Heterogeneous ¹ H and ¹³ C Parahydrogen-Induced Polarization of Acetate and Pyruvate Esters

Magnetic resonance imaging of [1-¹³ C]hyperpolarized carboxylates (most notably, [1-¹³ C]pyruvate) allows one to visualize abnormal metabolism in tumors and other pathologies. Herein, we investigate the efficiency of ¹ H and ¹³ C hyperpolarization of acetate and pyruvate esters with ethyl, propyl and allyl alcoholic moieties using heterogeneous hydrogenation of corresponding vinyl, allyl and propargyl precursors in isotopically unlabeled and 1-¹³ C-enriched forms with parahydrogen over Rh/TiO₂ catalysts in methanol-d₄ and in D₂ O. The maximum obtained ¹ H polarization was 0.6±0.2 % (for propyl acetate in CD₃ OD), while the highest ¹³ C polarization was 0.10±0.03 % (for ethyl acetate in CD₃ OD). Hyperpolarization of acetate esters surpassed that of pyruvates, while esters with a triple carbon-carbon bond in unsaturated alcoholic moiety were less efficient as parahydrogen-induced polarization precursors than esters with a double bond. Among the compounds studied, the maximum ¹ H and ¹³ C NMR signal intensities were observed for propyl acetate. Ethyl acetate yielded slightly less intense NMR signals which were dramatically greater than those of other esters under study.