5b00469
The development of abiological catalysts that can function in biological systems is an emerging subject of importance with significant ramifications in synthetic chemistry and the life sciences. Herein we report a biocompatible ruthenium complex [Cp(MQA)Ru(C3H5)]+PF6– 2 (Cp = cyclopentadienyl, MQA = 4-methoxyquinoline-2-carboxylate) and a general analytical method for evaluating its performance in real time based on a luciferase reporter system amenable to high throughput screening in cells and by extension to evaluation in luciferase transgenic animals. Precatalyst 2 activates alloc-protected aminoluciferin 4b, a bioluminescence pro-probe, and releases the active luminophore, aminoluciferin (4a), in the presence of luciferase-transfected cells. The formation and enzymatic turnover of 4a, an overall process selected because it emulates pro-drug activation and drug turnover by an intracellular target, is evaluated in real time by photon counting as 4a is converted by intracellular luciferase to oxyaminoluciferin and light. Interestingly, while the catalytic conversion (activation) of 4b to 4a in water produces multiple products, the presence of biological nucleophiles such as thiols prevents byproduct formation and provides almost exclusively luminophore 4a. Our studies show that precatalyst 2 activates 4b extracellularly, exhibits low toxicity at concentrations relevant to catalysis, and is comparably effective in two different cell lines. This proof of concept study shows that precatalyst 2 is a promising lead for bioorthogonal catalytic activation of pro-probes and, by analogy, similarly activatable pro-drugs. More generally, this study provides an analytical method to measure abiological catalytic activation of pro-probes and, by analogy with our earlier studies on pro-Taxol, similarly activatable pro-drugs in real time using a coupled biological catalyst that mediates a bioluminescent readout, providing tools for the study of imaging signal amplification and of targeted therapy.
Co-reporter:Srinivasan Ramakrishnan, Kate M. Waldie, Ingolf Warnke, Antonio G. De Crisci, Victor S. Batista, Robert M. Waymouth, and Christopher E. D. Chidsey
Inorganic Chemistry 2016 Volume 55(Issue 4) pp:1623-1632
Publication Date(Web):February 2, 2016
DOI:10.1021/acs.inorgchem.5b02556
The ruthenium hydride [RuH(CNN)(dppb)] (1; CNN = 2-aminomethyl-6-tolylpyridine, dppb = 1,4-bis(diphenylphosphino)butane) reacts rapidly and irreversibly with CO2 under ambient conditions to yield the corresponding Ru formate complex 2. In contrast, the Ru hydride 1 reacts with acetone reversibly to generate the Ru isopropoxide, with the reaction free energy ΔG°298 K = −3.1 kcal/mol measured by 1H NMR in tetrahydrofuran-d8. Density functional theory (DFT), calibrated to the experimentally measured free energies of ketone insertion, was used to evaluate and compare the mechanism and energetics of insertion of acetone and CO2 into the Ru–hydride bond of 1. The calculated reaction coordinate for acetone insertion involves a stepwise outer-sphere dihydrogen transfer to acetone via hydride transfer from the metal and proton transfer from the N–H group on the CNN ligand. In contrast, the lowest energy pathway calculated for CO2 insertion proceeds by an initial Ru–H hydride transfer to CO2 followed by rotation of the resulting N–H-stabilized formate to a Ru–O-bound formate. DFT calculations were used to evaluate the influence of the ancillary ligands on the thermodynamics of CO2 insertion, revealing that increasing the π acidity of the ligand cis to the hydride ligand and increasing the σ basicity of the ligand trans to it decreases the free energy of CO2 insertion, providing a strategy for the design of metal hydride systems capable of reversible, ergoneutral interconversion of CO2 and formate.
Co-reporter:Andrew J. Ingram; Katherine L. Walker; Richard N. Zare
Journal of the American Chemical Society 2015 Volume 137(Issue 42) pp:13632-13646
Publication Date(Web):October 7, 2015
DOI:10.1021/jacs.5b08719
Aerobic oxidation of alcohols are catalyzed by the Pd–acetate compound [LPd(OAc)]2(OTf)2 (L = neocuproine = 2,9-dimethyl-1,10-phenanthroline) to form ketones and the release of hydrogen peroxide, but the latter rapidly undergoes disproportionation. We employ a series of kinetic and isotope labeling studies made largely possible by electrospray ionization mass spectrometry to determine the role of intermediates in causing this complex chemical transformation. The data suggested that multiple catalytic paths for H2O2 disproportionation occur, which involve formation and consumption of multinuclear Pd species. We find that the trinuclear compound [(LPd)3(μ3-O)2]2+, which we have identified in a previous study, is a product of dioxygen activation that is formed during aerobic oxidations of alcohols catalyzed by [LPd(OAc)]2(OTf)2. It is also a product of hydrogen peroxide activation during disproportionation reactions catalyzed by [LPd(OAc)]2(OTf)2. The results suggest that this trinuclear Pd compound is involved in one of the simultaneous mechanisms for the reduction of oxygen and/or the disproportionation of hydrogen peroxide during oxidation catalysis. Electrospray ionization mass spectrometry of hydrogen peroxide disproportionation reactions suggested the presence of other multinuclear Pd–O2 species in solution. Theoretical calculations of these compounds yield some insight into their structure and potential chemistry.
Co-reporter:Gregg A. Barcan; Xiangyi Zhang
Journal of the American Chemical Society 2015 Volume 137(Issue 17) pp:5650-5653
Publication Date(Web):April 28, 2015
DOI:10.1021/jacs.5b02161
The design and generation of adaptable materials derived from structurally dynamic polymers provides a strategy for generating smart materials that can respond to environmental stimuli or exhibit self-healing behavior. Herein we report an expedient organocatalytic ring-opening polymerization of cyclic carbonates containing pendant dithiolanes (trimethylene carbonate/dithiolane, TMCDT) from poly(ethylene oxide) diols to generate water-soluble triblock (ABA) copolymers containing a central poly(ethylene oxide) block and terminal dithiolane blocks. Hydrogels generated from the triblock copolymers and a cross-linking dithiol exhibited dynamic behavior as a result of the reversible ring opening of the pendant 1,2-dithiolanes. These materials exhibit self-healing behavior, can be injected through a syringe, and rapidly recover their mechanical properties after a severe strain deformation. The dynamic properties of these gels can be modulated with the number of dithiolane units, pH, and temperature.
Co-reporter:James C. A. Flanagan, Eun Joo Kang, Nathaniel I. Strong, and Robert M. Waymouth
ACS Catalysis 2015 Volume 5(Issue 9) pp:5328
Publication Date(Web):August 10, 2015
DOI:10.1021/acscatal.5b00930
A room-temperature dimerization of crotonates into 2-ethylidene-3-methylpentanedioates provides a sustainable route to difunctional monomers for step-growth polymerizations. We report two such dimerizations: (1) an organocatalytic dimerization using the N-heterocyclic carbene 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IiPr2Me2) and (2) a rapid dimerization (under 15 s to full conversion) using potassium t-butoxide in THF. In addition to unsaturated diesters, the resulting dimers can be easily converted to other step-growth monomers; namely, their corresponding diacids and saturated diesters.Keywords: alkenoates; base catalysis; dimerization; monomer synthesis; N-heterocyclic carbenes
Co-reporter:Megan Buonaiuto, Antonio G. De Crisci, Thomas F. Jaramillo, and Robert M. Waymouth
ACS Catalysis 2015 Volume 5(Issue 12) pp:7343
Publication Date(Web):November 12, 2015
DOI:10.1021/acscatal.5b01830
A titanium electrode, modified with Ru(OH)x/TiO2, was prepared and observed to mediate both chemical transfer hydrogenation and electrochemical alcohol oxidation. Electro-oxidation of 2-propanol (1.3 M) at room temperature and pH 7.2 exhibits an onset of electrocatalytic current at 1000 mV vs RHE for the two-electron oxidation of 2-propanol to acetone. XPS characterization, cyclic voltammetry, and electrolysis experiments confirm that electrochemically active ruthenium species catalyzed the electro-oxidation of 2-propanol to acetone. These modified electrode surfaces maintain >60% original activity upon reuse, despite low loadings of ruthenium. The applied potentials are consistent with an electrocatalytic mechanism mediated by surface-immobilized Ru–oxo species (1380 mV vs RHE). These results indicate that heterogeneous transfer hydrogenation catalysts can function as alcohol electro-oxidation catalysts.Keywords: alcohol oxidation; electrocatalysis; heterogeneous catalysis; ruthenium; transfer hydrogenation
Co-reporter:Tyler S. Stukenbroeker, Jeffrey S. Bandar, Xiangyi Zhang, Tristan H. Lambert, and Robert M. Waymouth
ACS Macro Letters 2015 Volume 4(Issue 8) pp:853
Publication Date(Web):July 30, 2015
DOI:10.1021/acsmacrolett.5b00421
Cyclopropenimine superbases were employed to catalyze the ring-opening polymerization of lactide. Polymerization occurred readily in the presence and absence of alcohol initiators. Polymerizations in the absence of alcohol initiators revealed a competitive initiation mechanism involving deprotonation of lactide by the cyclopropenimine to generate an enolate. NMR and MALDI-TOF analysis of the poly(lactides) generated from cyclopropenimines in the absence of alcohol initiators showed acylated lactide and hydroxyl end groups. Model studies and comparative experiments with guanidine and phosphazene catalysts revealed the subtle influence of the nature of the superbase on competitive initiation processes.
