Using hyperpolarised NMR and DFT to rationalise the unexpected hydrogenation of quinazoline to 3,4-dihydroquinazoline

PHIP and SABRE hyperpolarized NMR methods are used to follow the unexpected metal-catalysed hydrogenation of quinazoline (Qu) to 3,4-dihydroquinazoline as the sole product. A solution of [IrCl(IMes)(COD)] in dichloromethane reacts with H(2) and Qu to form [IrCl(H)(2)(IMes)(Qu)(2)] (2). The addition of methanol then results in its conversion to [Ir(H)(2)(IMes)(Qu)(3)]Cl (3) which catalyses the hydrogenation reaction. Density functional theory calculations are used to rationalise a proposed outer sphere mechanism in which (3) converts to [IrCl(H)(2)(H(2))(IMes)(Qu)(2)]Cl (4) and neutral [Ir(H)(3)(IMes)(Qu)(2)] (6), both of which are involved in the formation of 3,4-dihydroquinazoline via the stepwise transfer of H(+) and H(–), with H(2) identified as the reductant. Successive ligand exchange in 3 results in the production of thermodynamically stable [Ir(H)(2)(IMes)(3,4-dihydroquinazoline)(3)]Cl (5).