The reaction of 1u afforded a 1.46:1 mixture of the azetidine 2u and the b-lactam 3U (Figure ). The 2:3 ratio was altered by varying N (up to 1.92:1, for a rationale, vide infra). The same trend was observed using 1v. Both substrates had two competing C(sp 3 )-H groups (green vs red -H in Figure ). Interestingly, the reaction of 1v-d gave exclusively 3V-d. The d-labelled b-lactam was accessed by steering the selectivity of the C-H vs C-D functionalization, which is often challenging. The result indicated that this step presents a large primary kinetic isotope effect. This, in turn, is consistent with a 1,5-HAT, for which significant kH/kD (> 7) were reported. The catalytic role of N4 was established monitoring two parallel reactions through time by 1 H NMR (Figure ). The rate of formation of 2i was sharply lower without N4.