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Homogeneous Catalysts Supported on Soluble Polymers: Biphasic Suzuki–Miyaura Coupling of Aryl Chlorides Using Phase‐Tagged Palladium–Phosphine Catalysts
Author(s) -
an der Heiden Markus,
Plenio Herbert
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200305562
Subject(s) - catalysis , phosphine , aryl , biphenyl , chemistry , palladium , heptane , suzuki reaction , organic chemistry , polymer chemistry , medicinal chemistry , alkyl
The Suzuki–Miyaura coupling of aryl chlorides and PhB(OH) 2 under biphasic conditions (DMSO/heptane) can be performed in almost quantitative yields over several cycles by means of polymeric Pd catalysts with soluble polyethylene glycol phase tags. Three sterically demanding and electron‐rich phosphines 1‐CH 2 Br,4‐CH 2 P(1‐Ad) 2 ‐ C 6 H 4 , and 2‐PCy 2 ,2′‐OH‐biphenyl, and 2‐P t Bu 2 ,2′‐OH‐biphenyl were covalently bonded to 2000 Dalton MeOPEG‐OH. The catalysts, which were formed in situ from Na 2 [PdCl 4 ], the respective polymeric phosphine, KF/K 3 PO 4 , and PhB(OH) 2 , efficiently couple aryl chlorides at 80 °C at 0.5 mol % loading, resulting in a >90 % yield of the respective biphenyl derivatives. The use of polar phase tags allows the efficient recovery of palladium–phosphine catalysts by simple phase separation of the catalyst‐containing DMSO solution and the product‐containing n ‐heptane phase. The high activity (TOF) of the catalyst remains almost constant over more than five reaction cycles, which involve the catalytic reaction, separation of the product phase from the catalyst phase, and addition of new reactants to initiate the next cycle. The Buchwald type biphenyl phosphines form the most active Pd catalysts, which are 1.3–2.8 times more active than catalysts derived from diadamantyl–benzylphosphine, but appear to be less robust in the recycling experiments. There is no apparent leaching of the catalyst into the heptane solution (<0.05 %), as evidenced by spectrophotometric measurements, and contamination of the product with Pd is avoided.