Reductive Elimination from Metal Phosphonate Complexes: Circumvention of Competing Protonolysis Reactions

The formation of MeP(O)(OPh)2 by reductive elimination from L2PdMe(P(O)(OPh)2) species
has been investigated. The electronic and steric effects of the supporting ligands were
investigated by studying reductive elimination reactions from a series of discrete complexes
containing nitrogen- and phosphorus-based ligands. The P(O)−C(sp3) bond-forming reaction
is slow when the intermediate species contains bidentate nitrogen ligands or small basic
monodentate phosphines. Analogous complexes bearing large bite angle diphosphines such
as dppf and Xantphos undergo reductive elimination at ambient temperature. The rate of
MeP(O)(OPh)2 formation by reductive elimination from (dppf)PdMe(P(O)(OPh)2) is not
affected by the identity or concentration of added ligand (excess dppf or PPh3), suggesting
that the reductive elimination occurs from a four- or three-coordinate intermediate. When
the rate of reductive elimination is slow, protonolysis reactions between L2PdMe(P(O)(OPh)2)
intermediates and HP(O)(OPh)2 leads to the formation of bis-phosphonate complexes. The
protonolysis reaction can be circumvented by the use of large bite angle phosphines such as
dppf and Xantphos, which lead to rapid rates of P(O)−C(sp3) bond formation. These results
demonstrate that the formation of P(O)−C(sp3) bonds by reductive elimination from L2PdRP(O)(OR)2 complexes is quite sensitive to the steric bulk of the supporting ligand and the
presence of excess hydrogen phosphonate.