Theoretical investigation on carbene-catalyzed [3 + 3] annulation of vi-nyl sulfoxonium ylide with enal for synthesis of 2-sulfenylidene-3-cyclohexen-1-one

The first theoretical investigation was provided by our DFT calculation on trazolum carbene-catalyzed [3 + 3] annulation of vinyl sulfoxonium ylide with enal. Free carbene NHC is formed via deprotonation by BF4 with enhanced nucleophilicity. NHC attacks positive carbonyl of enal with its negative carbon followed by proton transfer leading to Breslow intermediate, which is easily transformed to α, β-unsaturated acylazolium inter-mediate through dehydrogenation. Spontaneous electron delocalization in vinyl sulfoxonium ylide makes car-banion available at α-position of ester. Michael addition undergoes linking two substrates. Followed by pro-ton shift, the carbanion attacks positive carbonyl in crucial ring closure yielding six-membered carbocycle. The second proton shift generates isomer of desired 2-sulfenylidene-3-cyclohexen-1-one after release of proto-nated NHC.