A density functional theory study of the Stille cross-coupling via associative transmetabolism.: The role of ligands and coordinating solvents

An associative mechanism has been computationally characterized for the Stille cross-coupling of vinyl bromide and trim ethylvinylstannane catalyzed by PdL2 (L=PMe3,, AsMe3) with or without dimethylformamide as coordinating ligand. All the species along the catalytic cycles that start from both the cis- and the trans-PdL(Y)(vinyl)Br complexes (Y = L or S; L = PMe3, AsMe3 or PH3; S = DMF) have been located in the gas phase and in the presence of polar solvents. Computations support the central role of species trans-PdL(DMF)(vinyl)Br which react by ligand dissociation and stannane coordination in the rate-limiting transmetalation step via a puckered four-coordinate (at palladium) transition state comprised of Pd, Br, Sn and sp(2) C atoms. A donating solvent may enter the catalytic cycle assisting isomerization of cis-PdL2(vinyl)Br to trans-PdL(DMF)(vinyl)Br complexes via a pentacoordinate square pyramidal Pd intermediate. In keeping with experimental observations, the activation energies of the catalytic cycles with arsines as Pd ligands are lower than those with phosphines. Polytopal rearrangements from the three-coordinate T-shaped Pd complexes resulting from transmetalation account for the isomerization and the C-C bond formation on the reductive elimination step.