Mechanistic Insight into the Heterogeneous Hydrogenation of Furan Derivatives with the use of Parahydrogen

Parahydrogen-induced polarization (PHIP) was shown to be a useful and unique technique to study the mechanisms of catalytic reactions involving hydrogen. In this paper, PHIP was utilized for mechanistic investigation of the gas-phase hydrogenation of furan, 2,3-dihydrofuran, and 2,5-dihydrofuran over titania-supported Rh, Pd, and Pt catalysts. In the hydrogenation of all three substrates over the Rh/TiO2 catalyst, the PHIP technique revealed the possibility of pairwise addition of two H atoms from the same H-2 molecule with the formation of tetrahydrofuran molecules while retaining spin correlation between the added protons. In the hydrogenation of 2,3-dihydrofuran over the Rh/TiO2 catalyst, PHIP effects were detected not only for tetrahydrofuran but also for the reactant (2,3-dihydrofuran) at positions 2 and 3 of the heterocyclic ring. Such unexpected results are direct evidence for the pairwise replacement of the hydrogen atoms in 2,3-dihydorfuran. A probable mechanism for this pairwise replacement includes sequential steps of addition and elimination of hydrogen atoms. In contrast, if the hydrogenation of 2,5-dihydrofuran was performed over Rh/TiO2, PHIP effects were detected for all protons of 2,3-dihydrofuran, implying that 2,3-dihydrofuran could be formed from 2,5-dihydrofuran not only through isomerization of the C=C bond but also through dehydrogenation of 2,5-dihydrofuran to furan with subsequent semihydrogenation.