Acceleration of the rate of the Heck reaction through UV- and visible-light-induced palladium(II) reduction

Irradiation with light (UV and visible) increases the rate of the Heck reaction using homogeneous (palladium(II) acetate) and heterogeneous (Pd/Al₂O₃ and Pd/TiO₂) catalysts. The rate of the coupling of bromobenzene, chlorobenzene, and 4-chloroacetophenone with styrene was increased under light irradiation at temperatures between 90 and 160°C. Detailed investigations showed that light irradiation accelerates the reduction of the Pd^II precursor, as confirmed by ³¹PNMR spectroscopy, in which the insitu reduction of [Pd(OAc)₂(PPh₃)₂] to [Pd(PPh₃)_n] (n=2-4) in the presence of PPh₃ took only minutes under visible light irradiation and several hours in the dark. The role of light is, however, complex since it also influences other Pd^II reduction steps in conjunction with catalyst deactivation (Pdblack formation). ³¹PNMR spectroscopy showed the same active species, anionic palladium halide complexes, in both irradiated and unirradiated reactions. UV/Vis absorption spectroscopy of Pd(OAc)₂ and DFT calculations, theoretical UV/Vis spectra, and orbital calculations of [PdI₄]^2-, [PdBr₄]^2-, and [PdCl₄]^2-, showed that ligand-to-metal charge transfer (LMCT) is responsible for the accelerated reduction of Pd^II to Pd⁰ under light irradiation.