Co-reporter:Young A. Chang and Robert M. Waymouth
Polymer Chemistry 2015 vol. 6(Issue 29) pp:5212-5218
Publication Date(Web):10 Jun 2015
DOI:10.1039/C5PY00662G
The ring-opening polymerization (ZROP) of δ-valerolactone (VL) by N-heterocyclic carbenes (NHC) in the absence of alcohol initiators generates cyclic poly(valerolactones). A zwitterionic mechanism has been proposed where the propagating alkoxide anion undergoes ion-pairing with the acyl-imidazolium polymer chain end. To evaluate the consequences of zwitterionic ion-pairing, the influence of added LiCl to the NHC-mediated ZROP of δ-valerolactone (VL) in THF was investigated. In THF in the absence of LiCl, the molecular weights of the resultant p(VL) are higher than that predicted from the initial ratio of [VL]0/[NHC]0 and the molecular weight distributions are bimodal and broad (Mw/Mn = 1.35). In the presence of 1.0 M LiCl, the molecular weights of the p(VL) match that predicted from [VL]0/[NHC]0 and the molecular weight distributions are monomodal and narrow (Mw/Mn = 1.08–1.26). The presence of LiCl also influences both the rate of polymerization and the topology of the resultant poly(valerolactones). In the absence of LiCl, cyclic poly(valerolactones) are isolated upon aqueous methanol work-up, but at high LiCl concentrations, only linear chains are produced. These results imply that zwitterionic ion-pairs can be readily broken up by added LiCl.
Co-reporter:Gavin O. Jones, Young A. Chang, Hans W. Horn, Ashwin K. Acharya, Julia E. Rice, James L. Hedrick, and Robert M. Waymouth
The Journal of Physical Chemistry B 2015 Volume 119(Issue 17) pp:5728-5737
Publication Date(Web):April 7, 2015
DOI:10.1021/acs.jpcb.5b01595
Computational investigations with density functional theory (DFT) have been performed on the N-heterocyclic carbene (NHC) catalyzed ring-opening polymerization of ε-caprolactone in the presence and in the absence of a methanol initiator. Much like the zwitterionic ring opening (ZROP) of δ-valerolactone which was previously reported, calculations predict that the mechanism of the ZROP of caprolactone that occurs without an alcohol present involves a high-barrier step involving ring opening of the zwitterionic tetrahedral intermediate formed after the initial nucleophilic attack of NHC on caprolactone. However, the operative mechanism by which caprolactone is polymerized in the presence of an alcohol initiator does not involve the analogous mechanism involving initial nucleophilic attack by the organocatalytic NHC. Instead, the NHC activates the alcohol through hydrogen bonding and promotes nucleophilic attack and the subsequent ring-opening steps that occur during polymerization. The largest free energy barrier for the hydrogen-bonding mechanism in alcohol involves nucleophilic attack, while that for both ZROP processes involves ring opening of the initially formed zwitterionic tetrahedral intermediate. The DFT calculations predict that the rate of polymerization in the presence of alcohol is faster than the reaction performed without an alcohol initiator; this prediction has been validated by experimental kinetic studies.
Co-reporter:Timothy R. Blake
Journal of the American Chemical Society 2014 Volume 136(Issue 26) pp:9252-9255
Publication Date(Web):June 19, 2014
DOI:10.1021/ja503830c
The oxidative lactonization of N-substituted diethanolamines with the Pd catalyst [LPd(OAc)]22+[OTf–]2 generates N-substituted morpholin-2-ones. The organocatalytic ring-opening polymerization of N-acyl morpholin-2-ones occurs readily to generate functionalized poly(aminoesters) with N-acylated amines in the polyester backbone. The thermodynamics of the ring-opening polymerization depends sensitively on the hybridization of the nitrogen of the heterocyclic lactone. N-Acyl morpholin-2-ones polymerize readily to generate polymorpholinones, but the N-aryl or N-alkyl substituted morpholin-2-ones do not polymerize. Experimental and theoretical studies reveal that the thermodynamics of ring opening correlates to the degree of pyramidalization of the endocyclic N-atom. Deprotection of the poly(N-Boc-morpholin-2-one) yields a water-soluble, cationic polymorpholinone.
Co-reporter:Xiangyi Zhang and Robert M. Waymouth
ACS Macro Letters 2014 Volume 3(Issue 10) pp:1024
Publication Date(Web):September 23, 2014
DOI:10.1021/mz500525n
Bicyclic isothioureas 1 and 2 mediate controlled ring opening polymerizations (ROP) of lactides in the absence of protic initiators to afford high molecular weight polylactides (PLA) with narrow polydispersities. The cyclic structure of the resulting PLA was determined by dilute solution viscosity measurement and MALDI-TOF mass spectrometry. Compared to DBU initiator, isothioureas are more selective for producing cyclic PLA without appreciable linear contaminants. Mechanistic studies involving acyl amidinium support our hypothesis that DBU-initiated ZROP generates linear chains from a ketene aminal intermediate.
Co-reporter:Ashwin K. Acharya, Young A. Chang, Gavin O. Jones, Julia E. Rice, James L. Hedrick, Hans W. Horn, and Robert M. Waymouth
The Journal of Physical Chemistry B 2014 Volume 118(Issue 24) pp:6553-6560
Publication Date(Web):April 4, 2014
DOI:10.1021/jp500200b
Experimental and computational investigations of the zwitterionic ring-opening polymerization (ZROP) of δ-valerolactone (VL) catalyzed by the N-heterocyclic carbenes (NHC) 1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene (1) and 1,3,4,5-tetramethyl-imidazol-2-ylidene (2) were carried out. The ZROP of δ-valerolactone generates cyclic poly(valerolactone)s whose molecular weights are higher than predicted from [VL]0/[NHC]0. Kinetic studies reveal the rate of polymerization is first order in [VL] and first order in [NHC]. Density functional theory (DFT) calculations were carried out to elucidate the key steps involved in the ring-opening of δ-valerolactone and its subsequent oligomerization. These studies have established that the initial steps of the mechanism involve nucleophilic attack of the NHC on δ-valerolactone to form a zwitterionic tetrahedral intermediate. DFT calculations indicate that the highest activation barrier of the entire mechanism is associated with the ring-opening of the tetrahedral intermediate formed from the NHC and δ-valerolactone, a result consistent with inefficient initiation to generate reactive zwitterions. The large barrier in this step is due to the fact that ring-opening requires a partial positive charge to develop next to the directly attached NHC moiety which already bears a delocalized positive charge.
Co-reporter:Hayley A. Brown, Silei Xiong, Grigori A. Medvedev, Young A. Chang, Mahdi M. Abu-Omar, James M. Caruthers, and Robert M. Waymouth
Macromolecules 2014 Volume 47(Issue 9) pp:2955-2963
Publication Date(Web):April 29, 2014
DOI:10.1021/ma500395j
Investigations of the kinetics of zwitterionic ring-opening polymerization of ε-caprolactone by N-heterocyclic carbenes (NHC) were carried out to illuminate the key reaction steps responsible for the formation of high molecular weight cyclic poly(caprolactones). Modeling of both the decay in monomer concentration as well as the evolution of molecular weights and polydispersities were necessary to identify the key reaction steps responsible for initiation, propagation, cyclization and chain-transfer. Nucleophilic attack of the NHC on ε-caprolactone to generate reactive zwitterions is slow and reversible. The modeling indicates that less than 60% of the carbenes are transformed to active zwitterions, but that these zwitterions rapidly add monomer and cyclize by intramolecular backbiting of the terminal alkoxides on internal esters of the zwitterions. This cyclization event maintains the concentration of active zwitterions. The reactivation of cyclized chains by active zwitterions is a key step that leads to high molecular weight poly(caprolactones).
Co-reporter:Tyler S. Stukenbroeker, Diego Solis-Ibarra, and Robert M. Waymouth
Macromolecules 2014 Volume 47(Issue 23) pp:8224-8230
Publication Date(Web):November 17, 2014
DOI:10.1021/ma501764c
The zwitterionic ring-opening polymerization of 2-isopropoxy-2-oxo-1,3,2-dioxaphospholane (iPP) with N-heterocyclic carbenes (NHC) generates poly(alkylene phosphate)s with molecular weights of Mn = 55000–202000 Da. MALDI-TOF mass spectrometry provided clear evidence for cyclic poly(alkylene phosphate)s (poly(iPP)) for lower molecular weight fractions (m/z ≤ 3000). The cyclic topology of the higher molecular weight fractions was inferred by trapping of poly(iPP) in cross-linked 2-hydroxyethyl methacrylate (HEMA) hydrogels. Cross-linked HEMA hydrogels were generated in the presence of a high molecular weight (Mn = 202000 Da) poly(iPP) generated from the zwitterionic ring-opening polymerization of iPP with the NHC 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene 2. Extraction of the resulting gel with methanol for 11 days revealed that 36% of the poly(iPP) was retained in the gel, whereas a linear poly(iPP) was completely extracted under similar conditions. The retention of the poly(iPP)s in the gels is attributed to topological trapping of the cyclic poly(iPP) in the cross-linked network.
