Development and mechanistic investigation of the manganese(iii) salen-catalyzed dehydrogenation of alcohols

The first example of a manganese(III) catalyst for the acceptorless dehydrogenation of alcohols is presented. N,N'-Bis(salicylidene)-1,2-cyclohexanediaminomanganese(III) chloride (2) has been shown to catalyze the direct synthesis of imines from a variety of alcohols and amines with the liberation of hydrogen gas. The mechanism has been investigated experimentally with labelled substrates and theoretically with DFT calculations. The results indicate a metal-ligand bifunctional pathway in which both imine groups in the salen ligand are first reduced to form a manganese(III) amido complex as the catalytically active species. Dehydrogenation of the alcohol then takes place by a stepwise outer-sphere hydrogen transfer generating a manganese(III) salan hydride from which hydrogen gas is released.