Palladium Complexes with Bulky Diphosphine Ligands as Highly Selective Catalysts for the Synthesis of (Bio‑) Adipic Acid from Pentenoic Acid Mixtures

A series of sterically bulky diphosphines have been prepared, including P2 = trans-1,2-bis­[(di-tert-butylphosphino)­methyl]­cyclohexane (4), (2-methylenepropane-1,3-diyl)­bis­(di-tert-butylphosphine) (5), bis­[(di-tert-butylphosphino)­methyl]­dimethylsilane (6), and cis- and trans-11,12-bis­[(di-tert-butylphosphino)­methyl]-9,10-dihydro-9,10-ethanoanthracene (10 and 11). The corresponding palladium complexes of these ligands, P2Pd­(CF3CO2)2, have been synthesized and characterized. The solid-state structures of [Pd­(4)­(CF3CO2)2], [Pd­(5)­(CF3CO2)2], [Pd­(6)­(CF3CO2)2], and [Pd­(11)­(CF3CO2)2] were obtained by single-crystal X-ray diffraction and confirm the bidentate binding mode of the ligand and a square-planar coordination geometry with a minor distortion from the ideal. The diphosphines in combination with Pd­(OAc)2 have been applied in the hydroxycarbonylation of a mixture of pentenoic acid isomers to produce adipic acid with high selectivity (in several cases >95%). The (regio)­selectivity of the hydroxycarbonylation reaction is highly dependent on the P2 diphosphine ligand structure, particularly the steric bulk of the substituents on the diphosphine donor and the P–Pd–P bite angle in the complexes, with respectively tertiary alkyl phosphine substituents (tert-butyl, adamantyl) and a C4 backbone P–Pd–P bite angle >100° being the common features of highly adipic acid selective systems. It is suggested that the regioselectivity of hydroxycarbonylation becomes largely driven by the chelation of the carboxylic acid functionality of pentenoic acid substrates, when smaller size P substituents and/or when P2 ligands with smaller bite angles (