Our envisioned catalytic cycle (Fig. ) starts with the oxidation of the amine (I) by the excited photocatalyst. Analogous to previously reported mechanisms, 22 the thus formed amino radical cation (II) triggers rapid α-amino deprotonation, yielding a carbon-centered α-amino radical (III). This radical should then be converted to the desired carbanion intermediate (IV) by the reduced photocatalyst species, facilitated by the presence of an electron-withdrawing group. In accordance with this, N-(4methoxyphenyl)-N-methyl glycinate (1a) was chosen as a simple model substrate. The para-methoxy phenyl group (PMP) was selected as the amine protecting group, as it should not hamper the initial oxidation. Additionally, PMP can be easily removed in a subsequent step to yield the corresponding free amine. A methyl group was adopted as the second N-substituent to introduce another α-amino position in order to test the anticipated selectivity. Finally, D2O was employed as a potent carbanion trap and reasonably priced deuterium source to obtain the isotopically labelled compound. Gratifyingly, 5 mol% of 3DPA2FBN as photocatalyst in combination with 2 equivalents of K2CO3 as a mild base and DMA (0.1 M) as solvent gave the desired deuterated compound 1 in high yield (85% NMR-yield) and excellent deuterium incorporation (1.80 D/molecule) within 16 h (Table , entry 1). As designed and desired, the α-amino position of the amino ester is exclusively deuterated, while the primary α-amino position remains untouched. This is exceedingly difficult to achieve using a photocatalytic HAT process, where both positions are likely to be deuterated. 54 Reaction conditions: a 0.1 mmol substrate, 5 mol% 3DPA2FBN, 2 eq. K2CO3, 15 eq. D2O, 1 mL dry DMA (0.1 M), 451 nm, 25 °C, 16-24 h, 1 H NMR yield using dibromomethane as internal standard. Deuterium incorporation was measured by HRMS.