Mechanism of the Carboxylation of Cyclohexanone with Carbon Dioxide Catalyzed by 1,8-Diazabicyclo[5.4.0]undec-7-ene

The dependence of the rate of carboxylation upon the concentration of the reactant was investigated in dimethyl sulfoxide. The reaction rate depended upon the concentration of cyclohexanone and 1,8-diazabicyclo[5.4.0]undec-7-ene(DBU) in the first order, respectively. The reaction also followed first-order kinetics regarding the CO2 pressure in the range 50–400 Torr. From these reaction characteristics, it is suggested that carboxylation proceeds via a reaction between CO2 dissolved in dimethyl sulfoxide and the anion of cyclohexanone, rather than between the CO2 adduct with DBU and cyclohexanone. The results of carboxylation by the CO2 adduct with DBU and by CO2 dissolved in dimethyl sulfoxide also support the above conclusion. The formation of the cyclohexanone anion by a reaction of cyclohexanone with DBU was verified by the UV spectrum of a solution of concentrated DBU and cyclohexanone.