Catalytic Hydrogenation of Trivinyl Orthoacetate: Mechanisms Elucidated by Parahydrogen Induced Polarization

Abstract

Parahydrogen (pH₂ ) induced polarization (PHIP) is a unique method that is used in analytical chemistry to elucidate catalytic hydrogenation pathways and to increase the signal of small metabolites in MRI and NMR. PHIP is based on adding or exchanging at least one pH₂ molecule with a target molecule. Thus, the spin order available for hyperpolarization is often limited to that of one pH₂ molecule. To break this limit, we investigated the addition of multiple pH₂ molecules to one precursor. We studied the feasibility of the simultaneous hydrogenation of three arms of trivinyl orthoacetate (TVOA) intending to obtain hyperpolarized acetate. It was found that semihydrogenated TVOA underwent a fast decomposition accompanied by several minor reactions including an exchange of geminal methylene protons of a vinyl ester with pH₂ . The study shows that multiple vinyl ester groups are not suitable for a fast and clean (without any side products) hydrogenation and hyperpolarization that is desired in biochemical applications.

Keywords: geminal hydrogenation; homogeneous catalysis; hyperpolarization; parahydrogen; sidearm hydrogenation.