The alkylative carboxylation of allenamide catalyzed by an N-heterocyclic carbene (NHC)–copper(I) complex [(IPr)CuCl] with CO₂ and dialkylzinc reagents was investigated. The reaction of allenamides with dialkylzinc reagents (1.5 equiv) and CO₂ (1 atm.) proceeded smoothly in the presence of a catalytic quantity of [(IPr)CuCl] to afford (Z)-α,β-dehydro-β-amino acid esters in good yields. The reaction is regioselective, with the alkyl group introduced onto the less hindered γ-carbon, and the carboxyl group introduced onto the β-carbon atom of the allenamides. The first step of the reaction was alkylative zincation of the allenamides to give an alkenylzinc intermediate followed by nucleophilic addition to CO₂. A variety of cyclic and acyclic allenamides were found to be applicable to this transformation. Dialkylzinc reagents bearing β-hydrogen atoms, such as Et₂Zn or Bu₂Zn, also gave the corresponding alkylative carboxylation products without β-hydride elimination. The present methodology provides an easy route to alkyl-substituted α,β-dehydro-β-amino acid ester derivatives under mild reaction conditions with high regio- and stereoselectivtiy.

The CH bond carboxylation of various aromatic compounds with CO₂ was achieved by the deprotonative alumination with a mixed alkyl amido lithium aluminate compound iBu₃Al(TMP)Li followed by the NHC-copper-catalyzed carboxylation of the resulting arylaluminum species, which afforded the corresponding carboxylation products in high yield and high selectivity. In addition to benzene derivatives, heteroarenes such as benzofuran, benzothiophene, and indole derivatives are also suitable substrates. Functional groups such as Cl, Br, I, vinyl, amide, and CN could survive the reaction conditions. Some key reaction intermediates such as the copper aryl and isobutyl complexes and their carboxylation products were isolated and structurally characterized by X-ray crystallographic analyses, thus offering important information on the reaction mechanism.

The catalytic CH addition of pyridines to allenes has been achieved for the first time by using a half-sandwich scandium catalyst, thus constituting a straightforward and atom-economical route for the synthesis of alkenylated pyridine derivatives. The reaction proceeded regio- and stereoselectively, affording a new family of alkenylated pyridine compounds which are otherwise difficult to synthesize. A cationic Sc-η²-pyridyl species was isolated and confirmed to be a key catalyst species in this transformation.

Alkylative carboxylation of ynamides with CO₂ and dialkylzinc reagents using a N-heterocyclic carbene (NHC)–copper catalyst has been developed. A variety of ynamides, both cyclic and acyclic, undergo this transformation under mild conditions to afford the corresponding α,β-unsaturated carboxylic acids, which contain the α,β-dehydroamino acid skeleton. The present alkylative carboxylation formally consists of Cu-catalyzed carbozincation of ynamides with dialkylzinc reagents with the subsequent nucleophilic carboxylation of the resulting alkenylzinc species with CO₂. Dialkylzinc reagents bearing a β-hydrogen atom such as Et₂Zn and Bu₂Zn still afford the alkylated products despite the potential for β-hydride elimination. This protocol would be a desirable method for the synthesis of highly substituted α,β- dehydroamino acid derivatives due to its high regio- and stereoselectivity, simple one-pot procedure, and its use of CO₂ as a starting material.

The sequential hydroalumination or methylalumination of various alkynes catalyzed by different catalyst systems, such those based on Sc, Zr, and Ni complexes, and the subsequent carboxylation of the resulting alkenylaluminum species with CO₂ catalyzed by an N-heterocyclic carbene (NHC)–copper catalyst have been examined in detail. The regio- and stereoselectivity of the overall reaction relied largely on the hydroalumination or methylalumination reactions, which significantly depended on the catalyst and alkyne substrates. The subsequent Cu-catalyzed carboxylation proceeded with retention of the stereoconfiguration of the alkenylaluminum species. All the reactions could be carried out in one-pot to afford efficiently a variety of α,β-unsaturated carboxylic acids with well-controlled configurations, which are difficult to construct by previously reported methods. This protocol could be practically useful and attractive because of its high regio- and stereoselectivity, simple one-pot reaction operation, and the use of CO₂ as a starting material.