Co-reporter:Andrew J. Ingram;Dr. Diego Solis-Ibarra; Richard N. Zare; Robert M. Waymouth
Angewandte Chemie International Edition 2014 Volume 53( Issue 22) pp:5648-5652
Publication Date(Web):
DOI:10.1002/anie.201400134
Abstract
The activation of O2 is a key step in selective catalytic aerobic oxidation reactions mediated by transition metals. The bridging trinuclear palladium species, [(LPdII)3(μ3-O)2]2+ (L=2,9-dimethylphenanthroline), was identified during the [LPd(OAc)]2(OTf)2-catalyzed aerobic oxidation of 1,2-propanediol. Independent synthesis, structural characterization, and catalytic studies of the trinuclear compound show that it is a product of oxygen activation by reduced palladium species and is a competent intermediate in the catalytic aerobic oxidation of alcohols. The formation and catalytic activity of the trinuclear Pd3O2 species illuminates a multinuclear pathway for aerobic oxidation reactions catalyzed by Pd complexes.
Co-reporter:Hayley A. Brown and Robert M. Waymouth
Accounts of Chemical Research 2013 Volume 46(Issue 11) pp:2585
Publication Date(Web):June 21, 2013
DOI:10.1021/ar400072z
Cyclic polymers are an intriguing class of macromolecules. Because of the constraints of the cyclic topology and the absence of chain ends, the properties of these molecules differ from those of linear polymers in ways that remain poorly understood. Cyclic polymers present formidable synthetic challenges because the entropic penalty of coupling the chain ends grows exponentially with increasing molecular weight.In this Account, we describe recent progress in the application of zwitterionic ring-opening polymerization (ZROP) as a strategy for the synthesis of high molecular weight, cyclic polymers. Zwitterionic ring-opening polymerization involves the addition of neutral organic nucleophiles to strained heterocyclic monomers; under appropriate conditions, cyclization of the resultant macrozwitterions generates cyclic macromolecules. We discuss the mechanistic and kinetic features of these zwitterionic ring-opening reactions and the conditions that influence the efficiency of the initiation, propagation, and cyclization to generate high molecular weight cyclic polymers.N-Heterocyclic carbenes (NHC) are potent nucleophiles and relatively poor leaving groups, two features that are important for the generation of high molecular weight polymers. Investigations of the nature of the monomer and nucleophile have helped researchers understand the factors that govern the reactivity of these systems and their impact on the molecular weight and molecular weight distributions of the resulting cyclic polymers. We focus primarily on ZROP mediated by N-heterocyclic carbene nucleophiles but also discuss zwitterionic polymerizations with amidine, pyridine, and imidazole nucleophiles. The ZROP of N-carboxyanhydrides with N-hetereocyclic carbenes generates a family of functionalized cyclic polypeptoids. We can synthesize gradient lactone copolymers by exploiting differences in relative reactivity present in ZROP that differ from those of traditional metal-mediated polymerizations. These new synthetic methods have allowed us to investigate the influence of topology on the crystallization behavior, stereocomplexation, and solution properties of cyclic macromolecules.
Co-reporter:Hayley A. Brown ; Young A. Chang
Journal of the American Chemical Society 2013 Volume 135(Issue 50) pp:18738-18741
Publication Date(Web):December 5, 2013
DOI:10.1021/ja409843v
The zwitterionic ring-opening of 2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopentane (TMOSC) with N-heterocyclic carbenes generates high molecular weight cyclic p(TMOSC). The NHC-mediated polymerization of TMOSC with 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes, 1) generates the poly(carbosiloxane) p(TMOSC) with molecular weights from 27 000 < Mn < 80 000 Da (1.4 < Mw/Mn < 2.2) within 30 min at room temp. With the more nucleophilic carbene 1,3,4,5-tetramethyl-imidazol-2-ylidene (4), the ring-opening polymerization occurs within minutes at room temperature to generate cyclic p(TMOSC) with molecular weights up to Mn = 940 000 Da (Mw/Mn = 3.2). The resulting p(TMOSC)s are predominantly cyclic as evidenced by dilute solution viscosity studies and MALDI-TOF MS. DFT calculations provide support for both zwitterionic and neutral, cyclic intermediates.
Co-reporter:Kevin Chung ; Steven M. Banik ; Antonio G. De Crisci ; David M. Pearson ; Timothy R. Blake ; Johan V. Olsson ; Andrew J. Ingram ; Richard N. Zare
Journal of the American Chemical Society 2013 Volume 135(Issue 20) pp:7593-7602
Publication Date(Web):May 9, 2013
DOI:10.1021/ja4008694
The regio- and chemoselective oxidation of unprotected vicinal polyols with [(neocuproine)Pd(OAc)]2(OTf)2 (1) (neocuproine = 2,9-dimethyl-1,10-phenanthroline) occurs readily under mild reaction conditions to generate α-hydroxy ketones. The oxidation of vicinal diols is both faster and more selective than the oxidation of primary and secondary alcohols; vicinal 1,2-diols are oxidized selectively to hydroxy ketones, whereas primary alcohols are oxidized in preference to secondary alcohols. Oxidative lactonization of 1,5-diols yields cyclic lactones. Catalyst loadings as low as 0.12 mol % in oxidation reactions on a 10 g scale can be used. The exquisite selectivity of this catalyst system is evident in the chemoselective and stereospecific oxidation of the polyol (S,S)-1,2,3,4-tetrahydroxybutane [(S,S)-threitol] to (S)-erythrulose. Mechanistic, kinetic, and theoretical studies revealed that the rate laws for the oxidation of primary and secondary alcohols differ from those of diols. Density functional theory calculations support the conclusion that β-hydride elimination to give hydroxy ketones is product-determining for the oxidation of vicinal diols, whereas for primary and secondary alcohols, pre-equilibria favoring primary alkoxides are product-determining. In situ desorption electrospray ionization mass spectrometry (DESI-MS) revealed several key intermediates in the proposed catalytic cycle.
Co-reporter:Kristen R. Brownell ; Charles C. L. McCrory ; Christopher E. D. Chidsey ; Richard H. Perry ; Richard N. Zare
Journal of the American Chemical Society 2013 Volume 135(Issue 38) pp:14299-14305
Publication Date(Web):September 17, 2013
DOI:10.1021/ja4055564
Ruthenium transfer hydrogenation catalysts physisorbed onto edge-plane graphite electrodes are active electrocatalysts for the oxidation of alcohols. Electrooxidation of CH3OH (1.23 M) in a buffered aqueous solution at pH 11.5 with [(η6-p-cymene)(η2-N,O-(1R,2S)-cis-1-amino-2-indanol)]RuIICl (2) on edge-plane graphite exhibits an onset current at 560 mV vs NHE. Koutecky–Levich analysis at 750 mV reveals a four-electron oxidation of methanol with a rate of 1.35 M–1 s–1. Mechanistic investigations by 1H NMR, cyclic voltammetry, and desorption electrospray ionization mass spectrometry indicate that the electroxidation of methanol to generate formate is mediated by surface-supported Ru–oxo complexes.
Co-reporter:Elizabeth T. Kiesewetter and Robert M. Waymouth
Macromolecules 2013 Volume 46(Issue 7) pp:2569-2575
Publication Date(Web):March 29, 2013
DOI:10.1021/ma400116w
Octahedral group IV bis(phenolate) catalysts are highly active catalysts for the isospecific polymerization of 1-hexene and the copolymerization of ethylene with 1-hexene. These catalysts are active for the production of high molecular weight copolymers even at 130 °C. The copolymerization parameters for these complexes were determined; all of the bis(phenolate) complexes tested incorporate 1-hexene with high efficiency to give random copolymers. The complexes prepared from the more sterically demanding ligands showed higher molecular weights but similar comonomer incorporations to those prepared from the less sterically demanding ligands.
Co-reporter:Antonio G. De Crisci, Kevin Chung, Allen G. Oliver, Diego Solis-Ibarra, and Robert M. Waymouth
Organometallics 2013 Volume 32(Issue 7) pp:2257-2266
Publication Date(Web):March 21, 2013
DOI:10.1021/om4001549
Chiral palladium-based catalysts derived from pyridinyl oxazoline (pyOx) ligands catalyze the oxidation of alcohols, including 1,2-diols, triols, and tetraols, with high regio- and chemoselectivity. Screening of various chiral oxazoline-derived ligands for the oxidation of a model diol, 1,2-propanediol (1,2-PD), revealed that the nature of the ligand had a significant influence on the activity and chemoselectivity for oxidation of vicinal diols. The PyOx ligands containing an α-methyl substituent were the most active for the oxidation of 1,2-PD using benzoquinone as the terminal oxidant. Oxidation of vicinal diols and polyols occurs selectively at the secondary alcohol to afford α-hydroxy ketones in isolated yields of 62–87%. Chemoselective oxidation of meso-erythritol with the chiral [(S)-(α-Me(tert-Bu)PyOx)Pd(OAc)]2[OTf]2 afforded (S)-erthyrulose in 62% yield and 24% ee.
