THEORETICAL INVESTIGATION OF STEVENS REARRANGEMENT OF P AND AS YLIDES- MIGRATION OF H, CH3, CH=CH2, SIH3, AND GEH3 GROUPS ON P AND AS ATOMS

Ylides are considered as intermediates of the well-known Stevens rearr angement. To investigate the mechanism of this rearrangement, theoreti cal calculations for the rearrangements of monosubstituted ylides, ZH( 2)MCH(2) --> H(2)MCH(2)Z (Z = H, CH3, CH=CH2, SiH3, and GeH3; M = P an d As), were performed with the Moller-Plesset perturbation theory up t o the fourth order. The IRC calculations at the RMP2 level show that t he reaction mechanism is strongly dependent on the migrating group. Wh ile the methyl and vinyl migrations are antarafacial with large activa tion energies of 37-47 kcal/mol, the IRC of the SiH3 migration along w ith that of GeH3 migration displays that it is a suprafacial process. While the migration of the groups with the heavier elements of Si and Ge utilizes their hypervalency to have a small activation energy of 13 kcal/mol, the transition states for the migrations of the methyl and vinyl groups as well as the hydrogen atom are higher in energy. The RM P2 transition state for the methyl migration is less stable than the r adical dissociation limit, H2MCH2 + Z, the radical dissociation-recomb ination path being more favorable, and it may not exist as real. On th e other hand, the transition state for the vinyl migration as well as that for the hydrogen migration is slightly more stable than the disso ciation limit. If the entropy effect is properly taken into account, t he radical dissociation-recombination path would be more favorable tha n the concerted mechanism for these migrations as well.