Acyl anion free N-heterocyclic carbene organocatalysis

The capacity of N-heterocyclic carbenes (NHCs, i.e. 1) to act as organocatalysts for the benzoin condensation has been known for more than 60 years. Following Ukai’s initial observations, Breslow proposed a mechanism for this transformation analogous to the cyanide catalysed benzoin condensation. Central to his proposal was the action of thiazolium derived zwitterions, now more commonly formulated as NHCs, to generate acyl anion equivalent. Building upon these studies modern NHC organocatalysis has been dominated by reactions that involve acyl anion equivalent formation, followed by either direct reaction, or the relay of acyl anion reactivity. Pivotal to such chemistry is the NHCs excellent s-donor characteristics, and moderate p-acidity, leading to facile acyl anion equivalent formation.