Photochemical reduction of 1-methyl-3-carbamoyl-pyridinium by cyanoborohydride induced by outer-sphere charge transfer excitation

The NAD+ subunit 1-methyl-3-carbamoyl pyridinium cation (or 1-methyl nicotinamide cation, MNA+) forms an ion pair with BH3CN− which shows a new absorption (λmax=453 nm) which is assigned to an outer-sphere charge transfer (OSCT) transition from BH3CN− to MNA+. At r.t. this ion pair is not stable and undergoes a hydride transfer from BH3CN− to MNA+ with the formation of MNAH. At −77°C this ion pair is persistent but the hydride transfer occurs as a photoreaction which is induced by OSCT excitation.