Kinetics and Mechanism of Iridium-Catalyzed Dehydrogenation of Primary Amines to Nitriles

A family of primary amines has been catalytically dehydrogenated to nitriles by [C 6 H 3 -2,6-(OP t Bu 2 ) 2 ]IrH 2 using tert -butylethylene as a hydrogen acceptor. The catalytic mechanism has been investigated by a series of kinetic and isotopic labeling experiments, in addition to the isolation of intermediates along the reaction pathway. The turnover frequency exhibits a first-order dependence on the concentration of amine, an inverse first-order dependence on nitrile, and a zero-order dependence on tert -butylethylene. The mechanism of amine dehydrogenation is proposed to proceed from an iridium(I) nitrile complex, the catalyst resting state, via two preturnover limiting equilibria, followed by a slow β-hydride elimination event from a transient iridium(III) amido hydride species. Measuring rate constants for the stoichiometric dehydrogenation of isobutylamine over a 41 °C temperature range established activation parameters for β-hydride elimination of Δ H ⧧ 24.8(9) kcal/mol and Δ S ⧧ = −10(2) eu. Dehydrogenation of a series of para -substituted benzyl amines indicated enhanced conversions to nitriles for substrates bearing electron-donating substituents.