Methyl/phenyl exchange between palladium and a phosphine ligand. Consequences for catalytic coupling reactions

Article abstract of DOI:10.1021/ja00138a012

The methyl ligand of trans-CH₃Pd(PPh₃)₂I (1) exchanges with a phenyl from its PPh₃ ligand, initially giving PhPd(PPh₃)(PMePh₂)I (4). The PMePh₂ ligand of 4 then exchanges with a PPh₃ of 1 to give CH₃Pd(PPh₃)-(PMePh₂)I and with the PMePh₂ ligand of more 4 to give PhPd(PPh₃)₂I and trans-PhPd(PMePh₂)₂I. The observed rate constant for the disappearance of 1 is about 7 × 10^-5 s^-1 at 75 °C. The rearrangement is irreversible, does not involve a free phosphonium cation, and does not require phosphine dissociation. The rearrangement competes with transmetalation when 1 is treated with tin reagents, leading to coupling products incorporating phenyls from PPh₃. Relatively electropositive phosphine substituents seem reluctant to rearrange onto palladium.