Pyridinium‐Derived Mesoionic N‐Heterocyclic Olefins (py‐mNHOs)

Abstract

AbstractMesoionic polarization allows access to electron‐rich olefins that have found application as organocatalysts, ligands, or nucleophiles. Herein, we report the synthesis and characterization of a series of 3‐methylpyridinium‐derived mesoionic olefins (py‐mNHOs). We used a DFT‐supported design concept, which showed that the introduction of aryl groups in the 1‐, 2‐, 4‐, and 6‐positions of the heterocyclic core allowed the kinetic stabilization of the novel mesoionic compounds. Tolman electronic parameters indicate that py‐mNHOs are remarkably strong σ‐donor ligands toward transition metals and main group Lewis acids. Additionally, they are among the strongest nucleophiles on the Mayr reactivity scale. In reactions of py‐mNHOs with electron‐poor π‐systems, a gradual transition from the formation of zwitterionic adducts via stepwise to concerted 1,3‐dipolar cycloadditions was observed experimentally and analyzed by quantum‐chemical calculations.