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

Abstract

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 Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> 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.