Catalyst activity and selectivity in the isomerising alkoxycarbonylation of methyl oleate

Abstract

The synthesis of unsymmetrical diphosphine ligands (3 a-g) with an o-tolyl backbone and tert-butyl, adamantyl, cyclohexyl and isopropyl substituents on the phosphorus moiety is described (1,2-(CH2PR2) (PR′2)C6H4; 3 a: R=tBu, R′=tBu, 3 b: R=tBu, R′=Cy, 3 c: R=tBu, R′=iPr, 3 d: R=Ad, R′=tBu, 3 e: R=Ad, R′=Cy, 3 f: R=Cy, R′=Cy, 3 g: R=Ad, R′=Ad). The corresponding diphosphine-PdII ditriflate complexes [(P^P)Pd(OTf)2] (5 a-g) were prepared and structurally characterised by X-ray crystallography. These new complexes were studied as catalyst precursors in the isomerising methoxycarbonylation of methyl oleate, and were found to convert methyl oleate into the corresponding linear α,ω-diester (L) with 70-80 % selectivity. The products of this catalytic reaction with the known [{1,2-(tBu2PCH2) 2C6H4}Pd(OTf)2] complex (5 h) were fully analysed, and revealed the formation of the linear α,ω-diester (L, 89.0 %), the methyl-branched diester B1 (4.3 %), the ethyl-branched diester B2 (1.0 %), the propyl-branched diester B3 (0.6 %) and all diesters from butyl-to hexadecyl-branched diesters B4-B16 (overall 4.8 %) at 90 °C and 20 bar CO. The productivity of the catalytic conversion of methyl oleate with complexes 5 a-g varied with the steric bulk of the alkyl substituent on the phosphorus. Ligands with more bulky groups, like tert-butyl or adamantyl (e.g., 5 a, 5 d, 5 g), were more productive systems. The formation of the catalytically active hydride species [(P^P)Pd(H)(MeOH)]+ (6-MeOH) was investigated and observed directly for complexes 5 a-e and 5 g, respectively. These hydride species were isolated as the corresponding triphenylphosphine complexes (6-PPh3) and fully characterised, including by X-ray crystallography. The catalytic productivity of 6 a-PPh3 was virtually identical to that of 5 a, thereby confirming the efficient hydride formation of 5 a under catalytic conditions. Conversion of plant oils: The selective conversion of plant oils has been studied for a range of diphosphine-coordinated Pd II catalysts. An efficient conversion of the double bond deep in the methyl oleate chain to the terminal 1,19-diester (see scheme) was brought about with conveniently accessible α-tolyl-bridged diphosphines. Remarkably, the extreme steric bulk of the diphosphine appears more decisive for the efficient formation of the catalytically active species than for selectivity. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.