Water and a Borohydride/Hydronium Intermediate in the Borane-Catalyzed Hydrogenation of Carbonyl Compounds with H₂ in Wet Ether: A Computational Study

Abstract

We have computationally evaluated water as an active Lewis base (LB) and introduced the borohydride/hydronium intermediate in the mechanism of B(CF)-catalyzed hydrogenation of carbonyl compounds with H in wet/moist ether. Our calculations extend the known frustrated Lewis pair mechanism of this reaction toward the inclusion of water as the active participant in all steps. Although the definition of the zero-energy point interweaves in comparison of the scenarios with and without water, we will be able to show that (i) water (hydrogen bonded to its molecular environment) can, in principle, act as a reasonably viable LB in cooperation with the borane Lewis acid such as B(CF) but relatively a strong borane-water complexation can be the hindering factor; (ii) the herein-proposed borohydride/hydronium intermediates with the hydronium cation having three OH···ether hydrogen bonds or a combination of the OH···ether/OH···ketone hydrogen bonds appear to be as valid as the previously considered borohydride/oxonium or borohydride/oxocarbenium intermediates; (iii) the proton-coupled hydride transfer from the borohydride/hydronium to a ketone (acetone) has a reasonably low barrier. Our findings could be useful for better mechanistic understanding and further development of the aforementioned reaction.

Optimizing the Energetics of FLP-Type H2 Activation by Modulating the Electronic and Structural Properties of the Lewis Acids: A DFT Study

“…Recently, it was found that the free-energy profile may become exergonic through the formation of a particularly stabile "borohydride/hydronium" ion pair intermediate in the presence of water molecules. 58 The results shown in Figure 10 are in agreement with the "Hammond's postulate" that the transition-state structure in an exothermic reaction resembles the reactants and in an endothermic reaction seems like the products. Furthermore, the barrier heights correlate very well with the reaction free energies of H 2 splitting, which is in accordance with the "Bell− Evans−Polanyi (BEP) principle".…”

“…Recently, it was found that the free-energy profile may become exergonic through the formation of a particularly stabile “borohydride/hydronium” ion pair intermediate in the presence of water molecules. 58 …”