Computational Study of Isomerization Reactions of Silacyclopropene

CASPT2//CASSCF and hybrid DFT calculations have found that the barrier to the concerted ring opening of silacyclopropene ( 1 ) to vinylsilylene ( 2 ) is lower than the barrier to forming silylacetylene ( 5 ). The ΔZPE-corrected barriers, obtained by (12,12)CASPT2 calculations, are 32.2 and 38.7 kcal/mol, respectively, and the corresponding values obtained by B3LYP calculations are 35.8 and 42.2 kcal/mol. Rearrangement of 1 to silylvinylidene ( 3 ) is predicted to be the rate-determining step in the formation of 5 . The barrier to ring closure of 2 to 1 is computed to be 31.5 kcal/mol by (12,12)CASPT2 and 31.7 kcal/mol by B3LYP. The CASPT2 barrier height is essentially the same as that for rearrangement of 2 to 1-silaallene ( 4 ), but B3LYP predicts 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 generation of equal amounts of the two isotopomers of 1 from monodeuterated 2 is the result of a kinetically competitive equilibrium between 2 and 4 . Interconversion between 4 and 5 by two consecutive [1,2]-hydrogen shifts and involving formation of diradical 6 is found to be prevented by high energy barriers (>70 kcal/mol).