Ab initio molecular orbital study on three feasible mechanisms for substitution of the vinylic carbon in F2C = C(OMs)BMe3-.

A substitution on 2,2-difluorovinylic carbon Was investigated by using ab i nitio molecular orbital calculations. Three feasible-mechanisms, which are the S(N)1-like, the S(N)2-type and the addition-elimination mechanisms, wer e examined for a model berate, 2,2-difluoro-1-mesyloxyvinyl(trimethyl)borat e. Four TSs were obtained depending on the position of Li+ around-the vinyl borate although activation energies in the gas phase are rather high (ca. 3 0-40 kcal mol(-1)) in comparison with that expected from the experimental c onditions. It was confirmed at the SCRF-IPCM calculations that the solvent effect reduces the activation energy of one S(N)2-type mechanism very much (4.1 kcal mol(-1) at the B3LYP/6-31+G*//RHF/6-31+G* level of theory) while those-for the other mechanisms do not change very much. Therefore, the S(N) 2-type mechanism is applicable to the substitution reaction observed for th e vinylborate; (C) 1999 Elsevier Science Ltd, All rights reserved.