Catalysis with Stable Carbenes ( n ?→?π*)

This chapter focuses on the numerous ways N-heterocyclic carbene (NHCs) catalyze organic transformations, and highlights examples of new reactivity and mechanisms. The rapid expansion of NHC-catalyzed reactions has led to an increasing interest in evaluating the intrinsic chemical properties of NHCs. The chapter presents the many modes of reactivity NHCs can effect in umpolung, as well as in Bransted and Lewis base catalysis. The most studied reaction employing NHC umpolung catalysis is the benzoin reaction. The reaction of benzaldehyde to produce benzoin was first reported by Wohler and Liebig in 1832 using cyanide as catalyst. Efforts to render the benzoin reaction enantioselective began with Sheehan and Hunneman's work in 1966, when they described the use of chiral thiazolylidene with modest enantioselection. Several orthogonal reactivity manifolds can be accessed with NHC catalysis. Using α-oxidizable aldehydes allows the catalyst to formally reduce the a-carbon of the aldehyde, while concurrently oxidizing the aldehyde to the ester oxidation state.