Kinetics and Mechanisms of Homogeneous Catalytic Reactions. Part 12. Hydroalcoxycarbonylation of 1-Hexene Using Palladium/ Triphenylphosphine Systems as Catalyst Precursors

摘要

Systems prepared in situ by addition of n equivalents of triphenylphosphine to palladium dichloride in the presence of m equivalents of para-toluenesulfonic acid (TSA), PdCl2PPh3/mTSA (n and m varying between 2 and 10), were used as precatalysts for the olefin carbonylation (1-hexene, cyclohexene and styrene) with alcohols (MeOH, EtOH, n-PrOH and i-PrOH) to generate the corresponding esters (hydroalcoxycarbonylation), under mild reaction conditions. For 1-hexene carbonylation in presence of methanol (1-hexene hydromethoxycarbonylation), the most active system was PdCl2/6PPh3/5TSA at P(CO) = 50 atm and T = 125 °C, which was also active for the hydromethoxycarbonylation of other olefins (1-hexene > styrene > cyclohexene). This system was regioselective towards the linear product for 1-hexene and towards the branched product for styrene. A kinetic study of 1-hexene hydromethoxycarbonylation catalyzed by PdCl2/6PPh3/ 5TSA showed that the initial reaction rate (r_o) was first order on Pd and MeOH concentrations and fractional order with respect to CO concentration; for olefin concentration was found a saturation curve. These kinetic results, together with coordination chemistry and computational DFT studies, allow us to propose a catalytic cycle involving species of the type [Pd(H)(L)(PPh3)2]~(+n) (L = Cl, n = 0; L = CO, MeOH, olefin and PPh3, n = 1) as the catalytically active species and three sequential reactions: (1) olefin insertion into the Pd-H bond to yield Pd-alkyl species, (2) CO insertion into the Pd-C bond to generate Pd-acyl intermediates, and (3) the methanolysis of Pd-acyl species to produce the corresponding methyl esters, regenerate the active species and restart the cycle; the last reaction is considered the rate-determining step (rds) of the mechanism.