A new family of Y₄/M₂ and Y₅/M heterobimetallic rare-earth-metal/d-block-transition-metalpolyhydride complexes has been synthesized. The reactions of the tetranuclear yttriumoctahydride complex [{Cp′′Y(μ-H)₂}₄(thf)₄] (Cp′′=C₅Me₄H, 1-C₅Me₄H) with one equivalent of Group-6-metalpentahydride complexes [Cp*M(PMe₃)H₅] (M=Mo, W; Cp*=C₅Me₅) afforded pentanuclear heterobimetallic Y₄/Mpolyhydride complexes [{(Cp′′Y)₄(μ-H)₇}(μ-H)₄MCp*(PMe₃)] (M=Mo (2 a), W (2 b)). UV irradiation of compounds 2 a,b in THF gave PMe₃-free complexes [{(Cp′′Y)₄(μ-H)₆(thf)₂}(μ-H)₅MCp*] (M=Mo (3 a), W (3 b)). Compounds 3 a,b reacted with one equivalent of [Cp*M(PMe₃)H₅] to afford hexanuclear Y₄/M₂ complexes [{Cp*M(μ-H)₅}{(Cp′′Y)₄(μ-H)₅}{(μ-H)₄MCp*(PMe₃)}] (M=Mo (4 a), W (4 b)). UV irradiation of compounds 4 a,b provided the PMe₃-free complexes [(Cp′′Y)₄(μ-H)₄{(μ-H)₅MCp*}₂] (M=Mo (5 a), W (5 b)). C₅Me₄Et-ligated analogue [(Cp′′Y)₄(μ-H)₄{(μ-H)₅Mo(C₅Me₄Et)}₂] (5 a′) was obtained from the reaction of 1-C₅Me₄H with [(C₅Me₄Et)Mo(PMe₃)H₅]. On the other hand, the reaction of pentanuclear yttriumdecahydride complex [{(C₅Me₄R)Y(μ-H)₂}₅(thf)₂] (1-C₅Me₅: R=Me; 1-C₅Me₄Et: R=Et) with [Cp*M(PMe₃)H₅] gave the hexanuclear heterobimetallic Y₅/Mpolyhydride complexes [({(C₅Me₄R)Y}₅(μ-H)₈)(μ-H)₅MCp*] (6 a: M=Mo, R=Me; 6 a′: M=Mo, R=Et; 6 b: M=W, R=Me). Compound 5 a released two molecules of H₂ under vacuum to give [(Cp′′Y)₄(μ-H)₂{(μ-H)₄MoCp*}₂] (7). In contrast, compound 6 a lost one molecule of H₂ under vacuum to yield [{(Cp*Y)₅(μ-H)₇}(μ-H)₄MoCp*] (8). Both compounds 7 and 8 readily reacted with H₂ to regenerate compounds 5 a and 6 a, respectively. The structures of compounds 4 a, 5 a′, 6 a′, 7, and 8 were determined by single-crystal X-ray diffraction.

A series of half-sandwich scandium–dialkyl complexes that bear various auxiliary ligands have been examined for the copolymerization of isoprene (IP) with nonconjugated α,ω-dienes such as 1,5-hexadiene (HD) and 1,6-heptadiene (HPD). Significant ligand influence on the catalytic activity and selectivity has been observed. The thf-coordinated complex [(C₅Me₄SiMe₃)Sc(CH₂SiMe₃)₂(thf)] (2) and the methoxy side arm containing the half-sandwich complex [(C₅Me₄C₆H₄OMe-o)Sc(CH₂SiMe₃)₂] (3), in combination with an equivalent of [Ph₃C][B(C₆F₅)₄], can serve as excellent catalysts for the random cyclocopolymerizations of IP with HD and HPD. The resulting random HD–IP copolymers contain five-membered-ring methylene-1,3-cyclopentane (MCP), 3,4-polyisoprene (3,4-IP), and 1,4-polyisoprene (1,4-IP) units with controllable HD incorporation in a range of 17–82 mol %. The random HPD–IP copolymers possess six-membered-ring methylene-1,3-cyclohexane (MCH), 1,4-IP, and 3,4-IP units with HPD incorporation in a range of 11–55 mol %. By use of a catalyst that bears a phosphine oxide group [{C₅Me₄SiMe₂CH₂P(O)Ph₂}Sc(CH₂SiMe₃)₂] (5), the alternating copolymerizations of IP with HD and HPD have been achieved for the first time in which HD and HPD are completely cyclized to the MCP and MCH units, respectively. More remarkably, in the alternating copolymerization of HPD and IP by 5, the regio- and stereospecific cis-MCH selectivity reached as high as 99 %. The microstructures and compositions of these copolymers showed significant influences on their mechanical properties.

