Divergent C2 functionalization of N-Heteroarenes via nonclassical rearomatization

Site-selective functionalization of N-heteroarenes remains a persistent challenge due to their intrinsic aromatic stabilization and multiple reactive sites. We present a unified strategy that redirects the reactivity of N-alkenyloxy-heteroarenium salts to access two distinct product classes. Under Grignard-reagent conditions, pyridinium substrates undergo ultrafast C2-functionalization via a concerted, six-electron retro-hetero-ene rearrangement—a transformation not previously described—delivering ortho-substituted pyridines (alkyl, aryl, heteroaryl, alkynyl) in up to 93% yield. In contrast, quinolinium salts engage in a cascade involving C2‑addition, a 3,3‑sigmatropic rearrangement, cycloaddition and dehydration to furnish N‑bridged polycyclic frameworks efficiently. Notably, the reaction pathway is governed by substrate sterics; C8 substitution on quinoline switches the mechanism to the retro-hetero-ene route. The method is scalable, enables late-stage diversification of complexed scaffolds and site-selective deuterium labeling. Here we show that this strategy, supported by mechanistic and computational studies, establishes a versatile platform for heteroarene editing via nonclassical rearomatization.