In this Perspective, we describe recent advances on Pd-catalyzed oxidative cascade carbocyclizations. These cascade processes enable efficient construction of the molecular complexity and structural diversity of carbocyclic compounds via introducing diverse functionalities concomitant with multiple C–C bond-formations in one-pot operations. In many cases, these processes are facilitated by Pd-catalysts alone, while cocatalysis by combination of Pd catalyst and other catalysts are also discussed, since they represent a new entry to address the preparation of functionalized cyclic compounds with high efficiency and selectivity.Keywords: carbocyclization; cascade reaction; catalysis; oxidation; palladium;

A controllable stereoselective synthesis of tetrahydropyrrolo[2,1-a]isoquinoline derivatives bearing a sulfur moiety was demonstrated with high diastereoselectivity through a catalytic intramolecular acylsulfenylation of activated alkenes. This approach involved a catalytic thia-Michael addition triggered intramolecular aldol-type tandem sequence. Both cis- and trans-products can be readily prepared in moderate to high yields with excellent diastereoselectivities in a catalytically atom-economic fashion under the optimized mild reaction conditions.

The first example of the diastereoselective construction of polysubstituted 2,3-dihydrofurans incorporating contiguous quaternary and tertiary carbon centers via a Lewis base-catalyzed one-pot cascade sequence is described. The diversity and complexity of the final products can be efficiently constructed with high diastereoselectivities via this catalytic multistep process under mild reaction conditions.

•Diastereoselective synthesis of functionalized dihydroindolizidine derivatives bearing sulfur moiety.•Preparation of trans- and cis-dihydroindolizidine products in high yields with moderate to excellent diastereoselectivities.•Novel catalytic sulfur-Michael addition triggered intramolecular aldol-type tandem sequence.•Atom-economic transformations.A diastereoselective synthesis of functionalized dihydroindolizidine derivatives bearing sulfur moiety was reported with high stereoselectivity via a catalytic sulfur-Michael addition triggered intramolecular aldol-type tandem sequence. This protocol allowed to readily access trans- and cis-dihydroindolizidine products in high yields with moderate to excellent diastereoselectivities under the optimized mild reaction conditions, which also can be easily transformed into various substituted functionalized pyridines incorporating β-quaternary carbon centers.Download high-res image (98KB)Download full-size image

The first example of Cu-promoted cyclization of α-amino nitrile-tethered enynes incorporating an electron-deficient alkene component is described. A wide range of functionalized 3-azabicyclo[4.1.0]hepta-2,4-dienes and 4,5-dihydro-3H-azepines were prepared efficiently in a controllable manner. Moreover, the diverse cascade process enables efficient incorporation of tertiary amine moieties under mild reaction conditions. A possible reaction pathway is proposed on the basis of a series of control experiments.

The first example of Pd-catalyzed intramolecular C–H addition of indoles bearing cyanohydrin components at the C(3), C(2), and N(1) positions to nitriles is described. A wide range of functionalized partially saturated carbazoles, tetrahydropyrido[1,2-a]indole, and carbazoles can be prepared in good to excellent yields under the optimal conditions. In addition, fused polycyclic indoles with seven- or eight-membered rings can also be formed smoothly.

Phosphine-catalyzed [4+2] annulations of 2-(acetoxymethyl)buta-2,3-dienoates with α-aminonitriles have been developed, in which α-aminonitriles serve as C, N-bisnucleophilic reaction partners and 2-(acetoxymethyl)buta-2,3-dienoates as “C4 synthons” respectively. A number of α-aminonitriles could be successfully applied to giving multifunctional desired products using PPh3 as catalyst. This method provides a facile entry to access polysubstituted tetrahydropyridines bearing quaternary carbon centers. The possible reaction mechanism was also proposed.

A novel asymmetric catalytic approach for the construction of enantioenriched functionalized 1,2-dihydropyridines and pyridine derivatives incorporating adjacent quaternary and tertiary stereocenters has been reported. This process involved a metal-free catalytic asymmetric allylic alkylation and a stereospecifically nonoxidative aromatization approach for the desired chiral molecules.

A novel sulfa-Michael addition (SMA)-triggered tandem reaction was developed by combining a SMA reaction with a simultaneous rearomatization process utilizing a less reactive carbonyl group as an intramolecular electrophile partner, which provided a unique synthetic route to access various organosulfur compounds incorporating an N-aromatic heterocyclic motif and quaternary carbon centers.

A facile protocol to construct β-selective allylic 1, 2-dihydroisoquinoline with high E/Z selectivity from its γ-regioisomer through an unexpected Lewis base or thermo-initiated rearrangement is described. This rearrangement transformation proceeds under mild reaction conditions with efficiency and atom economy. The control experiments demonstrated that these rearrangement processes initiated by different modes followed two distinct mechanisms.

