Catalyst activity and selectivity in the isomerising alkoxycarbonylation of methyl oleate

Abstract

The synthesis of unsymmetrical diphosphine ligands (3a-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; 3a: R=tBu, R'=tBu, 3b: R=tBu, R'=Cy, 3c: R=tBu, R'=iPr, 3d: R=Ad, R'=tBu, 3e: R=Ad, R'=Cy, 3f: R=Cy, R'=Cy, 3g: R=Ad, R'=Ad). The corresponding diphosphine-Pd(II) ditriflate complexes [(P^P)Pd(OTf)2] (5a-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 (5h) 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 5a-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., 5a, 5d, 5g), 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 5a-e and 5g, 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 6a-PPh3 was virtually identical to that of 5a, thereby confirming the efficient hydride formation of 5a under catalytic conditions.