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 degrees C. Detailed investigations showed that light irradiation accelerates the reduction of the Pd-II precursor, as confirmed by P-31 NMR spectroscopy, in which the in situ 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 (Pd black formation). P-31 NMR 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-0 to Pd-0 under light irradiation.