Co-reporter:Hayley A. Brown, Antonio G. De Crisci, James L. Hedrick, and Robert M. Waymouth
ACS Macro Letters 2012 Volume 1(Issue 9) pp:1113
Publication Date(Web):August 23, 2012
DOI:10.1021/mz300276u
The ring-opening polymerization (ROP) of lactide with DBU (1,8-diazabicyclo[5.4.0] undec-7-ene) is described. Room temperature polymerization using the neutral amine catalyst DBU in the absence of any other initiator produces polymers with narrow polydispersities and shows a linear relationship between molecular weight and conversion. The resulting polymers were characterized and determined to be cyclic. DFT calculations support a mechanistic hypothesis involving a zwitterionic acyl amidinium intermediate.
Co-reporter:Hyunuk Kim, Johan V. Olsson, James L. Hedrick, and Robert M. Waymouth
ACS Macro Letters 2012 Volume 1(Issue 7) pp:845
Publication Date(Web):June 19, 2012
DOI:10.1021/mz3001397
A facile one-step synthesis of functionalized valerolactones was carried out by the conjugate addition of thiols to the α,β-unsaturated valerolactone 5,6-dihydro-2H-pyran-2-one. The resultant 3-mercaptovalerolactones undergo ring-opening polymerization in solution or in the melt to generate polyesters functionalized either with benzyl mercaptans or oligoethylene glycol pendant groups. The copolymerization of the 3-mercaptovalerolactones with ε-caprolactone generates random copolymers.
Co-reporter:Jeff Simon, Johan V. Olsson, Hyunuk Kim, Ian F. Tenney, and Robert M. Waymouth
Macromolecules 2012 Volume 45(Issue 23) pp:9275-9281
Publication Date(Web):November 21, 2012
DOI:10.1021/ma302311h
The ring-opening polymerization of glycerol-derived six-membered cyclic dimethylacetal dihydroxyacetone carbonate (MeO2DHAC) have been studied both in solution and bulk conditions with organic catalysts. The guanidine 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) was the most active catalyst in solution, whereas the thiourea/sparteine catalytic system displayed the most predictable kinetics. Ring-opening polymerization of MeO2DHAC or copolymerization with ε-caprolactone (CL) in the melt occurred readily with TBD as catalyst to afford random copolymers. Acetal deprotection afforded the polycarbonate poly(dihydroxyactone carbonate) (p(DHAC)) or poly(carbonate ester) copolymers p(DHAC-r-CL). The polycarbonate p(DHAC) is a high-melting thermoplastic with a melting point of 246 °C. The p(DHAC-r-CL) copolymers all displayed semicrystalline behavior as evidenced by DSC and WAXS analysis with Tg and Tm changing as a function of comonomer composition. These new materials could have potential use in biomedical applications or as biomass-derived thermoplastics.
Co-reporter:Eun Ji Shin, Alexandra E. Jones, and Robert M. Waymouth
Macromolecules 2012 Volume 45(Issue 1) pp:595-598
Publication Date(Web):December 15, 2011
DOI:10.1021/ma202184j
Co-reporter:Justin A. Edward, Matthew K. Kiesewetter, Hyunuk Kim, James C.A. Flanagan, James L. Hedrick, and Robert M. Waymouth
Biomacromolecules 2012 Volume 13(Issue 8) pp:
Publication Date(Web):July 31, 2012
DOI:10.1021/bm300718b
The ring-opening polymerization of substituted cyclic carbonates with 1-(3,5-bis-trifluoromethyl-phenyl)-3-cyclohexyl-thiourea (TU)/1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) organocatalysts afford highly functionalized oligocarbonates. The fluorescent alkaloid quinine can be readily incorporated into the oligocarbonates either by initiation from quinine or by ring-opening polymerization of a quinine-functionalized cyclic carbonate (MTC-Q). Copolymerization of MTC-Q with a boc-protected guanidinium cyclic carbonate affords, after deprotection, highly water-soluble cationic copolymers functionalized with both quinine and pendant guanidinium groups. When multiple quinine groups are attached to the oligomers, they exhibit minimal fluorescence due to self-quenching. Upon hydrolysis, the fluorescence intensity increases, providing a potential strategy for monitoring the hydrolysis rates in real time.
Co-reporter:Sören Roll;Elizabeth T. Kiesewetter
Journal of Polymer Science Part A: Polymer Chemistry 2012 Volume 50( Issue 13) pp:2604-2611
Publication Date(Web):
DOI:10.1002/pola.26028
Abstract
Octahedral group 4 bisphenolate ether complexes, activated by methylaluminoxane, were tested in propylene polymerization in the presence and absence of diethyl zinc. The resulting polypropylenes were analyzed thoroughly by means of differential scanning calorimetry and 13C NMR techniques. Despite structural similarity of the Hf and Zr complexes, the performance in propylene polymerization differs significantly in terms of productivity, isospecificity, and propensity to incorporate specific regioerrors. These catalysts are capable of generating high-molecular weight polypropylene (Mn = 130,000–360,000 g/mol) with isotacticities [mmmm] up to 97% and melting points as high as 165 °C when very bulky ligands are used. 13C NMR analysis revealed that the type and the number of regioerrors being incorporated into the polymer chain highly depend on the ligand and the metal. Polymerizations in the presence of diethyl zinc generate saturated low-molecular weight polypropylenes (Mn = 1700–9900 g/mol) and facilitated an end-group analysis, which revealed the presence of isobutyl and 2-methylbutyl groups. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012
Co-reporter:Matthew K. Kiesewetter ; Justin A. Edward ; Hyunuk Kim
Journal of the American Chemical Society 2011 Volume 133(Issue 41) pp:16390-16393
Publication Date(Web):September 22, 2011
DOI:10.1021/ja207465h
The catalytic condensation of cis-2-butene-1,4-diol with CpRu(MQA)(C3H5) (Cp = cyclopentadienyl, MQA = 4-methoxyquinoline-2-carboxylate) generates poly(2-butenediol), an unsaturated telechelic polyether diol with molecular weights between 400 and 4600 g/mol. This Ru(IV) allyl catalyst enchains 2-butene-1,4-diol primarily as the linear trans-2-butenyl ether (92%) along with vinyl branches (8%). These telechelic oligomers are useful chain extenders and macromonomers, as demonstrated by their use in the synthesis of poly(lactide)-b-poly(butenediol)-b-poly(lactide) triblock copolymers. Model studies support a proposed mechanism involving the formation of Ru(IV) allyl intermediates from allylic alcohols and chain growth by selective nucleophilic displacement at the terminus of the Ru(IV) allyl to generate trans-2-butenyl ether linkages.
Co-reporter:David M. Pearson;Nicholas R. Conley
Advanced Synthesis & Catalysis 2011 Volume 353( Issue 16) pp:3007-3013
Publication Date(Web):
DOI:10.1002/adsc.201100240
Abstract
The catalytic alkoxycarbonylation of 1,2-diols by (neocuproine)palladium(II) acetate (neocuproine=2,9-dimethyl-1,10-phenanthroline) or palladium(II) acetate/(−)-sparteine using N-chlorosuccinimide as the oxidant affords cyclic carbonates. The oxidative carbonylation of diols proceeds under mild conditions, requiring only 1 atm of carbon monoxide, and produces cyclic carbonates in moderate to good yields. Both 1,2- and 1,3-diols can be carbonylated using (neocuproine)Pd(OAc)2 and sodium dichloroisocyanuric acid, which serves as a competent oxidant and base for this system, to yield 5- and 6-membered cyclic carbonates.
Co-reporter:Kyung-sun Son;David M. Pearson;Sang-Jin Jeon
European Journal of Inorganic Chemistry 2011 Volume 2011( Issue 27) pp:4256-4261
Publication Date(Web):
DOI:10.1002/ejic.201100492
Abstract
The synthesis, characterization, and coordination chemistry of a series of mono-, di-, and trinuclear homo-/heterometallic complexes with the dinucleating ligand, N,N′-bis[(2-diphenyl-phosphanyl)phenyl]formamidine (pnnp)1 are reported. Treatment of (pnnp)PdMe (1) with NiCl2(dme) forms the heterobimetallic (pnnp)PdMe(μ-Cl)NiCl (3) complex, whereas reaction of 1 with Cu(CH3CN)4+PF6– yields the Pd···Cu···Pd heterometallic trinuclear complex (pnnp)Pd(Me)CuPd(Me)(pnnp)·PF6 (8). The reaction of pnnp with NiCl2(dme) (dme = 1,2-dimethoxyethane) produces the Ni dimer [(pnnp)NiCl(μ-Cl)]2 (5) or the dinuclear (pnnp)NiCl(μ-X)NiCl (X = Cl or OH) (4) complexes, depending upon the stoichiometry. The reaction of pnnp with CoII or FeII ions leads to the monometallic complexes (pnnp)CoCl2 (6) or (pnnp)FeCl2 (7), respectively. The dinuclear (pnnp)PdMe(μ-Cl)NiCl (3) and (pnnp)NiCl(μ-X)NiCl (X = Cl or OH) (4) complexes adopt A-frame structures with two square-planar d8 metal complexes hinged by the bridging ligand; the angles between the square planes correlate to the covalent radii, in which the smaller homo-bimetallic Ni complex adopts a more coplanar arrangement of the two Ni square planes. The heterometallic trinuclear complex (pnnp)Pd(Me)CuPd(Me)(pnnp)·PF6 (8) adopts an unusual structure around the Cu center, suggestive of a weak d8–d10 interaction between the Pd and Cu atoms. All complexes have been characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography, and their structural diversity is discussed.