The reactivity of the cubane-type rare-earth methylidene complex [Cp′Lu(μ₃-CH₂)]₄ (1, Cp′=C₅Me₄SiMe₃) with various unsaturated electrophiles was investigated. The reaction of 1 with CO (1 atm) at room temperature gave the bis(ketene dianion)/dimethylidene complex [Cp′₄Lu₄(μ₃-CH₂)₂(μ₃,η²-O-CCH₂)₂] (2) in 86 % yield through the insertion of two molecules of CO into two of the four lutetium–methylidene units. In the reaction with the sterically demanding N,N-diisopropylcarbodiimide at 60 °C, only one of the four methylidene units in 1 reacted with one molecule of the carbodiimide substrate to give the mono(ethylene diamido)/trimethylidene complex [Cp′₄Lu₄(μ₃-CH₂)₃{iPrNC(=CH₂)NiPr}] (3) in 83 % yield. Similarly, the reaction of 1 with phenyl isothiocyanate gave the ethylene amido thiolate/trimethylidene complex [Cp′₄Lu₄(μ₃-CH₂)₃{PhNC(S)=CH₂}] (4). In the case of phenyl isocyanate, two of the four methylidene units in 1 reacted with four molecules of the substrate at ambient temperature to give the malonodiimidate/dimethylidene complex [Cp′₄Lu₄(μ₃-CH₂)₂{PhN=C(O)CH₂(O)CNPh}₂] (5) in 87 % yield. In this reaction, each of the two lutetium–methylidene bonds per methylidene unit inserted one molecule of phenyl isocyanate. All the products have been fully characterized by NMR spectroscopy, X-ray diffraction, and microelemental analyses.

A series of rare-earth-metal–hydrocarbyl complexes bearing N-type functionalized cyclopentadienyl (Cp) and fluorenyl (Flu) ligands were facilely synthesized. Treatment of [Y(CH₂SiMe₃)₃(thf)₂] with equimolar amount of the electron-donating aminophenyl-Cp ligand C₅Me₄H-C₆H₄-o-NMe₂ afforded the corresponding binuclear monoalkyl complex [({C₅Me₄-C₆H₄-o-NMe(μ-CH₂)}Y{CH₂SiMe₃})₂] (1 a) via alkyl abstraction and CH activation of the NMe₂ group. The lutetium bis(allyl) complex [(C₅Me₄-C₆H₄-o-NMe₂)Lu(η³-C₃H₅)₂] (2 b), which contained an electron-donating aminophenyl-Cp ligand, was isolated from the sequential metathesis reactions of LuCl₃ with (C₅Me₄-C₆H₄-o-NMe₂)Li (1 equiv) and C₃H₅MgCl (2 equiv). Following a similar procedure, the yttrium- and scandium–bis(allyl) complexes, [(C₅Me₄-C₅H₄N)Ln(η³-C₃H₅)₂] (Ln=Y (3 a), Sc (3 b)), which also contained electron-withdrawing pyridyl-Cp ligands, were also obtained selectively. Deprotonation of the bulky pyridyl-Flu ligand (C₁₃H₉-C₅H₄N) by [Ln(CH₂SiMe₃)₃(thf)₂] generated the rare-earth-metal–dialkyl complexes, [(η³-C₁₃H₈-C₅H₄N)Ln(CH₂SiMe₃)₂(thf)] (Ln=Y (4 a), Sc (4 b), Lu (4 c)), in which an unusual asymmetric η³-allyl bonding mode of Flu moiety was observed. Switching to the bidentate yttrium–trisalkyl complex [Y(CH₂C₆H₄-o-NMe₂)₃], the same reaction conditions afforded the corresponding yttrium bis(aminobenzyl) complex [(η³-C₁₃H₈-C₅H₄N)Y(CH₂C₆H₄-o-NMe₂)₂] (5). Complexes 1–5 were fully characterized by ¹H and ¹³C NMR and X-ray spectroscopy, and by elemental analysis. In the presence of both [Ph₃C][B(C₆F₅)₄] and AliBu₃, the electron-donating aminophenyl-Cp-based complexes 1 and 2 did not show any activity towards styrene polymerization. In striking contrast, upon activation with [Ph₃C][B(C₆F₅)₄] only, the electron-withdrawing pyridyl-Cp-based complexes 3, in particular scandium complex 3 b, exhibited outstanding activitiy to give perfectly syndiotactic (rrrr >99 %) polystyrene, whereas their bulky pyridyl-Flu analogues (4 and 5) in combination with [Ph₃C][B(C₆F₅)₄] and AliBu₃ displayed much-lower activity to afford syndiotactic-enriched polystyrene.

