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

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)][BAr₄^F] (3c) and [(ĸ³-L4)Pd(CH₃)][BAr₄^F] (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)][BAr₄^F] (1b), [(ĸ²-L2)Pd(CH₃)(PPh₃)][BAr₄^F] (2b), [(ĸ²-L3)Pd(CH₃)(PPh₃)][BAr₄^F] (3b) and [(ĸ²-L4)Pd(CH₃)(PPh₃)][BAr₄^F] (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.