Carbene

A carbene is a neutral divalent carbon species in which the carbon atom has six electrons in its valence shell. Carbenes can exist in two main electronic states: singlet, with paired electrons in one orbital, and triplet, with two unpaired electrons in separate orbitals. This difference in spin states leads to distinct reactivity patterns, with singlet carbenes often behaving as electrophiles and triplet carbenes acting as diradicals.

Carbenes are typically highly reactive and short‑lived in solution, but they can be generated in situ or stabilized by adjacent heteroatoms or by coordination to metal centers. Free carbenes can be prepared under specialized conditions, such as in the gas phase, or generated transiently from precursors like diazo compounds or ketenes.

Reactions of carbenes include transfer to alkenes to form cyclopropanes (cyclopropanation), and insertion into X–H bonds (for example C–H or O–H insertions). They can also undergo rearrangements, such as the Wolff rearrangement of alpha‑diazoketones to ketenes. A widely used class of carbene transfer reagents is derived from diazo compounds in the presence of metal catalysts (Rh, Cu, etc.). The Simmons–Smith reaction uses a zinc–carbenoid to convert alkenes into cyclopropanes.

Metal–carbene complexes are central in organometallic chemistry. Two classic families are Fischer carbenes and Schrock carbenes: Fischer carbenes are typically electrophilic and associated with late transition metals, while Schrock carbenes are more nucleophilic and linked to early transition metals. Metal carbenes are key intermediates in olefin metathesis, facilitated by catalysts such as Grubbs and Schrock systems.

Persistent carbenes, notably N‑heterocyclic carbenes (NHCs), are stabilized carbenes that can be isolated and widely used as ligands in catalysis or as organocatalysts. The isolation of stable carbenes in the 1990s markedly expanded their practical applications.