Thiazolium Salts: From Thiamine to Cutting-Edge Organocatalysts

In this comprehensive review, the history of thiazolium salts as N-Heterocyclic Carbenes (NHC) in organocatalytic reactions is intended to be presented. Although the subject (i.e., NHC in organocatalysis) has been covered in several reviews in past years, the idea here stands on the exclusive use of thiazolium salts in such transformations. Indeed, thiazoliums and their related NHCs are the first identified as efficient mediators for the CC bond formation through its peculiar ability to reverse the classical reactivity (umpolung) of carbonyl groups. However, they are relatively quickly overtaken by other classes of NHCs generated from imidazolium and triazolium salts. Even though, thiazolylidenes are still better catalysts in some particular reactions affording outstanding results in terms of conversion and selectivity. Owing to the remarkably high number of reactions that are made possible thanks to the very characteristic properties of such NHCs, the aim of this review is to provide a full account on the discovery and evolution of thiazolium salts and thiazolylidenes in NHC organocatalysis. Information is first provided on their synthesis, structural evolution, physicochemical and electronic properties, and then reactivity and application in various organocatalytic methodologies.