Co-reporter:Eun Ji Shin;Hayley A. Brown;Silvia Gonzalez;Dr. Wonhee Jeong;Dr. James L. Hedrick;Dr. Robert M. Waymouth
Angewandte Chemie International Edition 2011 Volume 50( Issue 28) pp:6388-6391
Publication Date(Web):
DOI:10.1002/anie.201101853
Co-reporter:Eun Ji Shin;Hayley A. Brown;Silvia Gonzalez;Dr. Wonhee Jeong;Dr. James L. Hedrick;Dr. Robert M. Waymouth
Angewandte Chemie 2011 Volume 123( Issue 28) pp:6512-6515
Publication Date(Web):
DOI:10.1002/ange.201101853
Co-reporter:Eun Ji Shin, Wonhee Jeong, Hayley A. Brown, Bon Jun Koo, James L. Hedrick, and Robert M. Waymouth
Macromolecules 2011 Volume 44(Issue 8) pp:2773-2779
Publication Date(Web):March 18, 2011
DOI:10.1021/ma102970m
High molecular weight poly(ε-caprolactone)s (PCL) were synthesized via zwitterionic polymerization of ε-caprolactone initiated with N-heterocyclic carbenes. Ring-opening polymerization of ε-caprolactone (1 M) with carbenes 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (1) or 1,3,4,5-tetramethylimidazol-2-ylidene (3) affords cyclic PCL with molecular weights up to Mn = 114 000 g/mol. Crystallization of the cyclic PCL was investigated by synchrotron small-angle X-ray scattering experiments and differential scanning calorimetry. High molecular weight cyclic poly(ε-caprolactone) crystallizes with a similar lamellar thickness and long period spacing as linear poly(ε-caprolactone), but the crystallization of cyclic poly(ε-caprolactone) is faster than that of linear poly(ε-caprolactone) for molecular weights greater than 75 000 g/mol. These results imply that the polymer topology does not have a significant influence on the crystal structure or morphology but can have a significant influence on the rate of crystallization from the melt.
Co-reporter:David M. Pearson, Nicholas R. Conley, and Robert M. Waymouth
Organometallics 2011 Volume 30(Issue 6) pp:1445-1453
Publication Date(Web):February 18, 2011
DOI:10.1021/om101037k
The complex [(neocuproine)Pd(OAc)]2[OTf]2 (1) catalyzes the aerobic oxidation of 2-heptanol at room temperature, but competitive ligand oxidation leads to low catalyst lifetimes. In an effort to mitigate the oxidative degradation of the ligand, a variety of Pd complexes ligated by fluorinated phenanthrolines (bis-2,9-(trifluoromethyl)-1,10-phenanthroline (btfm-phen), 4-methyl-2-(trifluoromethyl)-1,10-phenanthroline (tfmm-phen), and 2-(o-difluorophenyl)-1,10-phenanthroline (odfp-phen)) were prepared and tested. Active catalyst precursors were generated by in situ conproportionation of (N−N)Pd(OAc)2 and [(N−N)Pd(NCCH3)2[OTf]2. Pd complexes derived from tfmm-phen catalyzed the aerobic oxidation of 2-heptanol at room temperature with up to 22 turnovers, nearly double that of 1.
Co-reporter:Elizabeth T. Kiesewetter ; Sören Randoll ; Madalyn Radlauer
Journal of the American Chemical Society 2010 Volume 132(Issue 16) pp:5566-5567
Publication Date(Web):April 1, 2010
DOI:10.1021/ja100627x
Octahedral group 4 bisphenolate ether complexes, activated by methylaluminoxane, are highly active and stereospecific α-olefin polymerization catalysts. X-ray crystallographic analysis reveals the Zr and Hf complexes to be closely isostructural; the bond lengths of the Hf complex are slightly shorter, but the maximum deviation is only 0.062 Å. Despite the structural similarity of the Hf and Zr complexes, the Hf complexes generate more highly stereoselective catalysts. In addition to the influence of the transition metal, the structure of the ligand has a large influence on the stereospecificity. Bis-tert-butyl phenyl substituted complexes of Hf and Zr, when activated by MAO at 50−80 °C, generate high molecular weight polypropylene (Mn = 130 000−360 000 g/mol) with isotacticities [mmmm] > 97% and melting points as high as 165 °C.
Co-reporter:Nahrain E. Kamber, Yasuhito Tsujii, Kate Keets and Robert M. Waymouth, Russell C. Pratt, Gregory W. Nyce and James L. Hedrick
Journal of Chemical Education 2010 Volume 87(Issue 5) pp:519-521
Publication Date(Web):March 23, 2010
DOI:10.1021/ed800152c
The depolymerization of the plastic polyethylene terephthalate (PET or PETE) is described in this laboratory procedure. The transesterification reaction used to depolymerize PET employs a highly efficient N-heterocyclic carbene catalyst derived from a commercially available imidazolium ionic liquid. N-heterocyclic carbenes are potent nucleophilic organic catalysts that provide an alternative to traditional metal alkoxide depolymerization catalysts. Deprotonation of the imidazolium generates an N-heterocyclic carbene, which catalyzed the glycolysis of PET in 1 h at atmospheric pressure in refluxing anhydrous tetrahydrofuran to afford bis(2-hydroxyethyl) terephthalate (BHET). BHET is characterized by NMR and IR spectroscopy and melting point determination. This laboratory is designed for individual student work where each student depolymerizes PET prepared from a common water bottle or other source. This experiment is well suited for an introductory organic chemistry laboratory course or a polymer chemistry laboratory course.Keywords (Audience): Second-Year Undergraduate; Keywords (Domain): Environmental Chemistry; Laboratory Instruction; Organic Chemistry; Polymer Chemistry; Keywords (Pedagogy): Hands-On Learning/Manipulatives; Keywords (Topic): Applications of Chemistry; Catalysis; Consumer Chemistry; Esters; Green Chemistry; Mechanisms of Reactions; Polymerization; Reactions; Synthesis;
Co-reporter:Kyung-sun Son and Robert M. Waymouth
Organometallics 2010 Volume 29(Issue 16) pp:3515-3520
Publication Date(Web):July 20, 2010
DOI:10.1021/om100503v
The selective oligomerization of ethylene to 1-hexene or 1-octene provides a selective route to these valuable α-olefins. Selective trimerization and tetramerization of ethylene with Cr catalysts have been proposed to occur by a mechanism involving metallacycle intermediates. We envisioned that the intermediacy of metallacycles could provide an opportunity to generate new classes of functionalized ethylene oligomers by chain transfer to a transmetalation reagent. Herein, we report the influence of transmetalation agents such as zinc alkyls on the selectivity of ethylene oligomerization with a Cr(PNP)Cl3/MAO system (PNP = Ph2PN(iPr)PPh2). Oligomerization of ethylene with Cr(PNP)Cl3/MAO at 200 psig of ethylene at 25 or 45 °C generates 1-hexene, 1-octene, C10−C22 oligomers, and polyethylene. The addition of ZnR2 increases the productivity and selectivity for C10−C22 oligomers and results in a decrease in the amount and molecular weight of the polyethylene generated. Analysis of the product distribution after quenching with D2O reveals that 1-hexene and 1-octene are unlabeled, but that all higher oligomers are deuteriated alkanes or alkenes, depending on the nature of the zinc alkyl (R = Me, Et, Bu). Mechanistic proposals are presented to explain the formation of differentially functionalized coproducts in the presence of zinc alkyls. These studies reveal that oligomerization in the presence of ZnR2 provides a new route to end-functionalized ethylene oligomers and new insights on the reactivity of metallacycle intermediates in the Cr-catalyzed oligomerization of ethylene.
Co-reporter:Matthew K. Kiesewetter and Robert M. Waymouth
Organometallics 2010 Volume 29(Issue 22) pp:6051-6056
Publication Date(Web):October 27, 2010
DOI:10.1021/om100892v
Kynurenic allyl ester reacts readily with [CpRu(NCCH3)3]PF6 in dry acetone to yield the RuIV allyl. The complex is an effective catalyst for the hydrolysis of methyl allyl carbonate in aqueous solution in air yet decomposes rapidly in aerobic solutions of methanol. Kinetic studies are consistent with rate-limiting attack of water (or methanol) on the RuIV allyl as a key step. A polystyrene-supported version of the catalyst was active for the catalytic hydrolysis of methylallyl carbonate, but kinetic studies indicate that the Ru leaches from polystyrene upon repeated catalytic cycles.
