Mechanistic considerations of guanidine-catalyzed reactions.

This feature article discusses the various mechanistic aspects of guanidine-catalyzed reactions. Guanidines are well known as strong organic bases; however, in the first section, three most common commercially available guanidines, TMG, TBD and MTBD, will be used to illustrate the use of guanidines as nucleophilic catalysts. In the second section, different modes of hydrogen bonding interactions of the conjugate acid of guanidine, the guanidinium, are discussed. Particularly interesting are the possibilities of mono-functional or bifunctional activation of a nucleophile and an electrophile by guanidinium.