Mechanism Study Of Cycloaddition Reactions Involved By Silylene And Disilene

Silylene, as the analogues of carbene, is an important reactive intermediates in organosilicon compounds, and while disilene is the dimer of silylenes. Both of them cycloaddition of silylenes and disilenes with a series of unsaturated compounds was considered as an efficient way to obtain various silicon containing heterocyclic compounds. Meanwhile, disilene was also considered as the molecular model of silicon surface to probed the specific reaction mechanism involved by the surface Si=Si dimer, since both of them have Si=Si bond.The present work include two parts. The main focus for part1is the mechanistic studies of [n+1](n=2,4) cycloaddition reaction between silylene and a series of unsaturated compounds including polar conjugated double bond constituted by C=N, C=O and C=C with various combinations. The corresponding potential energy surface have been successfully located at BMK/6-311G(3df,2p) level of theory. A systematic comparision of the mechanism difference of [2+1] and [4+1] cycloaddition between polar and non-polar conjugated double bond have been carried out from electronic structure level. It was found that the [4+1] cycloaddition is always more preferred than the [2+1] cycloaddition both thermodynamically and kinetically for unactivated silylene such as difulorosilylene. However, the dominant position of the [4+1] cycloaddition over [2+1] cycloaddition was found to be changed under certain chemical environment when the substrated silylene was shifted from unactived silylene (F2Si) to activated silylene (HFSi). The origin of these change have been discussed from electronic structure level.In part2, the mechanism of the [2+2] cycloaddition reaction between molecular Si=Si and C=C is our main focus. Tetramethyldisilene was selected as the model compound (Me2Si=SiMe2). The potential energy surface (PES) for [2+2] cycloaddition reaction between Me2Si=SiMe2and styrene C=C double bond from both concerted and diradical ways have been calculated at B3LYP/6-31G*level of theory. A detailed description of the actual reaction path along the reaction coordinates have been given. It was found that the diradical pathway was preferred, which was similar with the silicon surface case. Additionally, the effect of electronic substituent effect on C=C double bond to reactivity of both pathways have been systematically probed. The rate-determing step for both pathways was found to be accelerated by electron-withdrawing substituents and retard by electron-donating substituents. By a correlation analysis for the reaction barrier height and frontier orbtial energy gap, the energy gap between disilene HOMO and C=C LUMO was considered as to be the main contributor for the stabilization of the concerted TS. Electron-withdrawing group on C=C bond was considered to be able to reverse the reaction path from the diradical path to the concerted path.