posted on 2018-03-14, 00:00authored byD. Matthew Peacock, Quan Jiang, Patrick S. Hanley, Thomas R. Cundari, John F. Hartwig
We report the formation of phosphine-ligated
alkylpalladium(II)
amido complexes that undergo reductive elimination to form alkyl-nitrogen
bonds and a combined experimental and computational investigation
of the factors controlling the rates of these reactions. The free-energy
barriers to reductive elimination from t-Bu3P-ligated complexes were significantly lower (ca. 3 kcal/mol) than
those previously reported from NHC-ligated complexes. The rates of
reactions from complexes containing a series of electronically and
sterically varied anilido ligands showed that the reductive elimination
is slower from complexes of less electron-rich or more sterically
hindered anilido ligands than from those containing more electron-rich
and less hindered anilido ligands. Reductive elimination of alkylamines
also occurred from complexes bearing bidentate P,O ligands. The rates
of reactions of these four-coordinate complexes were slower than those
for reactions of the three-coordinate, t-Bu3P-ligated complexes. The calculated pathway for reductive elimination
from rigid, 2-methoxyarylphosphine-ligated complexes does not involve
initial dissociation of the oxygen. Instead, reductive elimination
is calculated to occur directly from the four-coordinate complex in
concert with a lengthening of the Pd–O bond. To investigate
this effect experimentally, a four-coordinate Pd(II) anilido complex
containing a flexible, aliphatic linker between the P and O atoms
was synthesized. Reductive elimination from this complex was faster
than that from the analogous complex containing the more rigid, aryl
linker. The flexible linker enables full dissociation of the ether
ligand during reductive elimination, leading to the faster reaction
of this complex.