Metallo Protic N -Allyl Enammonium Ylide 3-Aza-Cope Rearrangement Enabled by Resonance-Assisted H-Bonding: Direct Access to Functionalized Pyrroles

The 1,2-proton transfer reactions of high-energy metal-bound protic ammonium ylides have received significant attention in NH-insertion reactions. However, their sigmatropic reactions remain challenging due to rapid protodemetalation. Herein, we have designed a conceptually new class of Rh-bound protic N-allyl enammonium ylides for stereoselective 3-aza-Cope rearrangement. These transient ylides are generated in situ by the Rh-catalyzed reaction of vinyldiazo compounds and secondary (Z)-β-enamino carbonyl compounds under mild conditions. Mechanistic studies revealed that resonance-assisted hydrogen bonding (RAHB) promotes [3,3] rearrangement by disfavoring 1,2-proton transfer or direct C-C bond formation. The methodology enables direct synthesis of functionalized 4-vinylpyrroles and deuterated pyrroles. The pyrrole products were elaborated to the 7-arylindole and valuable tricyclic pyrroloindoline scaffold of a novel antitumor and UV-A-protecting agent, terreusinone.