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 for a carbene-stabilized palladium catalyst employing a density functional method with a gradient-corrected exchange-correlation functional. The crucial steps of the reaction, insertion of the olefin into the Pd-aryl bond and β-hydride elimination, have been investigated for a Pd catalyst ligated by two diaminocarbene ligands; these ligands have been chosen to model 1,3-dimethylimidazol-2-ylidene ligands which have been 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 investigated; its presence does not alter the reaction mechanism since its influence on the reaction energies is small. The insertion step has also been 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 both ligand systems, the calculated barriers to insertion are in the range 8.3-11.5 kcal/mol; the barrier to β-hydride elimination is calculated as 9.0 kcal/mol.