Mechanism of the ring-chain rearrangement in phosphiranes: Hydrogen versushalogen migration

Ab initio quantum chemical calculations including HF, MP2, CCSD(T), CASSCF( 10,10)/CASPT2, and B3LYP methods with the 6-31G(d,p) basis set were used to probe the mechanism of the ring-chain rearrangement of halogeno-phosphiran es. It is confirmed that the lowest energy interconversion between C-haloge nated-(X)-phosphiranes and vinylphosphines, with X = H, F, Cl, and Br, is a one-step process in which the C-P bond cleavage and X-sigmatropic migratio n from C to P occur in a concerted manner in a single transition structure. The migration of a hydrogen from CH(H) is slightly favored over that of CX (H), and thus, the cleavage of the C(X)-P bond is preferred. The energy bar rier for the whole process involving hydrogen migration in the parent phosp hirane is calculated to be about 45 +/- 5 kcal/mol. The migratory aptitude of the atoms X in the uncomplexed species is found as follows: H > Br > Cl > F, either in the gaseous phase or in aqueous and DMSO solutions. The solv ation enthalpies that were estimated using a polarizable continuum model (P CM) are rather small and do not modify the relative ordering of the energy barriers. Such a trend is at variance with recent experimental findings on metal-phosphinidene complexes in which only halogen migration was observed. This might arise from a peculiar effect of the metal fragments W(CO)(5) us ed in the experimental studies to stabilize the phosphorus species that ind uce a quite different mechanism. Calculations of the P-31 chemical shifts u sing the GIAO/B3LYP/6-311+G(d,p) method show a remarkable correlation betwe en the delta P-31(X) chemical shifts of X-phosphiranes and those of X-phosp hines (XCH2PH2), suggesting that the large beta substituent effect is not i nherent to the small rings.