Co-reporter:Kyung-Sun Son
Journal of Polymer Science Part A: Polymer Chemistry 2010 Volume 48( Issue 7) pp:1579-1585
Publication Date(Web):
DOI:10.1002/pola.23912
Abstract
A series of monocyclopentadienyl titanium complexes containing a pendant amine donor on a Cp group (A = CpTiCl3, B = CpNTiCl3, C = CpNTiCl2TEMPO, for Cp = C5H5, CpN = C5H4CH2CH2N(CH3)2, and TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) are investigated for styrene homopolymerization and ethylene–styrene (ES) copolymerization. When activated by methylaluminoxane at 70 °C, complexes with the amine group (B and C) are active for styrene homopolymerization and afford syndiotactic polystyrene (sPS). The copolymerizations of ethylene and styrene with B and C yield high-molecular weight ES copolymer, whereas complex A yields mixtures of sPS and polyethylene, revealing the critical role that the pendant amine has on the polymerization behavior of the complexes. Fractionation, NMR, and DSC analyses of the ES copolymers generated from B and C suggest that they contain sPS. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1579–1585, 2010
Co-reporter:Ron M. Painter;David M. Pearson ;Dr. Robert M. Waymouth
Angewandte Chemie 2010 Volume 122( Issue 49) pp:9646-9649
Publication Date(Web):
DOI:10.1002/ange.201004063
Co-reporter:Ron M. Painter;David M. Pearson ;Dr. Robert M. Waymouth
Angewandte Chemie International Edition 2010 Volume 49( Issue 49) pp:9456-9459
Publication Date(Web):
DOI:10.1002/anie.201004063
Co-reporter:Andrew P. Dove, Elizabeth T. Kiesewetter, Xavier Ottenwaelder and Robert M. Waymouth
Organometallics 2009 Volume 28(Issue 2) pp:405-412
Publication Date(Web):December 11, 2008
DOI:10.1021/om800571j
Complexes of the type Cp*TiX2(ONR′R′′) (Cp* = η5-C5Me5; X = Me, Cl; R = R′ = Et, TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl); X = R′ = Me, R′′ = tBu) were synthesized by several routes. Upon activation with [Ph3C]+[B(C6F5)4]− and AliBu3, these complexes generate highly active catalysts for propylene polymerization, although significant catalyst deactivation was observed. Activation with B(C6F5)3/AliBu3 or methylaluminoxane (MAO) resulted in reduced polymerization activity, the latter leading to increased catalyst lifetime. Model studies showed that the interaction of AlMe3 with Cp*Ti(Me)2(ONEt2) led to the formation of Cp*Ti(Me)2(η2-O(AlMe3)NEt2). The X-ray crystal structure confirmed that the hydroxylaminato ligand remained η2 bound to the titanium center with the AlMe3 bound to the complex through the oxygen atom of the hydroxylaminato ligand. Exchange reactions with organic ethers revealed the metalloether to be a comparable donor to PhOMe. Cp*Ti(Me)2(ONtBu(Me)) was revealed to be a weaker donor than Cp*Ti(Me)2(ONEt2); Cp*Ti(Me)2(TEMPO) did not bind to AlMe3. AliBu3 bound more weakly to Cp*Ti(Me)2(ONEt2) than AlMe3. Reaction of Cp*Ti(Me)2(ONEt2) and Cp*Ti(Me)2(ONtBu(Me)) with B(C6F5)3 resulted in clean formation of the zwitterionic contact ion pairs [Cp*Ti(Me)(η2-ONEt2)]+[MeB(C6F5)3]− and [Cp*Ti(Me)(η2-ONtBu(Me))]+[MeB(C6F5)3]−, respectively, whereas reaction of Cp*Ti(Me)2(η2-ONtBu(Me)) with [Ph3C]+[B(C6F5)4]− resulted in the clean formation of the solvent-separated ion pair [Cp*Ti(Me)(η2-ONtBu(Me))]+[B(C6F5)4]−. Reaction of Cp*Ti(Me)2(η1-TEMPO) with B(C6F5)3 results in the elimination of methane to result in the formation of the contact ion paired [Cp*Ti(η2-TEMPO)]+[MeB(C6F5)3]−, in which one of the TEMPO methyl groups has undergone C−H activation, resulting in a η2-bound TEMPO ligand, confirmed by 1H, gROESY, and 1H−15N HMBC NMR. Addition of AliBu3 or AlMe3 to the cationic [Cp*Ti(Me)(ONtBu(Me))]+[B(C6F5)4]− resulted in decomposition of the cations.
Co-reporter:David M. Pearson and Robert M. Waymouth
Organometallics 2009 Volume 28(Issue 13) pp:3896-3900
Publication Date(Web):June 9, 2009
DOI:10.1021/om900217s
The catalytic oxidation of methanol in the presence of the cationic [neocuproinePd(OAc)]2[OTf]2 (OTf = trifluoromethylsulfonate), 1, yields methyl formate. With benzoquinone as a terminal oxidant 4-hydroxyphenyl formate is also formed competitively. Mechanistic and labeling studies implicate formaldehyde and methyl hemiformal as key intermediates. Beta-hydrogen elimination from Pd-OCH3 or Pd-OCH2OCH3 intermediates is proposed as a key step in the generation of formaldehyde and methyl formate, respectively.
Co-reporter:Nahrain E. Kamber, Wonhee Jeong, Silvia Gonzalez, James L. Hedrick and Robert M. Waymouth
Macromolecules 2009 Volume 42(Issue 5) pp:1634-1639
Publication Date(Web):February 17, 2009
DOI:10.1021/ma802618h
We report the synthesis of poly(ε-caprolactone) utilizing N-heterocyclic carbene (NHC) organocatalysts. Sterically unencumbered NHCs were found to be highly effective for the living ring-opening polymerization of ε-caprolactone under mild conditions. The NHCs were able to produce poly(ε-caprolactone)s with controlled molecular weights, low polydispersities, and well-defined end groups derived from the alcohol initiator. Star-shaped poly(ε-caprolactone)s with three and four arms were prepared using NHC organocatalysts and characterized by 1H NMR and GPC.
Co-reporter:Sang-Jin Jeon and Robert M. Waymouth
Dalton Transactions 2008 (Issue 4) pp:437-439
Publication Date(Web):08 Nov 2007
DOI:10.1039/B715212D
Bimetallic Ni, Rh, and Ir complexes of pyrazolate biscarbene containing bulky substituents have been synthesized and characterized by X-ray crystallography; the Ni complex dimerizes to a highly congested L2Ni2 structure, whereas the corresponding Rh and Ir complexes form bimetallic LM2 structures.
Co-reporter:Simon H. Stelzig;Matthias Tamm
Journal of Polymer Science Part A: Polymer Chemistry 2008 Volume 46( Issue 18) pp:6064-6070
Publication Date(Web):
DOI:10.1002/pola.22918
Abstract
Monocyclopentadienyl titanium imidazolin-2-iminato complexes [Cp′Ti(L)X2] 1a (Cp′ = cyclopentadienyl, L = 1,3-di-tert-butylimidazolin-2-imide, X = Cl), 1b (X = CH3); 2 (Cp′ = cyclopentadienyl, L = 1,3-diisopropylimidazolin-2-imide, X = Cl); 3 (Cp′ = tert-butylcyclopentadienyl, L = 1,3-di-tert-butylimidazolin-2-imide, X = Cl), upon activation with methylaluminoxane (MAO) were active for the polymerization of ethylene and propylene and the copolymerization of ethylene and 1-hexene. Catalysts derived from imidazolin-2-iminato tropidinyl titanium complex 4 = [(Trop)Ti(L)Cl2] (Trop = tropidinyl, L = 1,3-di-tert-butylimidazolin-2-imide) were much less active. Narrow polydispersities were observed for ethylene and propylene polymerization, but the copolymerization of ethylene/hexene led to bimodal molecular weight distributions. The productivity of catalysts derived from the dialkyl complex 1b activated with [Ph3C][B(C6F5)4] or B(C6F5)3 were less active for ethylene/hexene copolymerization but yielded ethylene/hexene copolymers of narrower molecular weight distributions than those derived from 1a/MAO. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6064–6070, 2008
Co-reporter:Marc D. Scholten, James L. Hedrick and Robert M. Waymouth
Macromolecules 2008 Volume 41(Issue 20) pp:7399-7404
Publication Date(Web):September 25, 2008
DOI:10.1021/ma801281q
Group transfer polymerization (GTP) is an effective method for the synthesis of poly(methyl methacrylate) (pMMA) with controlled molecular weights and narrow polydispersities. Silyl ketene acetals initiate the polymerization of methyl methacrylate in the presence of either nucleophilic or Lewis acid catalysts. We report the use of N-heterocyclic carbenes (NHCs) as neutral nucleophilic catalysts for GTP of methyl methacrylate (MMA) and tert-butyl acrylate (TBA). For MMA, polymer molecular weights increase linearly with conversion and are predictable over a range of [M]0/[I]0 ratios. Polydispersities trend downward with conversion and approach 1.2 at >95% conversion. Significantly, NHCs were also shown to be effective for the controlled GTP of TBA, generating living polymers with predictable molecular weights, narrow molecular weight distributions (Mw/Mn ≤ 1.2), and living chain ends, as illustrated by chain-extension experiments.
