Due to the stabilization of the adjacent electron-withdrawing group, the reduced photocatalyst (E1/2(3DPA2FBN/3DPA2FBN •-) = -1.92 V vs. SCE) should be capable to accomplish this via the transfer of an electron to III. The formation of this anionic nucleophile is indicated by the successful trapping with D2O, T2O, and carbonyl compounds, which should all not efficiently react with a radical intermediate such as III. 28 The site-selectivity of the deuteration was confirmed by 1 H-and 2 H-NMR (Fig. and SI). As an example, unlabelled starting material 2 holds two potentially reactive α-amino C-H bonds. In the 1 H-NMR, the α-amino ester CH2-group shows a singlet resonance signal at δ 3.95 ppm (A), whereas the α-amino CH2-group of the ethylene substituent shows as a quartet resonance signal at δ 3.41 ppm (B), respectively. After the completed HIE reaction, singlet A disappears in the 1 H-NMR, while the quartet B stays unchanged, indicating a complete and selective deuteration of the α-amino ester position. The 2 H-NMR further supports this selectivity, with only a broad singlet resonance signal at δ 3.95 ppm corresponding to position A being observed (Fig. ).