An ab initio study on the structure and reactivity of 1,4-disilabenzene

Possible reaction pathways for the conversion of Dewar-type 1,4-disilabenzene (1) to 1,4-disilabenzene (2) are discussed by means of B3LYP/6-31G** density functional theory and multiconfigurational CASSCF(6,6)/6-31G** calculations. The activation energy for the symmetry-allowed conrotatory ring opening of 1 is 34.6 kcal mol^-1 at the B3LYP level and 47.7 kcal mol^-1 at the CASSCF level, and that for the symmetry-forbidden disrotatory ring opening is 36.6 kcal mol^-1 and 54.3 kcal mol^-1 at the same levels. In both calculations the conrotatory ring opening of 1 proceeds through a potential energy barrier that is 2.0-6.6 kcal mol^-1 lower than that of the disrotatory ring opening. Thus, we conclude that the conrotatory pathway is energetically more preferred than the disrotatory pathway. The Diels-Alder reaction of 1,4-disilabenzene and acetylene is also discussed. The activation energy for the Diels-Alder addition is computed to be 4.2 kcal mol^-1 at the B3LYP/6-31G** level. Thus, 1,4-disilabenzene is highly reactive, and once 1,4-disilabenzenes are produced, the Diels-Alder additions of 1,4-disilabenzenes with many kinds of dienophiles should readily take place.