Synthesis, Application and Kinetic Studies of Chiral Phosphite- Oxazoline Palladium Complexes as Active and Selective Catalysts in Intermolecular Heck Reactions

This study identifies new phosphite-oxazoline ligands that have been successfully applied in the palladium- catalyzed intermolecular asymmetric Heck reaction. The design of the new phosphite-oxazoline ligands derives from a previous successful generation of phosphine-oxazoline ligands, by replacing the phosphine group with several π- acceptor biaryl phosphite moieties. With these simple modifications, the new phosphite-based ligands, unlike previous phosphine-oxazoline, not only present a modular design with numerous potential phosphite groups available, but they are also air-stable solids, which can be made in the same number of synthetic steps as the phosphine analogues. The substitution of the phosphine by a biaryl phosphite group extended the range of substrates and triflates sources that can be coupled with regio-, enantioselectivities and activities comparable to the few best ones reported. In addition, the ligands that provided the best selectivities contained an isopropyl oxazoline moiety instead of the tert-butyl group found in the related phosphine-oxazoline ligands, which is made from a much more expensive precursor. In this paper we have also carried out kinetic studies and a Hammett plot analysis to determine the rate determining step of this system in the regime of interest. We suggest a likely explanation for the fast Heck reaction of the phosphite-oxazoline catalysts.