Carbene dimerization is a type of organic reaction

Organic reaction

Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemical reactions and redox reactions. In organic synthesis,...

 in which two carbene

Carbene

In chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is RR'C:, but the carbon can instead be double-bonded to one group. The term "carbene" may also merely refer to the compound H2C:, also called...

 or carbenoid

Carbenoid

In chemistry a carbenoid is a reactive intermediate that shares reaction characteristics with a carbene. In the Simmons-Smith reaction the carbenoid intermediate is a zinc / iodine complex that takes the form of...

 precursors react in a formal dimerization to an alkene

Alkene

In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...

. This reaction is often considered an unwanted side-reaction but it is also investigated as a synthetic tool. In this reaction type either the two carbenic intermediates react or a carbenic intermediate reacts with a carbene precursor. An early pioneer was Christoph Grundmann reporting on a carbene dimerisation in 1938. In the domain of persistent carbenes the Wanzlick equilibrium

Wanzlick equilibrium

The Wanzlick equilibrium is a chemical equilibrium between a relatively stable carbene compound and its dimer.-Original conjecture:In 1960, H.-W. Wanzlick and E...

 describes an equilibrium between a carbene and its alkene.

A reoccurring substrate is a diazo compound and more specifically an alpha-carbonyl diazo compound.For example ethyl diazoacetate

Ethyl diazoacetate

Ethyl diazoacetate is a diazo compound and a reagent in organic chemistry. It was discovered by Theodor Curtius in 1883. The compound can be prepared by reaction of the ethyl ester of glycine with sodium nitrite and sodium acetate in water....

 is converted to diethyl maleate using the ruthenium catalyst chloro(cyclopentadienyl)bis(triphenylphosphine)ruthenium

Chloro(cyclopentadienyl)bis(triphenylphosphine)ruthenium

Chlorobisruthenium is the organoruthenium half-sandwich compound with formula RuCl2. It as an air-stable orange crystalline solid that is used in a variety of organometallic synthetic and catalytic transformations.-Structure and Properties:The compound has idealized Cs symmetry...

 :

Grubbs' catalyst

Grubbs' catalyst

Grubbs' Catalyst is a transition metal carbene complex named after Robert H. Grubbs, the chemist who first synthesized it. There are two generations of the catalyst, as shown on the right. In contrast to other olefin metathesis catalysts, Grubbs' Catalysts tolerate other functional groups in the...

 is also effective In this reaction type the active intermediate is a transition metal carbene complex

Transition metal carbene complex

A transition metal carbene complex is a organometallic compound featuring a divalent organic ligand. The divalent organic ligand coordinated to the metal center is called a carbene. Carbene complexes for almost all transition metals have been reported. Many methods for synthesizing them and...

. A diazo cross-coupling reaction has also been reported between ethyl diazoacetate and methyl phenyldiazoacetate using the rhodium

Rhodium

Rhodium is a chemical element that is a rare, silvery-white, hard and chemically inert transition metal and a member of the platinum group. It has the chemical symbol Rh and atomic number 45. It is composed of only one isotope, 103Rh. Naturally occurring rhodium is found as the free metal, alloyed...

 catalyst [Rh₂(OPiv)₄].

A direct metal carbene dimerization has been used in the synthesis of novel Polyalkynylethenes