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 (<100°) are applied.