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.