Photocatalytic cross-coupling of amines and alcohols over Au/TiO2: Reaction atmosphere-regulating imines and secondary amines synthesis

The cross-coupling of alcohols with amines is an atomically efficient process for the synthesis of imines (CN) and secondary amines (CN), but developing efficient catalytic systems with CN and CN bonds precisely regulated under mild conditions remains a challenge. Here, utilizing the light as the only energy input, we achieved the efficient synthesis of imines and secondary amines by manipulating the hydrogen-transfer pathway over Au/TiO₂ with different reaction atmosphere. 100 % aniline conversion with 99 % secondary amine selectivity and 70 % aniline conversion with 99 % imine selectivity could be obtained, under N₂ and CO₂ atmosphere, respectively. Various control experiments and spectroscopies including in situ DRIFTS were tested. The investigations showed that UV-excited TiO₂ catalyzed the dehydrogenation of benzyl alcohol to produce hydrogen proton (H⁺), while the Au NPs catalyzed H⁺ reduction with the photogenerated electrons transferred from P25 to the interface of Au/TiO₂, forming Au-H species. The Au-H species hydrogenated imine to secondary amine in N₂ and the excellent hydrogen transfer capability of Au NPs was further proved compared with other metal counterparts. When replacing N₂ with CO₂, the CO intermediates derived from CO₂ reduction adsorbed on Au surface and inhibited the hydrogen transfer capability of the Au NPs, maintaining the CN bonds. This work provides a new strategy for CO₂ utilization and for tuning the CN and CN bonds in photocatalytic organic transformations with Au-based catalyst.