Molecular modeling of oxaphosphetane intermediates of wittig olefination reactions

The literature reveals that the structures of four unusual oxaphosphetanes have been established by x-ray diffraction studies, and the structure of one mythical oxaphosphetane ( 6 ) has been deduced by theoretical calculations. We have used these known structures to calibrate the MMX87 force field, the only necessary modification being the stretching term for the P-O bond. The bond length, 1_o, was set at 1.80Å, which is an “average” of the P-O axial bond distances of the stable oxaphosphetanes. The AMPAC (Ver. 1.0) molecular orbital package utilizing the MNDO hamiltonian was also applied to the same problem, starting with the minimized geometries of the modified MMX87 force field. Fixed bond lengths and full minimizations were performed. The computed geometries of the four-member ring of each of the four oxaphosphetanes of known structures were found to be in very good agreement with the values obtained by x-ray diffraction. Furthermore, the method was applied to the mythical oxaphosphetane, and the results of MMX87 and MNDO calculations were found to be in very good agreement with the results of ab initio calculations. The MMX force field and the MNDO semiempirical method have been used to calculate the geometries and the steric energies (or heats of formation) of diastereomeric oxaphosphetanes formed in a theoretically real Wittig reaction.