On the Circumambulatory Rearangement of Cyclopentadienylphosphanes — Retention versus Inversion at the Migration Phosphorus

According to a quantum chemical analysis based on energy optimized M NDO calculations the 1.5-sigmatropic reaction of cyclopentadienylphosphanes can occur via a transition state either with retention or inversion of configuration at phosphorus, depending on the ligands within the PR2 fragment. Electropositive R favour inversion and electronegative R retention of configura­ tion at phosphorus. In addition, given the same ligand R, retention (over inversion) com es to the fore with increasing atomic number of the central atom at the M R, unit, i.e. in the order M — N < P < A s < Sb.