Photocatalytic decarboxylative deuteration of lauric acid with heavy water for sustainable synthesis of deuterated alkanes†

As a simple model system for sustainable synthesis of deuterated alkanes, photocatalytic decarboxylative deuteration of lauric acid (dodecanoic acid) was explored employing heavy water (²H₂O, D₂O) as the deuterium source and titanium dioxide (TiO₂) photocatalysts loaded with metal cocatalysts (Au, Pt or Pd), without requiring any other reagents. The photocatalysts effectively facilitated the production of monodeuterated undecane ([²H₁]undecane, C₁₁H₂₃D). The alkyl radical degenerated from lauric acid through decarboxylation couples with the deuterium radical generated from heavy water, resulting in the formation of the deuterated undecane. Among the tested photocatalysts, a gold-loaded TiO₂ (Au/TiO₂) photocatalyst pre-dried before use achieved a 15.3% yield of deuterated undecane after 3 hours of photocatalytic reaction, with the deuteration ratio (R_d) in the obtained undecane reaching 85.1%. While extending the reaction time increased the overall yield, it led to a lower R_d. The R_d did not reach 100% since the alkyl radical intermediate also reacts with lauric acid or another alkyl radical to form non-deuterated undecane (C₁₁H₂₄) or docosane (C₂₂H₄₆), respectively as byproducts. The reaction mechanism of this photocatalytic system was elucidated using ESR measurement with radical trapping. This study offers a model methodology for the efficient synthesis of deuterated compounds, with potential applications in various fields such as pharmaceuticals.