N‐Heterocyclic Carbene Organocatalysis: Activation Modes and Typical Reactive Intermediates

The N-heterocyclic carbene (NHC)-catalyzed oxidative C–H deprotonations have attracted increasing attention; however, the general mechanism regarding this kind of oxidative organocatalysis remains unclear. In this paper, the competing mechanisms and origin of the stereoselectivity of the NHC-catalyzed oxidative γ-C(sp3)–H deprotonation of alkylenals and cascade [4 + 2] cycloaddition with alkenylisoxazoles were systematically investigated for the first time using density functional theory (DFT). The computed results indicate that the oxidation of the Breslow intermediate by 3,3′,5,5′-tetra-tert-butyl diphenoquinone (DQ) via a hydride transfer to oxygen (HTO) pathway is the most favorable among the four competing pathways. In addition, the analyses demonstrate that oxidant DQ plays a double role, i.e., strengthening the acidity of the hydrogen of the γ-carbon of alkylenal and forming π···π interactions with conjugated C═C bonds to promote the γ-C(sp3)–H deprotonation. The NHC catalyst acts as a Lewis base, ...

Insights into the N-Heterocyclic Carbene (NHC)-Catalyzed Oxidative γ-C(sp3)–H Deprotonation of Alkylenals and Cascade [4 + 2] Cycloaddition with Alkenylisoxazoles

N-Heterocyclic carbenes (NHCs) are now well-established organocatalysts for a large number of asymmetric and non-asymmetric transformations. In the last 15 years, there has been significant interest in using NHCs in kinetic resolution (KR), dynamic kinetic resolution (DKR) and asymmetric desymmetrization reactions for the stereoselective synthesis of enantioenriched compounds, with diverse substrates and activation modes being adopted to this end. This short review brings into focus the progress made on NHC-catalyzed KR, DKR, and asymmetric desymmetrization from 2004 until December 2018. The literature discussed in this article is classified on the basis of the type of reaction involving the NHC catalyst. 1 	Introduction 2 	Acylation Strategies 2.1 	O-Acylation 2.2 	N-Acylation 2.3 	C-Acylation 3 	Aldol-Acylation Processes 4 	Benzoin Reactions 5 	Stetter Reactions 6 	Miscellaneous Approaches 7 	Conclusion

Kinetic Resolution, Dynamic Kinetic Resolution and Asymmetric Desymmetrization by N-Heterocyclic Carbene Catalysis

An enantioselective functionalization of the indole NH group is developed. The reaction stereoselectivity is controlled by an N-heterocyclic carbene catalyst that adds to an aldehyde moiety at the ...

Enantioselective Indole N–H Functionalization Enabled by Addition of Carbene Catalyst to Indole Aldehyde at Remote Site

Catalytic, Enantioselective C2-Functionalization of 3-Aminobenzofurans Using N-Heterocyclic Carbenes.

https://dr.ntu.edu.sg/bitstream/10356/138257/2/Direct%20Activation%20of%20%ce%b2-sp3-Carbons%20of%20Saturated%20Carboxylic%20Esters%20as%20Electrophilic%20Carbons%20via%20Oxidative%20Carbene%20Catalysis.pdf

Direct activation of β-sp3-carbons of saturated carboxylic esters as electrophilic carbons via oxidative carbene catalysis

https://dr.ntu.edu.sg/bitstream/10356/138240/2/Kinetic%20Resolution%20of%201%2c2-Diols%20via%20NHC-Catalyzed%20Site-Selective%20Esterification.pdf

Kinetic Resolution of 1,2-Diols via NHC-Catalyzed Site-Selective Esterification.

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Direct activation of β-sp3-carbons of saturated carboxylic esters as electrophilic carbons via oxidative carbene catalysis

Kinetic Resolution of 1,2-Diols via NHC-Catalyzed Site-Selective Esterification.