A non-Woodward and Hoffmann reaction path for photochemical sigmatropic rearrangements

The semi-empirical MNDO-SCF method, extended with a limited (3 × 3) CI, has been used to calculate the photochemical (1, 3) shifts in propene, 2-propen-1-ol, 1-butene and the (1, 5) shifts in 1,3-pentadiene and 1,3-hexadiene. The suprafacial and antarafacial reactions as considered by Woodward and Hoffmann are compared with a planar shift which is based on the relaxation of the double bond towards a twisted geometry accompanied by a separation of charge. For the sigmatropic C-shifts in 1-butene and 1,3-hexadiene the reactions with retention and inversion of configuration of the migrating CH3-group are compared. It is shown that the proposed mechanism is strongly preferred to the mechanism predicted by the rules of conversion of orbital symmetry. This latter mechanism exhibits an energy barrier at an unexpected geometry located between the initially excited molecule and the supposed transition state. The theoretical results are in agreement with the known experimental data.