Mechanistic insight into B(C₆F₅)₃ catalyzed imine reduction with PhSiH₃ under stoichiometric water conditions

A DFT and experimental study on the mechanism of B(C₆F₅)₃ catalyzed imine reduction is performed using PhSiH₃ as reductant under stoichiometric water conditions. Ingleson's path B is reconfirmed here. And four novel (C₆F₅)₃B-OH₂ induced pathways (paths C2, C3, D2 and D3) entirely different from all the previous mechanisms were determined for the first time. They are all B(C₆F₅)₃ and water/amine catalyzed cycles, in which the nucleophilic water or amine catalyzed addition step between PhSiH₃ and the N-silicon amine cation is the rate-determining step of paths C2/D2 and C3/D3 with activation Gibbs free energy barriers of 23.9 and 18.3 kcal mol^-1 in chloroform, respectively, while the final desilylation of the N-silicon amine cation depends on an important intermediate, (C₆F₅)₃B-OH^-. The competitive behavior of the 5 paths can explain the experimental facts perfectly; if all the reactants and catalysts are added into the system simultaneously, water amount and nucleophiles (excess water and produced/added amine) provide on-off selectivity of the pathways and products. 1 eq. water leads to quick formation of (C₆F₅)₃B-OH^-, leading to B-II being turned off, and nucleophiles like excess water and produced/added amine switch on CD-II, leading to production of the amine. B-I' of Ingleson's path B is the only mechanism for anhydrous systems, giving N-silicon amine production only; B-I and C-I are competitive paths for systems with no more than 1 eq. water, producing the N-silicon amine and the [PhHC[double bond, length as m-dash]NHPh]⁺[(C₆F₅)₃B-OH]^- ion pair; and paths C2, C3, D2 and D3 are competitive for systems with 1 eq. water and nucleophiles like excess water or added/produced amine, directly giving amination products.