Exploring the reaction pathways of Pd(ii)-catalyzed cyclohexene oxidation with molecular oxygen: vinylic and allylic oxidation, disproportionation and oxidative dehydrogenation

Abstract

Palladium(II) salts are able catalysts to promote different oxidative transformations of cyclohexene in the presence of molecular oxygen: vinylic and allylic oxidation and disproportionation. In this paper, we assessed the main aspects that govern these reactions, by using palladium salts (i.e. PdCl₂, Pd(OAc)₂, Pd(acac)₂ and Pd(CF₃COO)₂) in protic solvents (i.e. CH₃COOH and CH₃OH). When carried out in CH₃COOH solutions at 333 K, the Pd(II)-catalyzed oxidation reactions preferentially convert cyclohexene to 2-cyclohexenil-1-acetate. Benzene was the secondary product. It was found that the efficiency of the palladium reoxidant followed the trend: Fe(NO₃)₃ > LiNO₃ > CuCl₂ > FeCl₃. Pd(OAc)₂ was the most active catalyst. Replacing Cu(OAc)₂ by Fe(NO₃)₃ notably enhances the conversion of cyclohexene and the selectivity of 2-cyclohexenyl-1-acetate, reducing significantly the reaction time from 22 to 3 h. Conversely, when performed in CH₃OH solutions, the reaction had its selectivity drastically changed; benzene and cyclohexane were the most selectively formed products (i.e. through disproportionation reaction). In this case, the reaction occurred at room temperature and in the absence of a reoxidant. Hence, Pd(CF₃OO)₂ is the most active and selective catalyst. The addition of the Fe(NO₃)₃ reoxidant and increasing the reaction temperature to 328 K resulted in the formation of 1,2-dimethoxycyclohexane and benzene as major products (i.e. allylic oxidation and oxidative dehydrogenation products, respectively). Catalytic cycles for these transformations were proposed based on experimental data and palladium chemistry.