Recent Advances in N‐Heterocyclic Carbene Catalyzed Oxidative Cyclization for the Formation of Heterocycles

Abstract

Oxidative N‐heterocyclic carbene catalyst mediated cyclization proved to be an efficient strategy for the construction of various N‐ and O‐ heterocycles. Among several metal‐free methods available for the construction of heterocyclic compounds, oxidative NHC catalysis proved to be an elegant and efficient strategy as several enantioenriched compounds with excellent enantioselectivities can be synthesized in good yields. The most common method of the generation of these heterocycles proceeds via the commonly encountered intermediate in oxidative NHC catalysis (i. e.) acyl azolium intermediate. This intermediate proves to be the active species as it can undergo reaction with suitable substrates leading to the formation of numerous N‐ and O‐ heterocycles. Herein, recent reports on different strategies that enable formal [2+2], [3+2], [3+3], [4+2] or [4+3] annulations between the incoming substrates and acyl azolium species or other active intermediates which can add to suitable substrate leading to various heterocycles were summarized.

Catalyst-controlled stereoselective carbon–heteroatom bond formations by N-heterocyclic carbene (NHC) organocatalysis

“…66,67 To date, various efficient approaches have been developed for the generation of α,β-unsaturated acyl azoliums that could be readily afforded from α,β-unsaturated acyl fluorides, esters, ynals, etc . 49,50,66…”

“…A diverse set of activation modes with various NHC-bound reactive intermediates have been developed. 49,50 For instance, enolates, 51,52 homoenolates 53,54 and dienolates [55][56][57][58] are widely utilized as versatile nucleophilic synthons that can undergo addition reactions with typical electrophilic amination reagents for the enantioselective formation of carbon-nitrogen bonds.…”

Oxidative N‐Heterocyclic Carbene Catalysis

“…[12] Since then, there has been an evergrowing interest in oxidative NHC-catalysis, and a selection of applications has been recently reviewed by Rafiński and Dzieszkowski. [13] In addition, Maheswari and co-workers focused their attention on representative examples in the field of NHCcatalyzed oxidative cyclization, [14] while Sundén and co-workers reviewed their own efforts in the field of aerobic oxidative NHCcatalysis. [15] However, in our view, advances in oxidative NHC-catalysis do not seem to have been totally covered by these last three papers, so we aim to give a new contribution to review the literature produced in this flourishing area (from 2014 to date).…”

“… [12] Since then, there has been an ever‐growing interest in oxidative NHC‐catalysis, and a selection of applications has been recently reviewed by Rafiński and Dzieszkowski. [13] In addition, Maheswari and co‐workers focused their attention on representative examples in the field of NHC‐catalyzed oxidative cyclization, [14] while Sundén and co‐workers reviewed their own efforts in the field of aerobic oxidative NHC‐catalysis. [15] …”

Redox-active molecules as organocatalysts for selective oxidative transformations – an unperceived organocatalysis field

“…The acylazolium cation in turn can undergo nucleophilic attack resulting in C-O and C-N coupling, or deprotonation followed by the functionalization of α-and β-positions of the starting aldehyde. NHC-catalyzed photochemical processes [64] and oxidative cyclizations with heterocycle formation [65] were reviewed previously.…”