Effect of hydrogen bonding on ligand substitution and its implication for the Heck reaction

Abstract Simple hydrogen-bond donating ammonium cations are found to hydrogen-bond with the halo ligand of a key intermediate in the Heck reaction, [Pd(diphosphine)PhBr], and promote the substitution of the halide by a phosphine ligand. Furthermore, upon introduction of a hydorgne bond donor (HBD), the palladium intermediate reacts with an electron-rich olefin to afford the expected Heck product, whereas no reaction was observed without the HBD. These observations support the hypothesis that HBDs accelerate the Heck α-arylation via facilitating halide dissociation from palladium and thereby the ionic pathway.