Margaret Anne (Peggy) Fredricks - Acceleration of the Rate of the Heck Reaction through UV- and Visible-Light-Induced Palladium(II) Reduction

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      DOI: 10.1002/cctc.201000137


      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 PdII precursor, as confirmed by 31P 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 PdII reduction steps in conjunction with   catalyst deactivation (Pd black formation). 31P  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 PdII to Pd0 under light   irradiation.

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