The reaction mechanisms forming lithio silole, 2, from silyl 1,4-dilithio 1,3-butadiene, 1, were examined theoretically at the CCSD/6-31+G(d)//B3LYP/6-31+G(d) level of theory in the gas phase. To account for the solvent effects, the reactions in diethyl ether were examined using the polarizable continuum model of the integral equation formalism (IEFPCM) with the united atom topological (UA0) cavity model at the IEFPCM-CCSD/6-31+G(d)//IEFPCM-B3LYP/6-31+G(d) level of theory. Without hexamethyl-phosphoramide (HMPA) as a cosolvent, the lithio silole, 2, was not produced due to the higher activation barrier, which is comparable to the homolytic cleavage of C?C and/or C?S bonds. On the other hand, the reaction could be feasible if HMPA solvates strongly or dissociates two Li+ cations from the reaction system. This suggests that HMPA plays a decisive role in the reaction. The optimized structures of the stationary point species on the potential energy surfaces in diethyl ether were similar to those in the gas phase, suggesting that the solvent effects on the structures of stationary species were not so large. On the other hand, theΔ E ZPVE ≠ values in diethyl ether showed larger changes from the corresponding values in the gas phase.