The stereochemistry of transition metal catalyzed formation of Z-vinyl aziridines was experimentally validated by use of a (2+2)-cycloaddition with DMAD (see Yudin et al. DOI:10.1021/ja0632557). Although the cis-cyclobutene product (top right, above) was formed, questions remained as to the mechanism of this reaction. Our quantum chemical calculations (B3LYP/6-31+G(d,p)) showed that the cycloaddition occurred through a zwitterionic pathway with an intermediate having a sufficiently short lifetime to avoid isomerization to the presumably more stable trans-cyclobutene. Further, the subsequent electrocyclic ring opening was investigated and found to be torquoselective for the methyl-in product.
Siebert, M.R.; Yudin, A.K. and Tantillo, D.J. Organic Letters, 2008, 10, 57-60. “Cycloaddition/Ring Opening Reaction Sequences of N-Alkenyl Aziridines: Influence of the Aziridine Nitrogen on Stereoselectivity.” 10.1021/ol702623d.