Photocatalytic transfer hydrogenation of Schiff bases with propan-2-ol by suspended semiconductor particles loaded with platinum deposits

Abstract

Photoirradiation at > 300 nm of platinized titanium(IV) oxide (TiO₂-Pt) or cadmium(II) sulfide loaded with platinum(IV) oxide suspended in propan-2-ol has been shown to lead to transfer hydrogenation of Schiff bases [N-benzylidenebenzylamine (BdBA) and N-benzylideneaniline] to the corresponding secondary amines [dibenzylamine (DBA) and N-benzylaniline, respectively]. Dehydrogenation of propan-2-ol also occurred as a side reaction. The transfer hydrogenation required both the photoirradiation and suspended semiconductor particles, indicating that photoexcited electrons and positive holes induces the reaction. Platinization of the semiconductor particles was also necessary and TiO₂-Pt prepared by impregnation followed by H₂ reduction showed the highest yield of DBA from BdBA. The most efficient catalyst was prepared by reduction at an H₂ flow rate of 35 cm³ min^–1 and had small Pt deposits dispersed uniformly on each TiO₂ particle. Addition of molecular sieve 3A to the reaction mixture improved the selectivity to DBA from BdBA by absorbing contaminant water and thereby inhibiting the hydrolysis of BdBA and subsequent formation of N-benzylpropyl-2-amine.