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Aldimine umpolung using N-heterocyclic carbenes (NHCs) is less explored compared to the well-known polarity reversal of aldehydes. Described herein is the NHC-catalyzed aldimine umpolung/semipinacol rearrangement cascade for the atom- and pot-economic synthesis of fluorescent active, N–H unprotected indoxyl derivatives. Moreover, conditions are identified for the NHC-catalyzed enantioselective synthesis of α-iminols by the umpolung of aldimines. The nucleophilic aza-Breslow intermediates are intercepted with carbonyl electrophiles. Preliminary DFT studies shed light on the rearomative proton transfer coupled aryl migration in an ortho-quinonemethide intermediate facilitating the semipinacol rearrangement. In addition, the antibacterial activity of the synthesized indoxyls has been evaluated.
Das et al. (Mon,) studied this question.