The acid–base reaction of [Ln(CH₂SiMe₃)₃(thf)₂] with Cp′H gave the corresponding half-sandwich rare earth dialkyl complexes [(Cp′)Ln(CH₂SiMe₃)₂(thf)] (1-Ln: Ln=Sc, Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; Cp′=C₅Me₄SiMe₃) in 62–90 % isolated yields. X-ray crystallographic studies revealed that all of these complexes adopt a similar overall structure, in spite of large difference in metal-ion size. In most cases, the hydrogenolysis of the dialkyl complexes in toluene gave the tetranuclear octahydride complexes [{(Cp′)Ln(μ-H)₂}₄(thf)_x] (2-Ln: Ln=Sc, x=0; Y, x=1; Er, x=1; Tm, x=1; Gd, x=1; Dy, x=1; Ho, x=1) as the only isolable product. However, in the case of Lu, a trinuclear pentahydride [(Cp′)₂Lu₃(μ-H)₅(μ-CH₂SiMe₂C₅Me₄)(thf)₂] (3), in which the CH activation of a methyl group of the Me₃Si unit on a Cp′ ligand took place, was obtained as a major product (66 % yield), in addition to the tetranuclear octahydride [{(Cp′)Lu(μ-H)₂}₄(thf)] (2-Lu, 34 %). The use of hexane instead of toluene as a solvent for the hydrogenolysis of 1-Lu led to formation of 2-Lu as a major product (85 %), while a similar reaction in THF yielded 3 predominantly (90 %). The tetranuclear octahydride complexes of early (larger) lanthanide metals [{Cp′Ln(μ-H)₂}₄(thf)₂] (2, Ln=La, Ce, Pr, Nd, Sm) were obtained in 38–57 % isolated yields by hydrogenolysis of the bis(aminobenzyl) species [Cp′Ln(CH₂C₆H₄NMe₂-o)₂], which were generated in-situ by reaction of [Ln(CH₂C₆H₄NMe₂-o)₃] with one equivalent of Cp′H. X-ray crystallographic studies showed that the fine structures of these hydride clusters are dependent on the size of the metal ions.

The acid-base reactions between the rare-earth metal (Ln) tris(ortho-N,N-dimethylaminobenzyl) complexes [Ln(CH₂C₆H₄NMe₂-o)₃] with one equivalent of the silylene-linked cyclopentadiene-amine ligand (C₅Me₄H)SiMe₂NH(C₆H₂Me₃-2,4,6) afforded the corresponding half-sandwich aminobenzyl complexes [{Me₂Si(C₅Me₄)(NC₆H₂Me₃-2,4,6)}Ln(CH₂C₆H₄NMe₂-o)(thf)] (2-Ln) (Ln=Y, La, Pr, Nd, Sm, Gd, Lu) in 60–87 % isolated yields. The one-pot reaction between ScCl₃ and [Me₂Si(C₅Me₄)(NC₆H₂Me₃-2,4,6)]Li₂ followed by reaction with LiCH₂C₆H₄NMe₂-o in THF gave the scandium analogue [{Me₂Si(C₅Me₄)(NC₆H₂Me₃-2,4,6)}Sc(CH₂C₆H₄NMe₂-o)] (2-Sc) in 67 % isolated yield. 2-Sc could not be prepared by the acid-base reaction between [Sc(CH₂C₆H₄NMe₂-o)₃] and (C₅Me₄H)SiMe₂NH(C₆H₂Me₃-2,4,6). These half-sandwich rare-earth metal aminobenzyl complexes can serve as efficient catalyst precursors for the catalytic addition of various phosphine PH bonds to carbodiimides to form a series of phosphaguanidine derivatives with excellent tolerability to aromatic carbon-halogen bonds. A significant increase in the catalytic activity was observed, as a result of an increase in the metal size with a general trend of La>Pr, Nd>Sm>Gd>Lu>Sc. The reaction of 2-La with 1 equiv of Ph₂PH yielded the corresponding phosphide complex [{Me₂Si(C₅Me₄)(NC₆H₂Me₃-2,4,6)}La(PPh₂)(thf)₂] (4), which, on recrystallization from benzene, gave the dimeric analogue [{Me₂Si(C₅Me₄)(NC₆H₂Me₃-2,4,6)}La(PPh₂)]₂ (5). Addition of 4 or 5 to iPrNCNiPr in THF yielded the phosphaguanidinate complex [{Me₂Si(C₅Me₄)(NC₆H₂Me₃-2,4,6)}La{iPrNC(PPh₂)NiPr}(thf)] (6), which, on recrystallization from ether, afforded the ether-coordinated structurally characterizable analogue [{Me₂Si(C₅Me₄)(NC₆H₂Me₃-2,4,6)}La{iPrNC(PPh₂)NiPr}(OEt₂)] (7). The reaction of 6 or 7 with Ph₂PH in THF yielded 4 and the phosphaguanidine iPrNC(PPh₂)NHiPr (3 a). These results suggest that the catalytic formation of a phosphaguanidine compound proceeds through the nucleophilic addition of a phosphide species, which is formed by the acid-base reaction between a rare-earth metal o-dimethylaminobenzyl bond and a phosphine PH bond, to a carbodiimide, followed by the protonolysis of the resultant phosphaguanidinate species by a phosphine PH bond. Almost all of the rare earth complexes reported this paper were structurally characterized by X-ray diffraction studies.