Carbenoid

Carbenoid is a reactive intermediate in organic synthesis that behaves as a latent carbene. In many reactions the carbenoid transfers a one-carbon fragment to a substrate, producing cyclopropanes or inserting into bonds, but unlike a free carbene it is generated in situ and is stabilized by a metal center or by adjacent substituents, which modulate its reactivity and selectivity.

Carbenoids are typically formed from diazo compounds under metal catalysis to give metal carbenoids (for example Rh, Cu, or Zn species), or from specialized reagent systems such as the Simmons–Smith methylene transfer, in which the iodomethyl zinc carbenoid generated from CH2I2 and Zn/Cu transfers CH2 to alkenes to give cyclopropanes. The Furukawa modification uses diethylzinc to improve efficiency and selectivity. Another important family comprises sulfur ylide–based carbenoids used in the Corey–Chaykovsky reactions, which generate methylene carbenoids that convert carbonyl compounds to epoxides or enable cyclopropanation.

Applications of carbenoids include stereoselective cyclopropanation of olefins, insertion into C–H or O–H bonds, and epoxidation or ring-closure reactions. The latent nature of carbenoids offers practical advantages in controlling reactivity, but they can be highly reactive and require careful handling and reaction design to avoid side reactions such as dimerization or rearrangement.

Usage of the term emphasizes the transient, often metal-stabilized character of the reactive fragment rather than a free carbene.