Catalytic Asymmetric Benzylation of Azomethine Ylides Enabled by Synergistic Lewis Acid/Palladium Catalysis

New routes towards azomethine ylide generation from prolines to synthesize diverse N-heterocycles: a DFT supported endo-selective mechanism

Azomethine ylides are generated using either organocatalysts or metal catalysts via a ballet of decarboxylative C-N coupling choreographed by prolines. These strategies enable diastereoselective [3 + 2] cycloaddition, C-C coupling, and ring annulation, providing sustainable routes. The synthesized pyrrolizines and other heterocycles have potential applications in the development of crucial biomolecules and pharmaceuticals. The endoselectivity of the azomethine ylide is realized and supported through DFT calculations.

The Development of Aldehyde Catalytic System

Aldehyde catalysts have proven to be highly effective in facilitating and accelerating a wide range of challenging transformations in organic chemistry. This article is structured into three main sections, focusing on the utilization of aldehydes as organocatalysts, the aldehydes/transition metals catalytic systems, and photochemical initiators. Finally, we provide a concise summary of the advancements in this fascinating research field, offering our perspectives and insights.

Asymmetric redox benzylation of enals enabled by NHC/Ru cooperative catalysis

The development of general methods for asymmetric benzylation of prochiral carbon nucleophiles remains a challenge in organic synthesis. The merging of ruthenium catalysis and N-heterocyclic carbene (NHC) catalysis for asymmetric redox benzylation of enals has been achieved, which opens up strategic opportunities for the asymmetric benzylation reactions. A wide range of 3,3'-disubstituted oxindoles with a stereogenic quaternary carbon center widely existing in natural products and biologically interesting molecules is successfully obtained with excellent enantioselectivities [up to 99% enantiomeric excess (ee)]. The generality of this catalytic strategy was further highlighted by its successful application in the late-stage functionalization of oxindole skeletons. Furthermore, the linear correlation between ee values of NHC precatalyst and the product elucidated the independent catalytic cycle of either the NHC catalyst or the ruthenium complex.

Substrate-Dependent Regioselectivity: Pd/PTC Cooperatively Catalyzed Domino Heck/Allylation of Allenamides with α-Carbon of Carbonyl Compounds

A Pd/phase-transfer catalyst cooperatively catalyzed domino Heck/allylation reaction is first reported, which represents interesting substrate-dependent regioselectivity. Under the same conditions, Ts-protected N-(2-iodophenyl)allenamides produced only linear allylation products, while Cbz, Ac, or Boc-protected N-(2-iodophenyl)allenamides and N-(2-iodobenzoyl)allenamides with various compounds generated branch allylation products with an exocylic C═C bond and two vicinal stereocenters. Up-scale syntheses and diverse fused cyclization transformations of products were then carried out. The enantioselective version for the domino process was studied.

Catalytic asymmetric Tsuji-Trost α-benzylation reaction of N-unprotected amino acids and benzyl alcohol derivatives

Catalytic asymmetric Tsuji–Trost benzylation is a promising strategy for the preparation of chiral benzylic compounds. However, only a few such transformations with both good yields and enantioselectivities have been achieved since this reaction was first reported in 1992, and its use in current organic synthesis is restricted. In this work, we use N-unprotected amino acid esters as nucleophiles in reactions with benzyl alcohol derivatives. A ternary catalyst comprising a chiral aldehyde, a palladium species, and a Lewis acid is used to promote the reaction. Both mono- and polycyclic benzyl alcohols are excellent benzylation reagents. Various unnatural optically active α-benzyl amino acids are produced in good-to-excellent yields and with good-to-excellent enantioselectivities. This catalytic asymmetric method is used for the formal synthesis of two somatostatin mimetics and the proposed structure of natural product hypoestestatin 1. A mechanism that plausibly explains the stereoselective control is proposed.

The catalytic asymmetric benzylations of prochiral nucleophiles are very limited. Here, the authors disclose an asymmetric α−benzylation of N-unprotected amino acids with benzyl alcohol derivatives by a chiral aldehyde-involved catalytic system.