Competition between one-step and two-step mechanism in polar [3+2] cycloadditions of (Z)-C-(3,4,5-trimethoxyphenyl)-N-methyl-nitrone with (Z)-2-EWG-1-bromo-1-nitroethenes

The selectivity and mechanistic aspects of [3 + 2] cycloaddition between (Z)-C-(3,4,5-trimethoxyphenyl)-N-methyl-nitrone and (Z)-2-EWG-1-bromo-1-nitroethenes was explored and explained on the basis of DFT computational study. It was found that in the light of Molecular Electron Density Theory, these processes should be treated as evidently polar. Detailed exploration of reaction channels shows that in weakly polar solvents, conversion of addents into adducts always proceeds according to a one-step mechanism, however the synchronicity of TSs may be very different. On the other hand, in a more polar environment, more favored should be a stepwise mechanism with a zwitterionic intermediate. (C) 2018 Elsevier B.V. All rights reserved.