Bis-alkoxycarbonylation of styrene by pyridinimine palladium catalysts

Pyridinimine-modified Pd(II) complexes of general formulae (N-N′)Pd(Y)₂ catalyze the methoxycarbonylation of styrene to give dimethyl phenylsuccinate as the largely major product [N-N′ = py-2-C(R) = N(2,6-R′C₆H₃), R = H, Me; R′ = Me, i-Pr; 6-Mepy-2-C(H)=N[2,6-(i-Pr)₂C₆H₃]; py-2-C(H)=N(C₆H₅); Y = acetate, trifluoroacetate]. The influence of various catalytic parameters on the overall conversion of styrene to carbonylated products and on the product selectivity has been studied by systematically varying the type of palladium initiator, the concentrations of organic oxidant (1,4-benzoquinone) and protic acid (p-toluenesulfonic acid), and the CO pressure. By an appropriate choice of the structure of the pyridinimine ligand and of the reaction parameters, turn-over numbers as high as 96 and selectivities in dimethyl phenylsuccinate as high as 98% were obtained. In particular, the overall conversion of styrene is controlled by the steric properties of the alkyl substituents on the imine aryl group, while the nature of the substituent (H or Me) on the imine carbon influences the selectivity. The addition of 2 equivalents of TsOH to the catalytic mixtures generally increased the styrene conversion but lowered the selectivity in dimethyl phenylsuccinate due to greater production of methyl 3,6-diphenyl-4-oxohexanoate. Further additions of TsOH (up to 6 equivalents) resulted in better selectivities and lower conversions for all precursors.