Scheme 1. Selected NHC-catalyzed reactions based on the umpolung
strategy (left) or the oxidative nonpolar inversion strategy (right).
The fact is ‘umpolung’ should not be the unique approach for NHCs as
organocatalyst.54-57 As
it is illustrated in Scheme 1, in the normal way without polarity
inversion under oxidative conditions, the reaction can be initiated by
conversion of aldehyde or aldimine to the Breslow or aza-Breslow
intermediate,58respectively, followed by a two electron oxidation to give azolium.
Finally, an inter- or intramolecular nucleophilic attack yields the
final product. For these reactions, the roles of NHCs play in
facilitating reactions to occur and/or manipulation of various
selectivities is an important issue that people are highly interested
in. In the ‘umpolung’ reactions, the main effect of NHCs has been
demonstrated to be increasing nucleophilicity of the substrate or
determining the selectivities owing to diverse non-covalent interactions
(NCIs) between substituents of the catalyst and the substrate in
different isomers. While for the nonpolar inversion pathway, acts of the
catalysts could be complicated, such as enhancing reducibility of the
substrate or even shows completely a diverse reaction route. Owing to
our long interesting in the NHC-catalyzed reactions but the lack of
insights into the roles of NHC and the co-effect of the oxidant in
reactions without polarity reverse, we would like to present our recent
theoretical study about the mechanism of an intramolecular oxidative
cyclization with NHC as the catalyst and
3,3’-5,5’-tetra-tert-butyl-4,4’-diphenoquinone
(DQ)59 as the oxidation
reagent. More importantly, some insights about how the catalyst and DQ
work together to enable the reaction have also been achieved.