The importance of N-heterocyclic carbene basicity in organocatalysis.

N-Heterocyclic carbenes (NHCs) are versatile species that figure prominently as catalysts. Despite their widespread use in organocatalysis, studies of the relationship between the basicity of NHCs and their catalytic ability are limited. Herein we review work on both the examination of NHC basicity as well as its impact on organocatalysis. The review is divided into three main parts: an overview of NHC basicity studies, both in solution and in the gas phase; the role of basicity in Umpolung-type catalysis; and the relationship between NHC basicity and its growing role as a Brønsted base catalyst. This review is not an exhaustive catalog of all NHC catalysis, but rather focuses on work that specifically examines and discusses the effect of NHC basicity on catalyst function.