A structure–activity relationship for pincer palladium(II) complexes — influence of ring-size of metallacycles on the activity in allylic alkylation

A series of new palladium(II) complexes derived from the well-known pincer complex [PdCl{(C6H3)(OPiPr2)2-2,6}] were synthesised: [PdX{(C6H3)(OPiPr2)2-2,6}] (X=Br−, I−, OAc−, OTf−). A novel PCP′ pincer ligand 1-(iPr2PO)-3-(iPr2POCH2)(C6H4) was prepared and complexed to palladium(II) to give [PdX{(C6H3)(OPiPr2)-2-(CH2OPiPr2)-6}] (X=Cl−, I−, OAc−, OTf−, BF4−). The X-ray structure of [PdCl{(C6H3)(OPiPr2)-2-(CH2OPiPr2)-6}] was solved and is discussed. These complexes were applied to the catalytic reaction of cinnamyl acetate with sodium dimethyl malonate in order to evaluate the influence of the ligand structure and co-ordinating or non-co-ordinating anions on the regioselectivity. A detailed analysis shows that palladium(II) complexes of the unsymmetrical PCP′ bis(phosphinito) ligand are much more active when compared to related complexes of the symmetrical PCP bis(phosphinito) ligand. The origin of this difference in activity is discussed.