Reaction mechanism of the Stevens rearrangement of iminophosphorane and iminoarsorane

Iminophosphoranes and iminoarsoranes are isoelectronic compounds of ylides. It is expected that they can perform the Stevens rearrangement as observed for the ylides. In order to investigate the mechanism of this rearrangemen t, theoretical calculations of mono-substituted iminophosphoranes and imino arsoranes with the Moller-Plesset perturbation theory up to the fourth orde r were performed for the rearrangement reactions of ZH(2)MNH -->H(2)MNHZ (Z =H, CH3, CH=CH2, SiH3 and GeH3: M=P and As). The migrations of hydrogen ato m, methyl, and vinyl groups were antarafacial with large activation energie s as readily expected based on the Woodward-Hoffmann rule. On the contrary, the rearrangement of the silyl group along with that of the germyl group d isplayed that it was a suprafacial process with a small activation energy. Structural. and molecular orbital considerations in the transition states c learly demonstrated that the hypervalency of the Si and Ge atoms lead to th e suprafacial migration with the small activation energies, While the migra tions of hydrogen atom, methyl, and vinyl groups of iminophosphorane requir ed an activation energy about 20 kcal/mol higher than those of the correspo nding ylide compounds, the activation energies for the rearrangement of the silyl and germyl groups were comparable. The origin of this behavior is di scussed based on the difference of the M-Z bond energies between ylide and iminophosphorane as well as their structural features.