Reaction Mechanism of the Stevens Rearrangement of Iminophosphorane
Kei MAKITA, Jugo KOKETSU*, Yoshihiko NINOMIYA and Nobuaki KOGA†
Department of Applied Chemistry, College of Engineering, Chubu University;
1200 Matsumoto, Kasugai-shi 487-8501 Japan
† Graduate School of Human Informatics, Nagoya University; Nagoya-shi 464-8601 Japan
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 rearrangement, theoretical calculations of mono-substituted iminophosphoranes and iminoarsoranes with the Møller-Plesset perturbation theory up to the fourth order were performed for the rearrangement reactions of ZH2MNH →H2MNHZ (Z=H, CH3, CH=CH2, SiH3 and GeH3: M=P and As). The migrations of hydrogen atom, methyl, and vinyl groups were antarafacial with large activation energies 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 displayed that it was a suprafacial process with a small activation energy. Structural and molecular orbital considerations in the transition states clearly demonstrated that the hypervalency of the Si and Ge atoms lead to the suprafacial migration with the small activation energies. While the migrations of hydrogen atom, methyl, and vinyl groups of iminophosphorane required an activation energy about 20 kcal/mol higher than those of the corresponding ylide compounds, the activation energies for the rearrangement of the silyl and germyl groups were comparable. The origin of this behavior is discussed based on the difference of the M-Z bond energies between ylide and iminophosphorane as well as their structural features.
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