Electronic control of the regiochemistry in the Heck reaction.

The migratory insertion of propene into the Pd-phenyl bond of a set of neutral palladium(II) complexes with N-N, P-O, and N-O chelating ligands has been investigated by using density functional methods. In contrast to cationic palladium(II)-diimine complexes, which favor 1,2-insertion, the more electron-rich neutral complexes lead to 2,1-insertion. Trans influences are substantial in the unsymmetrical ligand systems, primarily affecting the insertion barrier. Introducing steric effects leads toward improving 1,2-selectivity, due to destabilization of the 2,1-transition state. Electronic and steric influences can be systematically combined to control the regiochemistry in the Heck reaction.