Stereoselective hydrogenation of tert-butylphenols over charcoal-supported rhodium catalyst in supercritical carbon dioxide solvent

Abstract

Hydrogenation of 2-, 3-, and 4- tert-butylphenols was studied over a charcoal-supported rhodium catalyst in supercritical carbon dioxide (scCO 2) solvent, and the results were compared with those in organic solvents. In the hydrogenation of 4- tert-butylphenol, a higher cis ratio for 4- tert-butylcyclohexanol (0.79) was obtained in scCO 2 (10 MPa) than in 2-propanol (0.70) and cyclohexane (0.64) under similar conditions of hydrogen pressure (2 MPa) and temperature (313 K). In the case of 2- tert-butylphenol, the cis ratio for 2- tert-butylcyclohexanol was as high as 0.95 in both scCO 2 and 2-propanol (hydrogen pressure, 2 MPa; reaction temperature, 313 K). In the case of hydrogenation of 3- tert-butylphenol, the cis ratio decreased with the progression of consecutive hydrogenation of 3- tert-butylcyclohexanone intermediate. In addition, the stereoselectivity to cis- tert-butylcyclohexanols in scCO 2 was improved in the presence of hydrochloric acid. It was found that the protons of hydrochloric acid accelerated the hydrogenation of the intermediates, tert-butylcyclohexanones, to the corresponding cis- tert-butylcyclohexanols. The hydrogenation mechanism of tert-butylphenols, particularly the enhanced selectivity to cis- tert-butylcyclohexanols in scCO 2, is postulated based on the observed reaction profiles.