Carbene dimerization is a type of organic reaction in which two carbene or carbenoid precursors react in a formal dimerization to an alkene. 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.Grundmann, C. (1938), Über die Zersetzung der Diazoketone. Justus Liebigs Annalen der Chemie, 536: 29–36. In the domain of persistent carbenes the Wanzlick equilibrium 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 is converted to diethyl maleate using the ruthenium catalyst chloro(cyclopentadienyl)bis(triphenylphosphine)ruthenium:Highly stereoselective formation of cis-enediones from α-diazo carbonyl compounds catalysed by [RuCl(η5-C5H5)(PPh3)2] Walter Baratta and Alessandro Del Zotto Chem. Commun., 1997, 2163-2164
Grubbs' catalyst is also effective Maleates from diazoacetates and dilactones from head-to-head dimerisation of alkenyl diazoacetates using Grubbs' 2nd-generation ruthenium carbene catalyst David M. Hodgson and Deepshikha Angrish Chem. Commun., 2005, 4902-4904 In this reaction type the active intermediate is a transition metal carbene complex. A diazo cross-coupling reaction has also been reported between ethyl diazoacetate and methyl phenyldiazoacetate using the rhodium catalyst [Rh2(OPiv)4].
A direct metal carbene dimerization has been used in the synthesis of novel Polyalkynylethenes Barluenga, J., de Sáa, D., Gómez, A., Ballesteros, A., Santamaría, J., de Prado, A., Tomás, M. and Suárez-Sobrino, A. (2008), Metal Carbene Dimerization: Versatile Approach to Polyalkynylethenes. Angewandte Chemie International Edition, 47: 6225–6228.
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