Synthesis and Investigation of New Macrocyclic Diphosphine−Palladium(0) Complexes Based on the Barbiturate Binding Receptor

A series of diphosphine ligands possessing a barbiturate-binding receptor have been synthesized with the goal of preparing new palladium(0) complexes for the Heck reaction, which position the alkene with respect to the metal center and thereby control the regioselectivity of the insertion step. Some of the diphosphines prepared were found to efficiently form macrocyclic bisphosphine palladium(0) complexes even though a 26-membered cycle is produced. A significant solvent effect for the oxidative addition of the Pd0 complexes with phenyl iodide was noted in the case of one of the diphosphine ligands. This descrepancy may well be accounted for by the ability of the ligand when complexed to Pd0 to possess different conformational preferences in the solvents tested, which influences the bite angle to the metal center. The receptors possessing an isophthaloyl connector bind barbital with affinities corresponding to those of the previously reported open receptors. However, upon complexation with Pd(dba)2, none ...