Carbene dimerization explained

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.^[1] 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:^[2]

Grubbs' catalyst is also effective ^[3] 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].^[4]

A direct metal carbene dimerization has been used in the synthesis of novel Polyalkynylethenes ^[5]

Notes and References

1. Grundmann, C. (1938), Über die Zersetzung der Diazoketone. Justus Liebigs Annalen der Chemie, 536: 29–36.
2. 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
3. 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
4. Hansen, J. H., Parr, B. T., Pelphrey, P., Jin, Q., Autschbach, J. and Davies, H. M. L., Rhodium(II)-Catalyzed Cross-Coupling of Diazo Compounds. Angewandte Chemie International Edition, n/a.
5. 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.