The first example of an intramolecular asymmetric reductive amination of a dialkyl ketone with an aliphatic amine has been developed for the synthesis of Suvorexant (MK-4305), a potent dual Orexin antagonist under development for the treatment of sleep disorders. This challenging transformation is mediated by a novel Ru-based transfer hydrogenation catalyst that provides the desired diazepane ring in 97% yield and 94.5% ee. Mechanistic studies have revealed that CO2, produced as a necessary byproduct of this transfer hydrogenation reaction, has pronounced effects on the efficiency of the Ru catalyst, the form of the amine product, and the kinetics of the transformation. A simple kinetic model explains how product inhibition by CO2 leads to overall first-order kinetics, but yields an apparent zero-order dependence on initial substrate concentration. The deleterious effects of CO2 on reaction rates and product isolation can be overcome by purging CO2 from the system. Moreover, the rate of ketone hydrogenation can be greatly accelerated by purging of CO2 or trapping with nucleophilic secondary amines.