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

Abstract

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), PdCl2/nPPh3/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. Systems prepared in situ by addition of triphenylphosphine to palladium dichloride in the presence of para-toluenesulfonic acid (TSA), PdCl2/nPPh3/mTSA were used as precatalysts for the olefin carbonylation with alcohols (MeOH, EtOH, n-PrOH and i-PrOH) to generate the corresponding esters (hydroalcoxycarbonylation), under mild reaction conditions. For 1-hexene hydromethoxycarbonylation, the most active system was PdCl2/6PPh3/5TSA at P(CO) = 50 atm and T = 125 oC. Kinetic, 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, n = 1) or [Pd(H)(PPh3)3]+ as the catalytically active species.