Tuning ligand structure in chiral bis(phosphite) and mixed phosphite–phosphinite PCP-palladium pincer complexes

Abstract

A range of chiral resorcinol bis(phosphite) and phosphite–phosphinite ligands were produced and their propensity to form palladiumPCP-pincer complexes examined. The ease of base-assisted C–H palladation of the ligands falls in the order bis(phosphinite) > phosphite–phosphinite > bis(phosphite). The catalytic activity of the complexes in the asymmetric allylation of benzaldehyde with allyl tributyltin was examined and it was found that, contrary to expectations, ligands with 3,3′-disubstituted BINOL residues show poorer activity and stereoselectivity than unsubstituted BINOL analogues. In addition the order of activity of the pincer complexes was established as bis(phosphite) > phosphite–phosphinite > bis(phosphinite). Crystal structures of representative examples of a 3,3′-disubstituted BINOL, mono- and bis(phosphite) ligands based on 2,4-di-tert-butyl resorcinol and Pd complexes of two of the chiral complexes are presented.