Metal cocatalyst mediated photocatalytic dehydrogenative-condensation and direct condensation cross-coupling of aniline and alcohol

Cross coupling of aniline and alcohol for the synthesis of imine and secondary amines represents a highly atom efficient processes, yet achieving high selectivity requires specialized catalysts and harsh reaction conditions to avoid over-hydrogenation of as-produced imines and dehydrogenation of secondary amines. Here we employ metal cocatalysts to mediate photocatalytic activation of alcohols, thus separating the stepwise dehydrogenative-condensation process to imine and the direct condensation process to secondary amine. The Rh/TiO₂ promotes the photocatalytic dehydrogenation of alcohol into aldehyde and molecular hydrogen, resulting in rapid condensation of aldehyde with aniline to yield imine. In contrast, Fe/TiO₂ activates the alcohol mildly without forming aldehyde and hydrogen atoms under irradiation, allowing a direct dehydrative coupling of the activated alcohol with aniline to produce secondary amine. Both Rh/TiO₂ and Fe/TiO₂ photocatalysts show high performance in terms of conversion, selectivity, stability and a wide substrate scope, rendering the strategy promising for applications.