Role of diagonal silicon-carbon interaction in the [2+2] cycloaddition of silene and acetylene

Mechanistic aspects of a series of thermal reactions that start from acylpolysilane are discussed from B3LYP density functional theory calculations. The chemistry includes the 1,3-silyl migration on acylpolysilane that leads to the formation of silene and the [2 + 2] cycloaddition between silene and acetylene that gives rise to silacyclobutene, an organosilicon four-membered ring compound. Calculated energies for these reactions are reasonable as chemical processes that proceed under thermolysis conditions. There are two kinds of reaction pathways in the 1,3-silyl migration on acylpolysilane with respect to the stereochemistry of the migrating silyl group; one is a retention pathway and the other is an inversion pathway. The retention pathway is energetically preferred to the inversion pathway, being fully consistent with previous results about similar reactions. Silene thus formed undergoes [2 + 2] cycloaddition with acetylene in a two-step manner, resulting in the formation of silacyclobutene. The reason that this symmetry-forbidden reaction occurs under thermolysis conditions is considered from intrinsic reaction coordinate calculations as well as orbital interaction analyses. Because the orbital amplitude is significant on the Si atom of silene, the interaction,in the region between the silene Si atom and the diagonal acetylene C atom is dominant in the initial stages of the ring-closure process. Consequently, the formation of the diagonal Si-C bond has a preference over that of the proper Si-C bond finally formed in the four-membered ring. This initial process is important in the avoidance of the symmetry restriction arising from the unfavorable HOMO-LUMO overlap in the [2 + 2] cycloaddition.