Ring-opening mechanism of disilacyclopropylidenoids and trisilacyclopropylidenoid: A theoretical study

Ab-initio and density functional theory calculations have been performed to investigate the ring-opening reactions of 1-bromo-1-lithiodisilirane (5), 3-bromo-3-lithiodisilirane (6), and 1-bromo-1-lithiotrisilirane (7) to 1,2-disilaallene (9), 1,3-disilaallene (10), and trisilaallene (11), respectively. Formally, the ring-opening mechanism of silacyclopropylidenoids may be either concerted or stepwise involving the intermediacy of a free silacyclopropylidene or cyclopropylidene. The ring-opening of 5 to 9 can proceed in both concerted and stepwise mechanism, where high activation energy barriers need to be overcome in order to open the silacyclopropylidene ring and to generate 9. In contrast, the ring-opening reactions of 6 and 7 can occur in a concerted fashion. The activation energy barrier for the isomerization of 6 to the complex of 10 with LiBr was determined to be only 2.3 kcal/mol at the B3LYP/6-31G(d) level, and the reaction is highly exothermic, by 37.0 kcal/mol, which makes this reaction for a promising strategy for the synthesis of 1,3-disilaallenes. However, the barrier for the conversion of 7 to 11 is calculated to be quite high, 27.5 kcal/mol, and in this case the reaction is endothermic, by 8.8 kcal/mol.