Mesoionic N‐Heterocyclic Olefins, Imines, Thiones, Phosphinidenes and Their Application in Catalysis

Transition metal compounds are widely known for their ability to activate small molecules and are excellent catalysts for a wide range of reactions. However, environmentally friendly and cost-effective alternatives are highly desirable due to their toxicity and low abundance. In recent years, there has been a significant surge in the study of main-group-based species, mainly p-block element-derived compounds, which revealed reactivity resembling that of transition metal complexes. Recently, the mesoionic N-heterocyclic carbenes (mNHCs) adducts of p-block elements, containing exocyclic fragments such as ═CH₂ (mNHOs), ═NR (mNHIs), ═PR (mNHPs), and ═S (mNHTs), have evolved as ligands for coordination chemistry, reactive species for the activation of small molecules, as well as efficient metal-free catalysts for various challenging organic transformations, driven by their strong nucleophilicity. This minireview documents recent breakthroughs in the synthesis, reactivity, and catalytic applications of mNHOs, mNHIs, mNHTs, and mNHPs.