The 3 N4-1 EnT can occur through an encounter complex between 3 N4 and the substrate, taking advantage of the long lifetime of the former. Alternatively, a supramolecular adduct, such as seems more likely because the latter is more stable than the corresponding separate species 1j and N4 by 4.1 kcal/mol in DG. The complexation was confirmed by NMR titration (spectra in SI). In addition to their role in the substrate activation, the N co-catalysts allow the formation of products through energetically favored pathways (blue vs black paths for the complexed vs isolated manifolds, respectively, Figure ). For 2, the transition state is 4.3 kcal/mol more stable and The derivatization of a-vinyl azetidines 4 2 and 3 was used to extend the accessible chemical space (Figure ). The carbamate group of 2i can be easily deprotected, delivering the NH-free azetidine 4 with an excellent yield. The amide reduction on 3R furnished spirocyclic azetidine 5. The azetidine 4 has many applications (Scheme 1b) and it can be derivatized with electrophiles. For instance, the use of an isocyanate gives urea 6; treatment with phenylglyoxyl chloride allowed us to trigger a visiblelight-promoted 36 formal allylic isomerization (7). This provided a valuable 6 vinylidene-azetidine with two contiguous quaternary carbons. The vinyl group can be treated with a peracid, delivering epoxide 8 with good diastereocontrol. A mild alkyne hydration 37 leaves the heterocyclic core unscathed (9). Finally, fused bicyclic products 10 and 11, featuring a prized headbridging nitrogen nucleus, 1b,e could be smoothly prepared via ring-closing methathesis.