The direct γ-carbon functionalization of α,β-unsaturated esters via N-Heterocyclic Carbene (NHC) catalysis is disclosed. This catalytically generated nucleophilic γ-carbon undergoes highly enantioselective additions to hydrazones. The resulting δ-lactam products can be readily transformed to optically enriched pipecolic acid derivatives.
Many hands make light work: In an organocatalytic asymmetric sulfonation of enones, the activation of a sulfonyl imine by an N-heterocyclic carbene (NHC) catalyst led to the release of a sulfinic anion, which underwent nucleophilic addition to the enone. The enantioselectivity of the process was controlled by a chiral thiourea/amine co-catalyst through anion recognition and hydrogen-bonding interactions. Tol=p-tolyl.
A carbene-catalyzed
reductive coupling reaction of carboxylic esters
and substituted Hantzsch esters is disclosed. Key steps of this reaction
include one-electron reduction of a carbene catalyst-bound acyl azolium
intermediate to generate the corresponding radical intermediate for
subsequent alkylation reactions. The reaction is promoted by irradiation
with visible light without the involvement of transition-metal photocatalysts.
Mechanistic studies suggest that direct photoexcitation of the in
situ formed acyl azolium intermediate is likely responsible for this
light-induced one-electron-reduction process. Photoexcitation converts
the acyl azolium intermediate to a single-electron oxidant, enabling
single-electron oxidation of Hantzsch esters to generate radical intermediates.
Our reactions work well for a broad range of aryl carboxylic ester
and Hantzsch ester substrates. Sophisticated structures, including
those present in medicines, can be incorporated into ketone molecules
using our approach via very mild conditions that tolerate various
functional groups.
The combined use of gold as transition metal catalyst and N-heterocyclic carbene (NHC) as organic catalyst in the same solution for relay catalytic reactions was disclosed. The ynamide substrate was activated by gold catalyst to form unsaturated ketimine intermediate that subsequently reacted with the enals (via azolium enolate intermediate generated with NHC) effectively to form bicyclic lactam products with excellent diastereo-and enantio-selectivities.T he gold and NHC coordination and dissociation can be dynamic and tunable events,a nd thus allow the co-existence of both active metal and carbene organic catalysts in appreciable concentrations,f or the dual catalytic reaction to proceed.
N‐Heterocyclic carbene (NHC) organocatalysis has been developed as an important approach in modern organic synthesis. Versatile activation modes within NHC organocatalysis have been established with countless transformations being realized in both efficient and selective fashion. We would like to provide an overview on the key progresses achieved within this field in the past two decades. Since numerous excellent reviews have been documented within this area, we will mainly focus on the scientific development of this research field based on the basic reaction modes and typical reaction intermediates.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.