Abstract
The interplay between metals and N-heterocyclic carbenes (NHCs) has provided a window of opportunities for the development of novel catalytic strategies within the past few years. The recent successful combination of Brønsted acids with NHCs has added a new dimension to the field of cooperative catalysis, enabling the stereoselective synthesis of functionalized pyrrolidin-2-ones as valuable scaffolds in heterocyclic chemistry. This Commentary will briefly highlight the concept of N-heterocyclic carbene/Brønsted acid cooperative catalysis as a new and powerful methodology in organic chemistry.
Highlights
Carbenes have been the topic of intensive research for more than 150 years, and they continue to attract considerable attention from chemists to date
Selected examples of the successful deployment of cooperative catalysis in organic synthesis comprise the preparation of chiral γ-lactams from N-acyl hydrazones and α,β-unsaturated aldehydes [18], the enantioselective synthesis of cyclopentenes from α,β-unsaturated aldehydes and α,β-unsaturated ketones [19], and the preparation of cyclopentanes through the reaction of enals and β,γ-unsaturated α-ketoesters [20]
The N-heterocyclic carbenes (NHCs) and the conjugate acid can be present in sufficient quantities to promote a new form of cooperative catalysis
Summary
Carbenes have been the topic of intensive research for more than 150 years, and they continue to attract considerable attention from chemists to date. A interesting application comprises the use of “umpolung” reactions [11] (inversion of polarity) catalyzed by N-heterocyclic carbenes, such as the benzoin condensation and the Stetter reaction In these reactions, the NHC effects an “umpolung” of the normal carbonyl reactivity, and the electrophilic aldehyde carbon atom becomes nucleophilic and can attack a variety of electrophiles. Carbenes can act as ligands for metals, inhibiting the individual reactivity of each component, recent studies have shown that in some cases combinations of metals (Lewis acids) and carbenes can be used as a novel catalytic system [15,16,17] This type of cooperation has been demonstrated to feature a unique reactivity that is difficult to achieve by using one of both catalysts individually. Selected examples of the successful deployment of cooperative catalysis in organic synthesis comprise the preparation of chiral γ-lactams from N-acyl hydrazones and α,β-unsaturated aldehydes [18], the enantioselective synthesis of cyclopentenes from α,β-unsaturated aldehydes and α,β-unsaturated ketones [19], and the preparation of cyclopentanes through the reaction of enals and β,γ-unsaturated α-ketoesters [20]
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