Ligand-accelerated C-H activation reactions: evidence for a switch of mechanism.
Initial rate studies have revealed dramatic acceleration in aerobic Pd(II)-catalyzed C-H olefination reactions of phenylacetic acids when mono-N-protected amino acids are used as ligands. In light of these findings, systematic ligand tuning was undertaken, which has resulted in drastic improvements in substrate scope, reaction rate, and catalyst turnover. We present evidence from intermolecular competition studies and kinetic isotope effect experiments that implies that the observed rate increases are a result of acceleration in the C-H cleavage step. Furthermore, these studies suggest that the origin of this phenomenon is a change in the mechanism of C-H cleavage from electrophilic palladation to proton abstraction.
Engle
Keary M
KM
Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, USA.
Wang
Dong-Hui
DH
Yu
Jin-Quan
JQ
D002244:Carbon / Q000737:chemistry*; D002384:Catalysis; D006859:Hydrogen / Q000737:chemistry*; D007700:Kinetics; D008024:Ligands
D008024:Ligands; D002244:Carbon; D006859:Hydrogen