Co-reporter:Kyung-sun Son, Frank Jöge and Robert M. Waymouth
Macromolecules 2008 Volume 41(Issue 24) pp:9663-9668
Publication Date(Web):November 17, 2008
DOI:10.1021/ma801853f
The copolymerization of ethylene and styrene at 120 °C was investigated with a series of titanocenes: Cp′TiCl2L [1, 2: Cp′ = cyclopentadienyl (Cp) versus 3, 4: Cp′ = {2-(dimethylamino)ethyl}cyclopentadienyl (CpN); 1, 3: L = Cl versus 2, 4: L = 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)], in combination with methylaluminoxane (MAO). At 120 °C, titanocenes bearing a pendant amine ligand (3 and 4) are effective catalysts for the ethylene-styrene (ES) copolymerization, whereas complexes lacking the pendant group (1 and 2) afford only mixtures of polyethylene (PE) and atactic polystyrene (aPS). At these temperatures, copolymerization with complexes 3 and 4 yields mixtures of ES copolymers and atactic polystyrene that are not readily separated by solvent extraction or gel permeation chromatography as both components of the mixture exhibit similar molecular weights and solubility in THF and acetone. The source of the atactic polystyrene was the autopolymerization of styrene as deduced by carrying out the copolymerization in the presence of a catalytic chain transfer agent, cobalt tetraphenylporphyrin (Co(tpp)). The addition of Co(tpp) caused a decrease in the molecular weight of the atactic polystyrene but had no effect on the molecular weight of the ES copolymer.
Co-reporter:Stefan Benson;Bridgett Payne
Journal of Polymer Science Part A: Polymer Chemistry 2007 Volume 45(Issue 16) pp:3637-3647
Publication Date(Web):21 JUN 2007
DOI:10.1002/pola.22113
Two new N-heterocyclic carbene enolate nickel(II) allyl complexes have been prepared and their activity towards ethylene polymerization was investigated. It was found that in the presence of diethyl zinc, the carbene enolate complex bearing a nitro substituent produces highly linear polyethylene of modest molecular weight and high polydispersity. The influence of the reaction parameters on catalytic activity and the characteristics of the resulting polymer were investigated through systematic variation of the time, temperature, and diethyl zinc concentration. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45:3637–3647, 2007
Co-reporter:Darcy A. Culkin Dr.;Wonhee Jeong;Szilárd Csihony Dr.;Enrique D. Gomez;Nitash P. Balsara Dr.;James L. Hedrick Dr.;Robert M. Waymouth Dr.
Angewandte Chemie International Edition 2007 Volume 46(Issue 15) pp:
Publication Date(Web):2 MAR 2007
DOI:10.1002/anie.200604740
Closing the loop: The zwitterionic ring-opening polymerization of lactide in the presence of N-heterocyclic carbenes generates well-defined cyclic poly(lactide) (see scheme). The polymerization is rapid and produces macrocyclic polymers with high purity and exceptional control of molecular weight and polydispersity.
Co-reporter:Darcy A. Culkin Dr.;Wonhee Jeong;Szilárd Csihony Dr.;Enrique D. Gomez;Nitash P. Balsara Dr.;James L. Hedrick Dr.;Robert M. Waymouth Dr.
Angewandte Chemie 2007 Volume 119(Issue 15) pp:
Publication Date(Web):2 MAR 2007
DOI:10.1002/ange.200604740
Der Ring wird geschlossen: Die zwitterionische Ringöffnungspolymerisation von Lactid in Gegenwart N-heterocyclischer Carbene liefert gut definiertes cyclisches Polylactid (siehe Schema). Die Polymerisation verläuft schnell und resultiert unter außergewöhnlicher Kontrolle des Molekulargewichts und der Polydispersität in hoch reinen makrocyclischen Polymeren.
Co-reporter:Olivier Coulembier;Matthew K. Kiesewetter;Andrew Mason;Philippe Dubois ;James L. Hedrick Dr.;Robert M. Waymouth
Angewandte Chemie 2007 Volume 119(Issue 25) pp:
Publication Date(Web):15 MAY 2007
DOI:10.1002/ange.200700522
Verzweigung möglich: Ein einstufiger Zugang zu verzweigten Copolymeren wird vorgestellt. Primäre Amine erwiesen sich als difunktionelle Initiatoren der Ringöffnungspolymerisation in Gegenwart von 1, wobei sie die Polymerisation zweier Ketten begünstigen, was das einfache Einführen von Verzweigungspunkten in Blockcopolymere ermöglicht.
Co-reporter:Olivier Coulembier;Matthew K. Kiesewetter;Andrew Mason;Philippe Dubois ;James L. Hedrick Dr.;Robert M. Waymouth
Angewandte Chemie International Edition 2007 Volume 46(Issue 25) pp:
Publication Date(Web):15 MAY 2007
DOI:10.1002/anie.200700522
Branch manager: A one-step approach to branched copolymers is described. Primary amines are found to function as bifunctional initiators for ring-opening polymerization in the presence of 1 to promote the polymerization of two chains, thereby enabling the facile introduction of branch points in block copolymers.
Co-reporter:Andrew P. Dove, Hongbo Li, Russell C. Pratt, Bas G. G. Lohmeijer, Darcy A. Culkin, Robert M. Waymouth and James L. Hedrick
Chemical Communications 2006 (Issue 27) pp:2881-2883
Publication Date(Web):06 Jun 2006
DOI:10.1039/B601393G
New sterically encumbered N-heterocyclic carbene catalysts were synthesized and used to polymerize rac-lactide to give highly isotactic polylactide or meso-lactide to give heterotactic polylactide.
Co-reporter:Kuo-Wei Huang;Adam P. Cole;Bradley M. Kraft;Robert M. Waymouth
Helvetica Chimica Acta 2006 Volume 89(Issue 8) pp:1589-1595
Publication Date(Web):30 AUG 2006
DOI:10.1002/hlca.200690159
A novel titanium complex with a chelating nitroxide ligand, dichloro-2-{[(hydroxy-κO)phenylamino-κN)phenylmethyl}phenolato(2−)-2κO}(tetrahydrofuran)titanium (1), was synthesized and characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray crystallography. In the presence of methylaluminoxane (MAO), 1 displayed moderate activity for the polymerization of propylene.
Co-reporter:Benjamin E. Ketz, Xavier G. Ottenwaelder and Robert M. Waymouth
Chemical Communications 2005 (Issue 45) pp:5693-5695
Publication Date(Web):20 Oct 2005
DOI:10.1039/B511202H
Two novel N-heterocylic carbene enolate nickel complexes have been prepared and shown to be active for ethylene and propylene polymerization to yield linear polymers.
Co-reporter:Andrew P. Dove, Xiangjin Xie and Robert M. Waymouth
Chemical Communications 2005 (Issue 16) pp:2152-2154
Publication Date(Web):04 Mar 2005
DOI:10.1039/B418778D
Half sandwich complexes of titanium bearing η1 or η2 bound nitroxide ligands are highly active catalysts for the polymerisation of propylene to high molecular weight atactic poly(propylene).
Co-reporter:Joyce Hung
Journal of Polymer Science Part A: Polymer Chemistry 2005 Volume 43(Issue 17) pp:3840-3851
Publication Date(Web):14 JUL 2005
DOI:10.1002/pola.20862
A new disilyl-bridged complex, [(N-tert-butylamido)(3-indenyl)tetramethyldisilyl]titanium dichloride (3), was synthesized and activated with methylaluminoxane (MAO) for propylene homopolymerization and ethylene/propylene and ethylene/1-hexene copolymerizations. A polypropylene with a slight isotactic enrichment was obtained. The number of regioerrors present in the polypropylene was somewhat smaller than that found in most polypropylenes made from monosilyl-bridged [(N-tert-butylamido)(3-indenyl)dimethylsilyl]titanium dichloride. The regioerrors detected in the copolymers obtained from 3/MAO were on the order of the amounts observed in polymers made with the monosilyl-bridged constrained geometry catalysts. Ethylene copolymers of propylene and 1-hexene had random sequence distributions and showed significant comonomer incorporation. Because of the presence of regioerrors, a modified method for determining the monomer composition and sequence distribution was developed from the direct measurement of the monomer content from the number of methylene and methine carbons per polymer chain, regardless of propylene inversion. An estimate of the error in the copolymerization reactivity ratio determination for regioirregular ethylene/α-olefin copolymers was obtained by the calculation of the reactivity ratios from monomer dyad sequences, with consideration given to the contribution of major regioirregular sequences. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3840–3851, 2005
Co-reporter:Olivier Coulembier;Andrew P. Dove Dr.;Russell C. Pratt Dr.;Alan C. Sentman Dr.;Darcy A. Culkin Dr.;Laetitia Mespouille;Philippe Dubois ;James L. Hedrick Dr.
