Evaluation of the Mechanism of Heterogeneous Hydrogenation of α,β-Unsaturated Carbonyl Compounds via Pairwise Hydrogen Addition

Heterogeneous hydrogenation of alpha,beta-unsaturated carbonyl compounds was addressed using the parahydrogen-induced polarization (PHIP) technique. PHIP effects were observed in hydrogenation of C=C bond of acrolein and crotonaldehyde over different supported metal catalysts, demonstrating the existence of a pairwise route of hydrogen addition to the substrate. Hydrogenation of acrolein over Pd-Sn/Al2O3, Pd-Sn/TiO2, Pd-Zn/TiO2, and Pd/TiO2 catalysts with parahydrogen also led to the polarization of the proton of CHO group of propanal. This was explained by C(O)-H bond dissociation which represents a side process on the catalyst surface. Formation of polarized cis- and trans-2-butenes was detected in hydrogenation of acrolein with parahydrogen over several Rh-based catalysts. This observation is made possible only due to the high NMR signal enhancement provided by PHIP. It was also found that hydrogenation of acetone and propanal with parahydrogen leads to polarized propane formation as a result of C-O bond hydrogenolysis.