Combined process of cyclohexyl cyclohexanecarboxylate synthesis from cyclohexanol and CO catalyzed by the Pd(OAc)2–PPh3–p-toluenesulfonic acid system

Abstract

Objectives. To study the possibility of combining acid-catalytic cyclohexanol dehydration and alkoxycarbonylation of the formed cyclohexene with cyclohexanol and carbon(II) oxide in a single reactor in order to achieve high yields of the target cyclohexyl cyclohexanecarboxylate product under mild conditions using the Pd(OAc)2–PPh3–p-toluenesulfonic acid catalytic system.Methods. The combined process took place in a toluene medium in a periodic steel reactor designed to operate at elevated pressure, equipped with a glass insert, a magnetic stirrer, and a sampler, as well as gas input and discharge devices. The reaction mass with the components of the catalytic system was placed in a glass reactor inside a steel autoclave. The reaction mass samples obtained during the combined process were analyzed by gas–liquid chromatography with a flame ionization detector.Results. The possibility of combining cyclohexanol dehydration catalyzed by p-toluenesulfonic acid monohydrate and formed cyclohexene alkoxycarbonylation with cyclohexanol and CO during catalysis by the Pd(OAc)2–PPh3–p-toluenesulfonic acid system in a single reactor was demonstrated. Under mild conditions (temperature 110°C; CO pressure 2.1 MPa), the target product yield reached 64.8% in 5 h. However, the combined process is complicated by the formation of a cyclohexanecarboxylic acid by-product formed as a result of the cyclohexyl cyclohexanecarboxylate hydrolysis and the cyclohexene hydroxycarbonylation.Conclusions. The reactions of intramolecular acid-catalytic cyclohexanol dehydration and formed cyclohexene alkoxycarbonylation catalyzed by the Pd(OAc)2–PPh3–p-toluenesulfonic acid system can be combined in a single reactor. p-Toluenesulfonic acid can simultaneously act as a catalyst for the cyclohexanol dehydration and a co-catalyst of the palladium–phosphine system of cyclohexene alkoxycarbonylation. The involvement of cyclohexene, representing a product of reversible cyclohexanol dehydration, in the alkoxycarbonylation reaction is a factor in shifting the dehydration reaction equilibrium towards the formation of cyclohexene. Cyclohexanecarboxylic acid is a by-product of the proposed combined process. A factor in the reduction of target product yield is water formed as a result of cyclohexanol dehydration due to the involvement of the latter in the hydrolysis reaction and the course of the cyclohexene hydroxycarbonylation.