Borrowing Hydrogen Mechanism in Amine Alkylation by Single Atom Nickel Catalysts

The alkylation of amines with alcohols by borrowing hydrogen (BH) is a versatile synthetic route to various relevant compounds, including drugs and agrochemicals. The atom economy of this reaction, with water as the sole byproduct, entails a complex mechanism in which alcohol activation can follow multiple competing pathways. The interest in single atom catalysts (SACs) combining the advantages of homogeneous and heterogeneous systems motivated this computational study on BH catalyzed by nickel SACs. We found that the oxidation, charge, and spin states of the system, as well as the geometry of the support, all have a strong influence on the BH mechanism. The pyrrolic support was the most reactive in a dicationic state with nearly degenerate singlet and triplet configurations. The free energy of the catalyst hydrogenation by H2 was strongly correlated to its reactivity, thus being a useful descriptor. Further, the mechanism followed an unusual reaction pathway in which the support takes and delivers the H equivalents, with the metal playing a spectator role. This study provides useful insights for the further development of BH SACs.