Speculations on new mechanisms for Heck reactions

A mechanism for the olefination reaction is proposed, involving Pd-II/Pd-IV, in which a key step is reversible nucleophilic attack on the Pd-II-coordinated olefin to give an electron-rich sigma-alkyl- or, with carbonate, a chelated sigma-dialkyl-complex, which then oxidatively adds the organic halide e.g. ArX. Loss of nucleophile, migration of Ar from Pd-IV to coordinated olefin, beta-hydrogen elimination and loss of HX then gives the product of olefination and regenerates the Pd-II catalyst.