Bis(amino)cyclopropenylidenes as Organocatalysts for Acyl Anion and Extended Umpohmg Reactions

摘要

The use of N-heterocyclic carbenes (NHCs) as organocatalysts has gained considerable attention in recent years because of their unique properties and modes of reactivity. In addition to their use as acyl anion equivalents in the reversal of reactivity (umpolung) of aldehydes, they have also been utilized in the extended umpolung of α-function-alized aldehydes, in the umpolung of activated olefins, and as nucleophilic catalysts. As a result of this versatility, significant effort has gone into the design of NHC catalysts that are tailored to particular reactions and reaction types. To date, the most commonly used precatalysts all feature a five-membered nitrogen-containing heterocyclic core, as present in 2-thiazolium, 1,2,4-triazolium, imidazolium, and imidazo-linium salts. The usefulness of each of these NHC families is now well established and new substitution patterns are constantly being introduced to improve or modulate reactivity. Although many studies have shown the catalytic properties of NHCs to be greatly influenced by often subtle steric and electronic effects, the search for new properties and complementary reactivity would benefit from the introduction of entirely new catalytic architectures.