Hydrogenation of Unsaturated Six-Membered Cyclic Hydrocarbons Studied by the Parahydrogen-Induced Polarization Technique

Parahydrogen-induced polarization (PHIP) is an efficient technique for mechanistic investigations of homogeneous and heterogeneous catalytic hydrogenations. Herein, heterogeneous gas phase hydrogenation of six-membered cyclic hydrocarbons (benzene, toluene, cyclohexene, 1,3-cyclohexadiene and 1,4-cyclohexadiene) over Rh/TiO₂, Pd/TiO₂, and Pt/TiO₂ catalysts was studied using PHIP. As expected, cyclohexene hydrogenation led to the formation of cyclohexane which because of its symmetry should not exhibit any PHIP effects. However, the presence of ¹³C nuclei at natural abundance (1.1%) breaks molecular symmetry, resulting in the observation of ¹³C satellite signals exhibiting PHIP effects in the ¹H NMR spectra. In experiments with cyclohexene, the reactant’s NMR signals were also polarized, demonstrating the possibility of cyclohexene dehydrogenation to 1,3-cyclohexadiene and subsequent hydrogenation to cyclohexene. In the hydrogenation of 1,3-cyclohexadiene and 1,4-cyclohexadiene, all NMR signals of cyclohexene exhibited PHIP effects, implying migration of CC bonds in 1,4-cyclohexadiene and cyclohexene. At the same time, upon hydrogenation of benzene and toluene the reaction products were those with saturated cycles exclusively (cyclohexane and methylcyclohexane, respectively), and their NMR signals were not polarized. The absence of PHIP effects for arene hydrogenation can be explained by a difference in the reaction mechanism compared to cyclohexane and cyclohexadienes hydrogenations, along with the larger extent to which hydrogen atoms undergo migration on the catalyst surface facilitated by lower catalyst coverage with an adsorbed substrate in case of arenes.