Hydroesterification of 1-hexene



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University of Cape Town

Hydroesterification is the carbonylation of an olefin and the successive addition of an alcohol or water yielding an ester or acid. A variety of soluble palladium complexes are widely used for the hydroesterification of olefins due to their high activity and selectivity. Branched and linear carboxylic acid esters are formed in these reactions and the regioselectivity is strongly dependent on the catalytic system and reaction conditions used. For potential industrial applications, there is a need to understand the effect of catalyst and reaction parameters on the initial rate to enhance the yield of the desired isomer and to develop a suitable rate equation. Standard carbonylation catalysts such as C02(CO)8, Fe(CO)s and Ni(CO)4 have been used to prepare fatty-acid esters. More recently, other catalysts based on Pd, Pt, Rh and Ru found widespread use because of their better performance under milder reaction conditions. Shell has intensively studied the use of palladium and ruthenium as halogen-free carbonylation catalysts. The palladium systems typically consist of palladium acetate, tertiary phosphines and strong acids such as mineral acids or acids with weak or non-coordinating anions such as sulfonic acids. Although a number of reaction conditions have been reported for homogeneously catalysed transformations of olefins to esters, it has not been clear what the key intermediate of the Pd(II)-catalysed hydroesterification is. The proposals for the mechanism originate from the accepted cobalt-catalysed hydroformylation mechanism with modifications from the Heck formulations. Two mechanistic pathways have been proposed viz. the hydride mechanism and the alkoxy mechanism where the catalytic cycle starts from the insertion of the olefin into a Pd-H or a Pd-COOMe species respectively. It is well known that the active species could either be a hydride Pd-complex or an alkoxy Pd-complex (in the presence of an alcohol). In this study, the kinetics of the hydroesterification of 1-hexene was investigated. The catalyst system used in this study was generated in situ from a mixture of Pd(OAch/PPh3/methanesulfonic acid. Reactions were carried out in a 100ml Hastelloy autoclave at 90°C and 70 bar carbon monoxide pressure. The performance of the in-situ generated, catalytic system was evaluated in terms of activity (turnover frequency, TOF) and the selectivity towards the various product compounds. For these reactions, the TOF was constant in the initial 15 minutes whereupon it decreased significantly. The following reaction and catalyst variables were studied: 1-hexene concentration, carbon monoxide pressure, methanol concentration, temperature; palladium, ligand and acid concentrations. The reaction rate was approximated to be first-order with respect to 1-hexene concentration. An increase in the carbon monoxide above 10 bar and palladium concentration strongly inhibited the hydro esterification of 1-hexene. On varying the methanol concentration, the TOF passed a pronounced maximum at ca. 7.7 mol/litre. The reaction became inhibited by methanol at high methanol concentrations. A similar trend was observed for the selectivity for the formation of esters, which passed a maximum at ca. 10 mol/litre. The molar ratio of triphenylphosphine to palladium showed a significant influence in the catalyst activity and product selectivity. The TOF passed a maximum at an excess of 30 indicating the need for a large excess of the ligand for the formation of the catalytically active complex. The selectivity for ester formation also passed a maximum with increasing ligand concentration. The presence of an acid was found to be necessary to form the catalytically active cationic species.

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