Theoretical insight into the solvent effect on the stoichiometric reduction of carbonyl compounds by ammonia borane and N-methyl amine borane

摘要

For the traditional reduction of ketones and aldehydes, NH3BH3 (AB) and N-methyl amine borane (M(n)AB) have been effective reducing agents. However, the reaction process is indefinite and different mechanisms have been proposed; also the solvent effect, which is closely related to the mechanism, has not been considered seriously. Here we employ density functional theory to carry out a comprehensive study on the mechanism. The calculated free energy of the concerted double hydrogen transfer process is lower than the hydroboration mechanism by 4.7 kcal/mol, which indicates that reduction of carbonyl by AB is likely due to be the concerted double hydrogen transfer in both aprotic (tetrahydrofuran) and protic (MeOH) solvents. For the reduction by M(n)AB, the corresponding free energies of all reactions are higher than those of AB. Meanwhile, the reduction of benzaldehyde by M(n)AB (n = 1, 2) also favors a concerted double hydrogen transfer rather than hydroboration.