Excited‐State Basicity Diverts the Site‐Selectivity of Aromatic Deuteration: Application to the Late‐Stage Labelling of Pharmaceuticals.

Isotope labelling, particularly with deuterium (2H), plays a critical role in drug discovery due to its ease of incorporation and its potential to switch unwanted metabolic transformations. Deuterium incorporation can enhance drug stability, affect pharmacokinetics, and alter metabolism pathways. Current deuterium labeling methods focus on hydrogen isotope exchange (HIE), and typically rely on the use of transition metal catalysis. Herein, we present a metal‐free approach for aromatic HIE, utilizing photoexcitation in deuterated hexafluoroisopropanol (HFIP‐d1). By exploiting the enhanced basicity of excited‐state aromatics, this method achieves selective deuteration at positions often inaccessible by traditional methods. The approach is efficient and was demonstrated across a broad number of complex drug molecules. Transient absorption spectroscopy confirms the formation of deuterated arenium ions.