Valence isomerization of 2-phospha-4-silabicyclo[1.1.0]butane: a high-level ab initio study.

The rearrangements for 2-phospha-4-silabicyclo[1.1.0]butane, analogous to the valence isomerization of the hydrocarbons bicyclobutane, 1,3-butadiene, and cyclobutene, were studied at the (U)QCISD(T)/6-311+G**//(U)QCISD/6-31G* level of theory. The monocyclic 1,2-dihydro-1,2-phosphasiletes are shown to be the thermodynamically preferred product, in contrast to the isomerization of the hydrocarbons, which favors the 1,3-butadiene structure. Furthermore, an unprecedented direct isomerization pathway to the 1,2-dihydro-1,2-phosphasiletes was identified. This pathway is competitive with the isomerization via the open-chain butadienes and becomes favorable when electron-donating substituents are present on silicon. Figure 2-Phospha-4-silabicyclo[1.1.0]butane can isomerize directly into the more stable P,Si-cyclobutene via an unprecedented [sigma2s+sigma2a] process, which becomes favorable over the isomerization via the P,Si-butadiene when electron-donating substituents are present on silicon.