Mechanism of alkene insertion into the Pd-Ar bond in the Heck reaction

The regularities in the regioselectivity of phenylation of alkenes (the Heck reaction) were studied by the multiple regression method using equations of the MO perturbation theory and HSAB principle. The addition of Ar to sterically nonhindered alkenes was established to be independent of the effective charge distribution on the carbon atom of the vinyl group and correlate best of all with the contributions of their AO to LUMO of the alkene. This is indicative of the orbital control of the reaction, and the stage of the electron transfer from the palladium atom to the alkene molecule plays the key role.