Mechanism of C−P Reductive Elimination from <i>trans</i>-[Pd(CHCHPh)Br(PMePh₂)₂]

The (E)- and (Z)-styryl isomers of trans-[Pd(CHCHPh)Br(PMePh2)2] (1a) and [Pd(η2-PhCHCHPMePh2)Br(PMePh2)] (2a) were prepared, and their C−P reductive elimination (1a → 2a) and C−P oxidative addition (2a → 1a) behaviors examined. Kinetics and thermodynamics of the reactions are strongly affected by E/Z configurations of the styryl group and solvent polarity. Complex (E)-1a readily undergoes C−P reductive elimination in CD2Cl2 as a polar solvent in high selectivity. On the other hand, while the (Z)-isomer of 1a is unreactive toward reductive elimination, (Z)-2a undergoes C−P oxidative addition favorably in nonpolar benzene. X-ray diffraction analysis and DFT calculations for 1a and 2a provided reasonable accounts for these reaction features. Kinetic examinations revealed two types of C−P reductive elimination processes, which involve predissociation and association of the PMePh2 ligand, respectively.