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

The formation of MeP(O)(OPh) 2 by reductive elimination from L 2 PdMe(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(sp 3 ) 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 PPh 3 ), suggesting that the reductive elimination occurs from a four- or three-coordinate intermediate. When the rate of reductive elimination is slow, protonolysis reactions between L 2 PdMe(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(sp 3 ) bond formation. These results demonstrate that the formation of P(O)−C(sp 3 ) bonds by reductive elimination from L 2 PdRP(O)(OR) 2 complexes is quite sensitive to the steric bulk of the supporting ligand and the presence of excess hydrogen phosphonate.