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.