Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 16, Issue 35, Pages 10844-10853Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201000138
Keywords
catalyst activation; cross-coupling; heterocycles; kinetics; palladium
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Funding
- Natural Sciences and Engineering Research Council of Canada (NSERC)
- Ontario Research and Development Challenge Fund (ORDCF)
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A series of Pd-N-heterocyclic carbene (Pd-NHC) complexes with various NHC, halide and pyridine ligands (PEPPSI (pyridine, enhanced, precatalyst, preparation, stabilisation and initiation) precatalysts) were prepared, and the effects of these ligands on catalyst activation and performance were studied in the Kumada-Tamao Corriu (KTC), Negishi, and Suzuki-Miyaura cross-coupling reactions. The lowered reactivity of more hindered 2,6-dimethylpyridyl complex 4 in the Negishi and KTC reactions is consistent with slow reductive dimerisation of the organometallic reaction partner during precatalyst activation. Comparative rate studies of complexes 1, 4 and 5 in the KTC and Suzuki-Miyaura reactions verify that 4 activated more slowly than the others. A potential on/off mechanism of pyridine coordination to NHC-Pd-0 is also plausible, in which the more basic pyridine stays bound for longer.
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