Palladium(II) complexes bearing mixed N^N^X (X = O and S) tridentate ligands as pre-catalysts for the methoxycarbonylation of selected 1-alkenes

Abstract

The methoxycarbonylation of selected 1-alkenes catalyzed by various neutral and cationic palladium(II) complexes, containing mixed N^N^X (X = O and S) tridentate ligands 2-[(3,5-dimethyl-1H-pyrazol-1-yl)methyl]-6-(phenoxymethyl)pyridine (L1), 2-[(3,5-di-tert-butyl-1H-pyrazol-1-yl)methyl]-6-(phenoxymethyl)pyridine (L2), 2-[(3,5-dimethyl-1H-pyrazol-1-yl)methyl]-6-(phenylthiomethyl)pyridine (L3), 2-[(3,5-di-tert-butyl-1H-pyrazol-1-yl)methyl]-6-(phenylthiomethyl)pyridine (L4), has been investigated. Neutral complexes, [(ĸ2-L1)Pd(CH3)(Cl)] (1a), [(ĸ2-L2)Pd(CH3)(Cl)] (2a), [(ĸ2-L3)Pd(CH3)(Cl)] (3a), [(ĸ2-L4)Pd(CH3)(Cl)] (4a), and the salts, [(ĸ3-L3)Pd(CH3)][BAr4F] (3c) and [(ĸ3-L4)Pd(CH3)][BAr4F] (4c), underwent complete decomposition during the reaction to palladium black and showed no catalytic activity. However, the addition of PPh3 to the reaction dramatically increased the catalytic activity. On the other hand, the salts, [(ĸ2-L1)Pd(CH3)(PPh3)][BAr4F] (1b), [(ĸ2-L2)Pd(CH3)(PPh3)][BAr4F] (2b), [(ĸ2-L3)Pd(CH3)(PPh3)][BAr4F] (3b) and [(ĸ2-L4)Pd(CH3)(PPh3)][BAr4F] (4b), showed good conversion of the selected olefins to branched and linear esters without PPh3. Addition of PPh3 to reactions with 1b-4b significantly improved catalytic activity. All decomposition of complexes led to the formation of the known palladium complexes, [Pd(PPh3)2(Cl)(CH3)] and [Pd(PPh3)2Cl2]. The decomposition of all palladium complexes could be followed by NMR studies and [Pd(PPh3)2Cl2] could be isolated from the crude methoxycarbonylation reaction.