Computational study of isomerization reactions of silacyclopropene

CASPT2//CASSCF and hybrid DFT calculations have found that the barrier to t he concerted ring opening of silacyclopropene (1) to vinylsilylene (2) is l ower than the barrier to forming silylacetylene (5). The Delta ZPE-correcte d barriers, obtained by (12,12)CASPT2 calculations, are 32.2 and 38.7 kcal/ mol, respectively, and the corresponding values obtained by B3LYP calculati ons are 35.8 and 42.2 kcal/mol. Rearrangement of 1 to silylvinylidene (3) i s predicted to be the rate-determining step in the formation of 5. The barr ier to ring closure of 2 to 1 is computed to be 31.5 kcal/mol by (12,12)CAS PT2 and 31.7 kcal/mol by B3LYP. The CASPT2 barrier height is essentially th e same as that for rearrangement of 2 to 1-silaallene (4), but B3LYP predic ts that the formation of 4 from 2 requires ca. 3 kcal/mol less than closure of 2 to 1. Our results thus support the hypothesis that the observed gener ation of equal amounts of the two isotopomers of 1 from monodeuterated 2 is the result of a kinetically competitive equilibrium between 2 and 4. Inter conversion between 4 and 5 by two consecutive [1,2]-hydrogen shifts and inv olving formation of diradical 6 is found to be prevented by high energy bar riers (>70 kcal/mol).