Lastly, the effect of the aromatic protecting group was investigated. Interestingly, secondary or tertiary amino esters with solely aliphatic substituents on the nitrogen do not incorporate deuterium (Scheme S8, Experiment-7). The inactivity of purely aliphatic amino esters does not seem to be mainly due to a higher oxidation potential, hindering its initial activation by the excited photocatalyst. Using acrylonitrile as a coupling partner, the corresponding amino radical could still be trapped, confirming that aliphatic amino esters can be oxidized by the excited 3DPA2FBN photocatalyst (Scheme S8, Experiment-8-9). Hence, we assume that the aromatic group is stabilizing the carbanion intermediate, presumably via a conjugation effect. This observation could be exploited for an intermolecular selectivity, where a PMP-protected amino ester is selectively deuterated in the presence of an aliphatic one (Fig. , Experiment-III). The idea could be further extended to an intramolecular selectivity (Fig. , Experiment-IV). A substrate containing both, a PMP-protected and an aliphatic amino ester/amide, was selectively labelled at the PMP-protected site, confirmed by 2 H-NMR (Fig. ). This is exceedingly difficult to achieve using other known methods.