ON C-C COUPLING BY CARBENE-STABILIZED PALLADIUM CATALYSTS - A DENSITY-FUNCTIONAL STUDY OF THE HECK REACTION

The mechanism of the Heck reaction has been computationally studied fo r a carbene-stabilized palladium catalyst employing a density function al method with a gradient-corrected exchange-correlation functional. T he crucial steps of the reaction, insertion of the olefin into the Pd- aryl bond and B-hydride elimination, have been investigated for a Pd c atalyst Ligated by two diaminocarbene ligands; these ligands have been chosen to model 1,3-dimethylimidazol-2-ylidene ligands which have bee n employed in experimental work. The reaction involves the cleavage of a Pd-halide bond and thus proceeds via positively charged complexes. For the insertion, the effect of a counterion has also been investigat ed; its presence does not alter the reaction mechanism since its influ ence on the reaction energies is small. The insertion step has also be en studied for a newly proposed bidentate ligand which coordinates to the Pd center via one carbene as well as one phosphine group. In this system, a Pd-P bond is broken, leading to neutral intermediates. For b oth ligand systems, the calculated barriers to insertion are in the ra nge 8.3-11.5 kcal/mol; the barrier to B-hydride elimination is calcula ted as 9.0 kcal/mol.