Biaryl phosphite-oxazoline ligands from the chiral pool: highly efficient modular ligands for the asymmetric Pd-catalyzed Heck reaction.

A highly modular library of readily available phosphite-oxazoline ligands L1-L21a-g was successfully applied in the asymmetric Pd-catalyzed Heck reactions of several substrates and triflates under thermal and microwave conditions. This ligand library contains three main ligand structures that have been designed by systematic modification of one of the most successful ligand families developed for this process. As well as studying the effect of these three ligand structures on the catalytic performance, we also evaluated the effect of modifying several ligand parameters in these ligand structures. The effectiveness of these ligands at transferring the chiral information into the product can be tuned by correctly choosing the ligand components. Both enantiomers of the Heck coupling products were obtained in excellent activities (conversion: >100% in 10 min), regioselectivities (>99%) and enantioselectivities (>99% ee). Under microwave-irradiation conditions, the reaction times were considerably shorter (full conversion was achieved in a few minutes) and the regio- and enantioselectivities were still excellent.