Vinylphosphirane-phospholene rearrangements: Pericyclic [1,3]-sigmatropic shifts or not?

The conversion of "free" and Cr(CO)₅-complexed 2-vinylphosphiranes into 3-phospholenes via [1,3]-sigmatropic shifts was studied with density functional theory and compared with the corresponding hydrocarbon system, that is, the vinylcyclopropane - cyclopentene rearrangement. All three systems behave similarly with subtle but important differences. No intermediate was found on any of the potential energy surfaces. 2-Vinylphosphiranes have smaller rearrangement barriers than vinylcyclopropane, and those carrying the Cr(CO)₅ group have still smaller ones. The rearrangement of both anti- and syn-2-vinylphosphiranes occurs in a concerted pericyclic manner with inversion of configuration at the migrating phosphorus, requiring, respectively, 29.3 and 36.7 kcal/mol, much in contrast to the 44.6 kcal/mol demanding diradical-like process for the hydrocarbon analogue. Epimerization at the phosphorus center (syn ↔ anti) requires ∼32.0 kcal/mol and occurs in a single step, reflecting a diradical-like ring opening-ring closure process that can occur in both a clockwise and counterclockwise fashion. Complexation of the phosphorus center by Cr(CO)₅ results in the substantial stabilization of reagents and products and further reduces the barriers for rearrangement. The anti isomer has the lowest barrier for the [1,3]-shift (ΔE = 20.5 kcal/mol), which is slightly less than that needed for P-epimerization and for conversion of the syn isomer, both of which are nonpericyclic processes. When a P-phenyl group is introduced, the diradical-like conversion of the syn isomer is favored over the anti isomer, in agreement with experimental reports. The influence of torquoselectivity is discussed for the rearrangements of these structures with their heavy substituents. The origin of the stabilization rendered by the Cr(CO)₅ group and its influence on the [1,3]-conversion are also analyzed. The DFT activation energies for the diradical-like [1,3]-sigmatropic shifts were verified with a multireference method.