Development of asymmetric N-heterocyclic carbene-catalyzed reactions
Date
2017
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Abstract
N-Heterocyclic carbenes (NHCs) are ubiquitous organocatalysts in a variety of asymmetric transformations. The benzoin and Stetter reactions, which couple aldehydes to other aldehydes or Michael acceptors, respectively are the most commonly reported reactivity manifold employing NHC catalysts. However, other umpolung reactivity pathways exist, for example, when α,β-unsaturated aldehydes are reacted with NHCs, the Breslow intermediate can react through the double bond of the aldehyde to functionalize at the beta position of the carbonyl. A process that has come to be known has homoenolate reactivity. A range of reactivity manifolds were investigated, including the asymmetric intermolecular Stetter reaction and an enantioselective NHC-catalyzed nucleophilic dearomatization of pyridiniums. In the dearomatization chemistry, a homoenolate equivalent is first generated from an enal and an NHC, which then adds to the pyridinium to generate 1,4-dihydropyridines with high enantioselectivity. This is a rare example of catalytic, asymmetric addition of a nucleophile to the activated pyridinium that prefers C-4 functionalization leading to the 1,4-dihydropyridine product. The asymmetric intermolecular Stetter reaction was also investigated in an attempt to broaden the scope of the reaction to include less activated Michael acceptors, specifically, α,β-unsaturated ketones. The coupling of heteroaryl aldehydes to enones could be achieved with appreciable levels of enantioselectivity (up to 80% ee), but reactivity remains a major challenge with this methodology.
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organic chemistry
n-heterocyclic carbenes
organocatalysis