A novel asymmetric synthetic approach for the construction of enantioenriched functionalized dihydroquinones incorporating adjacent quaternary and tertiary stereocenters has been reported, in which enantioenriched 3-allylic phthalides engaged in an unprecedented sulfa-Michael addition-triggered stereoselective ring-expansion reaction, and furnished the desired sulfur-incoporated dihydronaphthoquinones with high stereoselectivity.

A novel metal-free catalytic annulation was developed through a Lewis base-catalyzed asymmetric allylic alkylation and the ensuing unprecedented asymmetric intramolecular acylcyanation of alkenes. This protocol provides a unique and facile access to prepare enantioenriched densely functionalized dihydronaphthoquinones accompanied by enantiomerically pure 3,3-disubstituted phthalides bearing quaternary carbon centers.

Pyrrolidine rings are common moieties for pharmaceutical candidates and natural compounds, and the construction of these skeletons has received much attention. α-Amino nitriles are versatile intermediates in synthetic chemistry and have been widely used in the generation of multiple polyfunctional structures. Herein, a novel nucleophilic phosphine-catalyzed intramolecular Michael reaction of N-allylic substituted α-amino nitriles has been developed for the efficient construction of functionalized 2,4-disubstituted pyrrolidines (N-heterocyclic α-amino nitriles) via 5-endo-trig cyclization. Furthermore, the one-pot sequence of the synthesis of pyrrolidine and the subsequent transformations of the functionalized products have also been demonstrated.

A novel Lewis base-promoted rearrangement of allylic cyanohydrins has been developed, in which the cyano group was rearranged, directly coupled with the generation of new functional groups. This protocol provides a unique and facile way to prepare highly functionalized nitriles bearing 1,3-diketone moieties under mild reaction conditions. Furthermore, the synthetic transformations of the functionalized products have also been demonstrated.

The first asymmetric organocatalytic allylic alkylation of 1,2-dihydro-Reissert compounds and Morita–Baylis–Hillman (MBH) carbonates has been developed, which provided a novel protocol to construct enantioenriched functionalized 1,2-dihydroisoquinolines bearing vicinal quaternary and tertiary chiral centers at C-1 position (up to 94% ee, dr > 20:1).

The first tertiary amine catalyzed asymmetric allylic amination/cycloaddition cascade sequence has been developed, which provided a novel protocol to construct enantioenriched azapolyheterocycles under mild reaction conditions efficiently (up to 95% ee, endo/exo >20:1).

A novel phosphine-promoted intramolecular acylcyanation of α-substituted activated alkenes has been developed, which provides a unique access to densely functionalized acyclic ketones bearing β- quaternary carbon centers with a remarkable feature that both α- and β-positions of activated alkene are functionalized.

The first tertiary amine-catalyzed multicomponent tandem Strecker–allylic-alkylation (SAA) reaction has been developed, which provides a facile access to functionalized α-amino nitriles, which could be readily converted into α-methylene-γ-butyrolactams.

The first tertiary amine-catalyzed tandem cyanation–allyic alkylation (CAA) reaction of aldehydes, appropriate cyanide sources, and Morita–Baylis–Hillman (MBH) adducts has been developed, which provides a facile access to densely functionalized products containing O-substituted quaternary centers.

A novel sulfa-Michael addition (SMA)-triggered tandem reaction was developed by combining a SMA reaction with a simultaneous rearomatization process utilizing a less reactive carbonyl group as an intramolecular electrophile partner, which provided a unique synthetic route to access various organosulfur compounds incorporating an N-aromatic heterocyclic motif and quaternary carbon centers.

The first tertiary amine-catalyzed multicomponent tandem Strecker–allylic-alkylation (SAA) reaction has been developed, which provides a facile access to functionalized α-amino nitriles, which could be readily converted into α-methylene-γ-butyrolactams.

The first tertiary amine catalyzed asymmetric allylic amination/cycloaddition cascade sequence has been developed, which provided a novel protocol to construct enantioenriched azapolyheterocycles under mild reaction conditions efficiently (up to 95% ee, endo/exo >20:1).

The first asymmetric organocatalytic allylic alkylation of 1,2-dihydro-Reissert compounds and Morita–Baylis–Hillman (MBH) carbonates has been developed, which provided a novel protocol to construct enantioenriched functionalized 1,2-dihydroisoquinolines bearing vicinal quaternary and tertiary chiral centers at C-1 position (up to 94% ee, dr > 20:1).