Structure-activity relationship analysis of Pd-PEPPSI complexes in cross-couplings: a close inspection of the catalytic cycle and the precatalyst activation model.
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.
Nasielski
Joanna
J
Department of Chemistry, York University, 4700 Keele Street, Toronto, Ontario, M3J 1P3 Canada.
Hadei
Nilofaur
N
Achonduh
George
G
Kantchev
Eric Assen B
EA
O'Brien
Christopher J
CJ
Lough
Alan
A
Organ
Michael G
MG