Angewandte Chemie 2005 Volume 117(Issue 31) pp:
Publication Date(Web):11 JUL 2005
DOI:10.1002/ange.200500723
Kontrolle ist alles: Alkoholaddukte stabiler N-heterocyclischer Carbene sind ausgezeichnete Einkomponenten-Katalysatoren/Initiatoren für die Ringöffnungspolymerisation von Lactid (siehe Schema). Diese Polymerisation kann in einfacher Weise durch Temperaturmodulation reversibel beendet werden. Die Verwendung multifunktioneller Makroinitiatoren ermöglicht die Synthese komplexerer Polymerarchitekturen.
Co-reporter:Olivier Coulembier;Andrew P. Dove Dr.;Russell C. Pratt Dr.;Alan C. Sentman Dr.;Darcy A. Culkin Dr.;Laetitia Mespouille;Philippe Dubois ;James L. Hedrick Dr.
Angewandte Chemie International Edition 2005 Volume 44(Issue 31) pp:
Publication Date(Web):11 JUL 2005
DOI:10.1002/anie.200500723
It's all about control: Alcohol adducts of stable N-heterocyclic carbenes function as excellent single-component catalysts/initiators for the ring-opening polymerization of lactide (see scheme). This polymerization can be reversibly terminated simply by modulating the temperature. The use of multifunctional macroinitiators enables the synthesis of more complex polymer architectures.
Co-reporter:Szilárd Csihony;Tristan T. Beaudette;Alan C. Sentman;Gregory W. Nyce;Robert M. Waymouth;James L. Hedrick
Advanced Synthesis & Catalysis 2004 Volume 346(Issue 9-10) pp:
Publication Date(Web):21 SEP 2004
DOI:10.1002/adsc.200404097
The ring-opening polymerization of lactide with commercially available Bredereck-type reagents in the presence or absence of alcohol initiators was carried out affording polylactide with controlled molecular weight and narrow polydispersities. An anionic mechanism involving heterolytic cleavage to alkoxides is proposed, where these reagents function as latent anionic initiators for the ring-opening polymerization of lactide.
Co-reporter:Kuo-Wei Huang and Robert M. Waymouth
Dalton Transactions 2004 (Issue 3) pp:354-356
Publication Date(Web):14 Jan 2004
DOI:10.1039/B314027J
The η1-hydroxylamido half-titanocene complex, CpTiCl2(TEMPO) 1, hydrolyzes extremely efficiently to generate (CpTiClO)4 and the protonated hydroxylamine. The efficient hydrolysis chemistry provides selective syntheses of CpTi dialkoxide and diaryloxide complexes.
Co-reporter:Sarah E. Reybuck
Journal of Polymer Science Part A: Polymer Chemistry 2004 Volume 42(Issue 13) pp:3323-3331
Publication Date(Web):25 MAY 2004
DOI:10.1002/pola.20170
Ethylene/1-hexene copolymerizations with disiloxane-bridged metallocenes, rac- and meso-1,1,3,3-tetramethyldisiloxanediyl-bis(1-indenyl)zirconium dichloride (rac-1, meso-1) activated by modified methylaluminoxane were performed to investigate the influence of conformational dynamics on comonomer selectivity. Although 1H NOESY (nuclear Overhauser and exchange spectroscopy) analysis indicated that the most stable conformation for the meso isomer in solution was that in which both indenes project over the metal coordination site, this isomer showed higher 1-hexene selectivity in copolymerization (re = 140 ± 30, rh = 0.024 ± 0.004) than the rac isomer with only one indene over the coordination site (re = 240 ± 20, rh = 0.005 ± 0.001). The meso isomer showed high 1-hexene selectivity, a high product of reactivity ratios (rerh = 3.3 ± 0.5) and produced copolymers that could be separated into fractions with different ethylene content suggesting that the active species exhibited multisite behavior and populated conformations with different comonomer selectivities during the copolymerization. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3323–3331, 2004
Co-reporter:Gregory W. Nyce Dr.;Szilard Csihony Dr. ;James L. Hedrick Dr.
Chemistry - A European Journal 2004 Volume 10(Issue 16) pp:
Publication Date(Web):12 AUG 2004
DOI:10.1002/chem.200490054
Co-reporter:Gregory W. Nyce Dr.;Szilard Csihony Dr. ;James L. Hedrick Dr.
Chemistry - A European Journal 2004 Volume 10(Issue 16) pp:
Publication Date(Web):19 JUL 2004
DOI:10.1002/chem.200400196
The synthesis of N-heterocyclic carbene (NHC) adducts by condensation of diamines with appropriately substituted benzaldehydes is described. This simplified approach provides the NHC adduct without first having to generate the carbene followed by its protection. These adducts undergo thermal deprotection to generate N-heterocyclic carbene in situ. Adduct decomposition temperatures were investigated as a function of catalyst structure by using thermal analysis and spectroscopic techniques. Importantly, unlike adducts derived from chloroform, the new pentafluorobenzene-based adducts are more readily prepared and are stable at room temperature. The utility of these adducts as organic catalyst precursors for living ring-opening polymerization (ROP) of lactide, transesterification reactions, and the synthesis of N-heterocyclic carbene ligated organometallic complexes is also described.
Co-reporter:Adrien R. Lavoie, Michael H. Ho and Robert M. Waymouth
Chemical Communications 2003 (Issue 7) pp:864-865
Publication Date(Web):04 Mar 2003
DOI:10.1039/B300841J
The stereoselective copolymerization of cyclopentene (cP) and ethylene (E) to generate highly alternating polymers with isotactic cis 1,2-cyclopentene enchainment is reported.
Co-reporter:Mahesh K. Mahanthappa, Kuo-Wei Huang, Adam P. Cole and Robert M. Waymouth
Chemical Communications 2002 (Issue 5) pp:502-503
Publication Date(Web):11 Feb 2002
DOI:10.1039/B110147A
Two titanium compounds containing monoanionic ligands derived from TEMPO were synthesized and structurally characterized, demonstrating the flexibility of the coordination modes adopted by the ligand.
Co-reporter:Stephen C. Diehl
Israel Journal of Chemistry 2002 Volume 42(Issue 4) pp:393-401
Publication Date(Web):8 MAR 2010
DOI:10.1560/6N2N-BV7Q-MW0G-BR6J
Mono- and bis 2-arylindenyl zirconocene/methylalumoxane (MAO) catalysts are prepared on a crosslinked polystyrene resin using gel-phase organic reactions. The 2-arylindene ligands are bonded to the polystyrene backbone through a disiloxane linkage, and the zirconocene is prepared directly on the support in high yield. These supported 2-arylindenylzirconocenes in the presence of MAO show high activity and yield polypropylenes with microstructures similar to those produced by the analogous solution-phase catalysts.
Co-reporter:Jaewook Myung, James C.A. Flanagan, Robert M. Waymouth, Craig S. Criddle
Process Biochemistry (May 2016) Volume 51(Issue 5) pp:561-567
Publication Date(Web):1 May 2016
DOI:10.1016/j.procbio.2016.02.005
•Pure cultures of Type II methanotrophs enabled PHBV synthesis.•Cells incubated with both C1 substrates and odd-carbon VFAs produced PHBV.•In the absence of C1 substrates, neither P3HB nor PHBV was synthesized.•Oxidation of C1 substrates generates the energy required for 3HV incorporation.•Incorporation of 13C-labeled valerate was confirmed using NMR.Obligate methanotrophs capable of producing polyhydroxyalkanoates (PHAs), a sustainable alternative to non-biodegradable petroleum-based plastics, under nutrient-limited conditions are currently limited to synthesis of poly(3-hydroxybutyrate) (P3HB). Diversifying monomers would greatly expand the range of applications. Here we report the first pure culture evidence of methanotrophic synthesis of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). When grown with methane as sole substrate, the Type II obligate methanotroph Methylocystis parvus OBBP produces P3HB, but not PHBV, under nutrient-limiting conditions. We observed synthesis of PHBV when strain OBBP was incubated with C1 substrates (methane, methanol, or formate) and propionate or valerate. PHBV production was confirmed by gas chromatography, gel permeation chromatography, differential scanning calorimetry, and nuclear magnetic resonance with [1-13C]valerate. In the absence of C1-substrate oxidation, neither P3HB nor PHBV was synthesized. The mol% 3-hydroxyvalerate depended upon the amount of propionate or valerate consumed, and ranged from 0 to 40 mol%. Valerate addition resulted in a higher mol% 3-hydroxyvalerate and higher wt% PHA than propionate addition. To assess the generality of this capability among Type II obligate methanotrophs, we added methane and valerate to Methylosinus trichosporium OB3b, with similar results. We hypothesize that oxidation of C1 substrates generates the energy required for activation of co-substrates and incorporation of 3-hydroxyvalerate.Download full-size image
Co-reporter:Sang-Jin Jeon and Robert M. Waymouth
Dalton Transactions 2008(Issue 4) pp:NaN439-439
Publication Date(Web):2007/11/08
DOI:10.1039/B715212D
Bimetallic Ni, Rh, and Ir complexes of pyrazolate biscarbene containing bulky substituents have been synthesized and characterized by X-ray crystallography; the Ni complex dimerizes to a highly congested L2Ni2 structure, whereas the corresponding Rh and Ir complexes form bimetallic LM2 structures.
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