carbenoids

Carbenoids are reagents or reactive intermediates that behave as equivalents of carbenes in chemical transformations but are not free carbenes themselves. They generate or deliver carbene fragments in a controlled way, enabling carbene-like reactivity such as cyclopropanation, C–H insertion, and X–H insertion while often avoiding the hazards of generating a true carbene directly.

The most studied class is metal carbenoids, typically formed when diazo compounds interact with transition-metal catalysts (for example Rh(II) or Cu(I) complexes). In these systems a metal-carbene complex is generated that can transfer a carbene fragment to substrates, offering opportunities for stereoselective cyclopropanation, O–H and N–H insertion, and allylic or insertive transformations. The term rhodium carbenoid or copper carbenoid is commonly used in this context.

Other well-known carbenoids include those used in the Simmons–Smith methylene transfer reaction, where diiodomethylzinc or related species act as a methylene donor to alkenes to form cyclopropanes. Such reagents are often called carbenoids because they behave as carbene equivalents without free carbenes being present.

Donor–acceptor carbenoids describe stabilized carbenes generated from specially substituted diazo compounds or ylides, which broaden the scope and selectivity of carbene-type insertions.

Safety and handling considerations are important: diazo compounds and certain carbenoid precursors can be energetic or sensitive to heat, shock, or moisture, requiring appropriate controls and literature guidance.

In summary, carbenoids provide carbene-like reactivity through metal-stabilized or preformed donor–acceptor species, enabling diverse transformations with greater control and safety than free carbenes.