Associative transmetalation in the stille cross-coupling reaction to form dienes:: Theoretical insights into the open pathway

The open transmetalation mechanism for the Stille cross-coupling of vinylbromide and vinyl triflate with trimethylvinylstannane catalyzed by Pd(PMe3)(2) as well as the roles of a coordinating solvent molecule (DMF) and additive (LiCl) have been theoretically studied using density functional theory (DFT). The cyclic mechanism seems to be favored for vinyl bromides. In contrast, the open alternative is likely followed by triflates. An oxidative addition mechanism involving a rearrangement of the triflate group on the Pd(PMe3)(2)(eta(2)-vinyl triflate) complex has been characterized. The open transmetalation pathway for vinyl triflate involves the more electrophilic palladium species generated by the substitution of the triflate by a ligand Y (PMe3, DMF) on the oxidative addition intermediate. Moreover, LiCl as additive is shown to favor the oxidative addition step of triflates by forming an anionic trivalent species, [Pd(PMe3)(2)Cl](-). The square-planar trans-[Pd(PMe3)(2)(vinyl)Cl] complex generated by reaction with vinyl triflate corresponds formally to the product of direct oxidative addition of vinyl chloride to Pd(PMe3)(2), which then would follow the cyclic transmetalation pathway.