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Acceleration of the Rate of the Heck Reaction through UV‐ and Visible‐Light‐Induced Palladium(II) Reduction
Author(s) -
Fredricks Margaret Anne,
Drees Markus,
Köhler Klaus
Publication year - 2010
Publication title -
chemcatchem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.497
H-Index - 106
eISSN - 1867-3899
pISSN - 1867-3880
DOI - 10.1002/cctc.201000137
Subject(s) - palladium , chemistry , photochemistry , catalysis , heck reaction , irradiation , styrene , organic chemistry , copolymer , polymer , physics , nuclear physics
Irradiation with light (UV and visible) increases the rate of the Heck reaction using homogeneous (palladium(II) acetate) and heterogeneous (Pd/Al 2 O 3 and Pd/TiO 2 ) 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 31 P NMR spectroscopy, in which the in situ reduction of [Pd(OAc) 2 (PPh 3 ) 2 ] to [Pd(PPh 3 ) n ] ( n =2–4) in the presence of PPh 3 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). 31 P 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 [PdI 4 ] 2− , [PdBr 4 ] 2− , and [PdCl 4 ] 2− , showed that ligand‐to‐metal charge transfer (LMCT) is responsible for the accelerated reduction of Pd II to Pd 0 under light irradiation.