The feasibility of chiral N-heterocyclic carbene and nickel cooperative catalysis for creating asymmetric reactions has been convincingly demonstrated. The high efficiency of this catalytic system enables asymmetric allylic alkylation of isatin-derived enals with allylic carbonates and [3+3] annulation with racemic vinyl epoxides to provide straightforward access to highly enantioenriched 3,3-disubstituted oxindoles. The great practicality of this method in organic synthesis has been showcased by facile product modification and enantioselective synthesis of the key building block to access (-)-debromoflustramine B.

Asymmetric Redox Allylic Alkylation to Access 3,3՛-Disubstituted Oxindoles Enabled by Ni/NHC Cooperative Catalysis.

Structural, spectroscopic, quantum chemical, and molecular docking investigation of (E)-N'-(2,5-dimethoxybenzylidene)picolinohydrazide

Abstract N‐Heterocyclic carbene (NHC) catalysis is a by now consolidated organocatalytic platform for a number of synthetic (asymmetric) transformations via diverse reaction modes/intermediates. In addition to the typical umpolung processes involving acyl anion/homoenolate equivalent species, implementation of protocols under oxidative conditions greatly expands the possibilities of this methodology. Oxidative NHC‐catalysis allows for oxidative and oxygenative transformations through specific manipulations of Breslow‐type species depending upon the oxidant used (external oxidant or O2/air), the derived NHC‐bound intermediates paving the way to non‐umpolung processes through activation of carbon atoms and heteroatoms. This review is intended to update the state of the art in oxidative NHC‐catalyzed reactions that appeared in the literature from 2014 to present, with a strong focus to crucial intermediates and their mechanistic implications.

Oxidative N‐Heterocyclic Carbene Catalysis

Organo/Silver Dual Catalytic (3 + 2)/Conia-Ene Type Cyclization: Asymmetric Synthesis of Indane-Fused Spirocyclopenteneoxindoles.

In the title compound, C16H12N2O3, the dihedral angle between the mean planes of the naphthalene ring system and the furan ring is 21.3 (6)°. The mol­ecular structure is stabilized by an intra­molecular O—H⋯N hydrogen bond, which generates an S(6) graph-set motif.

https://journals.iucr.org/e/issues/2010/08/00/jj2034/jj2034.pdf

(2Z,N'E)-N'-[(2-Hy-droxy-1-naphth-yl)methyl-idene]furan-2-carbohydrazonic acid.

We report herein an unprecedented gold and oxidative NHC relay catalysis that enables highly enantioselective cascade annulation between readily available α-amino-ynones with enals. This method utilizes the in situ -generated pyrrolin-4-ones as a novel and versatile synthon, which engage α , β -unsaturated acylazolium intermediate generated from enals by oxidative NHC catalysis to produce pyrrole-fused lactones in high yield and excellent enantioselectivity. Synthetic utility of the lactone products is also demonstrated by facile conversion to densely-functionalized pyrroles and pyrrolin-4-ones in high yields with excellent stereopurity.

Enantioselective Cascade Annulation of α-Amino-ynones and Enals Enabled by Gold and Oxidative NHC Relay Catalysis.

The mol­ecule of the title Schiff base compound, C15H15N3O3, is non-planar. The pyridine ring is twisted with respect to the rest of the mol­ecule with a dihedral angle of 24.48 (4)°. Besides N—H⋯O and O—H⋯N hydrogen bonding, C—H⋯π inter­actions occur in the crystal structure.

N′-(3-Ethoxy-4-hydroxybenzylidene)isonicotinohydrazide

We present herein an unprecedented stereoselective synthesis of triaryl-2-pyrones with monoaxial or contiguous diaxes from readily available starting materials. This N-heterocyclic carbene catalysis method adopts an atroposelective annulation of 2-aryketones with ynals under oxidative conditions. The annulation includes the construction of one or two axes in a single operation, achieves step economy, and affords axially chiral triaryl-2-pyrones in moderate to good yields, with high to excellent enantioselectivities. DFT calculations of the relative energies of stereoisomers and rotational barriers were performed. 

Enantioselective Access to Triaryl-2-pyrones with Monoaxial or Contiguous C–C Diaxes via Oxidative NHC Catalysis

We report herein an unprecedented gold and oxidative NHC relay catalysis that enables highly enantioselective cascade annulation between readily available α-amino-ynones with enals. This method utilizes the in situ-generated pyrrolin-4-ones as a novel and versatile synthon, which engage with α,β-unsaturated acylazolium intermediates generated from enals by oxidative NHC catalysis to produce pyrrole-fused lactones in high yield and excellent enantioselectivity. Synthetic utility of the lactone products is also demonstrated by facile conversion to densely functionalized pyrroles and pyrrolin-4-ones in high yields with excellent stereopurity. 

Sheng-Min Liu

Papers

Enantioselective Cascade Annulation of α-Amino-ynones and Enals Enabled by Gold and Oxidative NHC Relay Catalysis.

N′-(3-Ethoxy-4-hydroxybenzylidene)isonicotinohydrazide

Enantioselective Access to Triaryl-2-pyrones with Monoaxial or Contiguous C–C Diaxes via Oxidative NHC Catalysis

Enantioselective Cascade Annulation of α‐Amino‐ynones and Enals Enabled by Gold and Oxidative NHC Relay Catalysis