Biaryl Phosphine Based Pd(II) Amido Complexes: The Effect of Ligand Structure on Reductive Elimination

Kinetic
studies conducted under both catalytic and stoichiometric
conditions were employed to investigate the reductive elimination
of RuPhos (2-dicyclohexylphosphino-2′,6′-diisopropoxybiphenyl)
based palladium amido complexes. These complexes were found to be
the resting state in Pd-catalyzed cross-coupling reactions for a range
of aryl halides and diarylamines. Hammett plots demonstrated that
Pd­(II) amido complexes derived from electron-deficient aryl halides
or electron-rich diarylamines undergo faster rates of reductive elimination.
A Hammett study employing SPhos (2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl)
and analogues of SPhos demonstrated that electron donation of the
“lower” aryl group is key to the stability of the amido
complex with respect to reductive elimination. The rate of reductive
elimination of an amido complex based on a BrettPhos-RuPhos hybrid
ligand (2-(dicyclohexylphosphino)-3,6-dimethoxy-2′,6′-diisopropoxybiphenyl)
demonstrated that the presence of the 3-methoxy substituent on the
“upper” ring of the ligand slows the rate of reductive
elimination. These studies indicate that reductive elimination occurs
readily for more nucleophilic amines such as N-alkyl
anilines, N,N-dialkyl amines, and primary aliphatic
amines using this class of ligands.