Persistent Carbene

A persistent carbene (also known as stable carbene or Arduengo carbene) is a type of carbene demonstrating particular stability. The best-known examples are diaminocarbenes with the general formula (R₂N)₂C:, where the 'R's are various functional groups. The groups can be bridged so that the carbon with unfilled orbitals is part of a heterocycle, such as imidazole or triazole.

Carbenes have long been known as very reactive and short lived molecules that could not be isolated, and were usually studied by observing the reactions they undergo. Stable carbenes had been proposed to exist by R. Breslow in 1957, and the first examples of stable carbenes coordinated to metal atoms were synthesized by H.-W. Wanzlick and collaborators. The isolation of a stable liquid dicarbene was reported in 1989 by G. Bertrand and others. In 1991, the group of A. Arduengo reported the synthesis of a stable, isolated, crystalline carbene.

Persistent carbenes are still fairly reactive substances, and many will undergo dimerisation, sometimes reversibly.

Persistent carbenes can exist in the singlet state or the triplet state, with the singlet state carbenes being more stable. The relative stability of these compounds is only partly due to steric hindrance by bulky groups. Some singlet carbenes are thermodynamically stable in the absence of moisture and (in most cases) oxygen, and can be isolated and indefinitely stored. Others are not thermodynamically stable and will dimerise slowly over days. The less stable triplet state carbenes have half-lives measured in seconds, and therefore can be observed but not stored.