Isolation of Anionic <i>N</i> ‐Heterocyclic Olefins and Their Versatile Reactivity

Abstract

Abstract N ‐Heterocyclic olefins (NHOs) have emerged as powerful bases and nucleophiles with broad applications in synthesis and catalysis. Here, we report a new synthetic approach to accessing their anionic derivatives through a formal ligand exchange reaction of metalated ylides or diazomethanes with free carbenes. The reaction was found to depend on the nature of the carbene, with more electrophilic carbenes reacting more readily via N 2 and PPh 3 extrusion, respectively. Spectroscopic and crystallographic analyses, combined with computational studies revealed that the anionic NHOs exhibit bent structures. In these structures, the anionic charge at the central carbon atom is partly delocalized into the carbene and the second substituent (Z). These stabilizing effects confer versatile reactivity at the central carbon atom, the nitrogen atom of the N ‐heterocyclic carbene (NHC) or at the Z substituent. While the predominant reactivity occurs at the central carbon atom—enabling the convenient synthesis of functionalized NHOs—anionic NHOs also undergo a thermally induced rearrangement to N ‐heterocyclic imines via a skeletal rearrangement involving ring‐opening of the N ‐heterocyclic carbene.