Control of Chemo- and Regioselectivity in the Palladium-Catalyzed Telomerization of Butadiene with Methanol − Catalysis and Mechanism

The palladium-catalyzed telomerization reaction of butadiene with methanol has been examined with the aim of controlling the chemoselectivity (telomerization vs. dimerization products) and regioselectivity (linear vs. branched telomerization product) of the reaction. We have shown that the reaction temperature, ligand-to-metal ratio and ratio of substrates exert a large influence on the selectivity of the reaction. Selectivities of up to 97% for the desired linear telomerization product 1 can be achieved below 50 °C by employing both low PPh3/Pd and butadiene/methanol ratios. Mono(phosphane)palladium(0)−diallyl ether complexes, Ar3P−Pd[(CH2=CHCH2)2O] (5), serve as new catalysts for the reaction. In order to gain a mechanistic understanding of the observed selectivity effects, we synthesized the phosphane(octadienyl)palladium(II) complexes 7a and 9a as model compounds for key reaction intermediates and examined their stoichiometric reactions with the methoxide nucleophile. Based on our results, we propose an extension of the known telomerization mechanism that accounts for the observed selectivity effects.