Introduction
Since the crystalline carbene 1,3-di-l-adamantyl-imidazol-2-ylidene was
firstly isolated and characterized by Arduengo et
al.,1,2extensive researches emerge involving synthesis of more stable
N-heterocyclic carbenes (NHCs) and applications of NHC as ligand of
metal-based catalyst or
organocatalyst.3Particularly, the success in catalytic benzoin condensations promotes
the NHC catalysis as an important alternative in constructing new C-C
bonds and C-X (X = N, O, S)
bonds.4-9 Inspired by
the ‘umpolung’ principle used in this
reaction,10,11the NHC-catalyzed Stetter
reaction,12 benzoin
reaction,13 homoenolate
reaction, and especially cycloaddition/annulation
reactions14-18 have
contributed a number of novel approaches to constructing various
heterocyclic structures. Inversion of the conventional reactivity of
substrates, typically such as aldehyde, ketone, ketene, and so on, is
the basic underline principle behind these reactions (Scheme
1).19-21 More
interestingly, catalysis using this concept has been extended to the
umpolung of Michael
acceptors22-25 and
aldimines.26-33Detailed theoretical studies reported by
our34-42 and other
groups43-51 provide
perspective at the molecular level to get insights into the catalytic
mechanism and chemo-, regio-, and/or
stereoselectivities.52,53