Migratory insertion is important elementary step in organometallic chemistry. In particular, the insertions of alkene or CO into metal-carbon bond are fundamental steps in the catalytic cycle of transition metal-catalyzed C-C bond forming reactions. The insertion of an alkene into palladium-carbon bond, which introduces a two-carbon unit, is the key step in Heck-Mizoroki reaction. On the other hand, the insertion of CO, which is a one-carbon unit insertion, is the base for transition metal-catalyzed carbonylation. However, the similar one-carbon unit insertion of carbene has received much less attention until very recently [1]. In this presentation, several novel cross coupling processes based on migratory insertion of palladium- or copper-carbene are discussed. These coupling reactions demonstrate new possibilities of migratory insertion process in the development of novel transition metal-catalyzed C-C bond forming reactions.
All the transformations so far developed in this area can be summarized. The organometallic species can be generated by oxidative addition or transmetallation. Then the reaction of organometallic species with diazo substrate generates metal carbene intermediate, which is followed by migratory insertion to form carbon carbon bond. Finally, the resulted organometallic species undergoes b–hydride elimination or protonation to complete the catalytic cycle. In essence, these transformations can be considered as the insertion of a metal carbene process into the catalytic cycle of classic cross-coupling reaction[2].
It is particularlly notable that tosylhydrazones can be used as corss coupling partner in this type of transformations [3]. This makes these coupling reactions potentially useful in organic synthesis, because tosylhydrazones can be easily prepared from the corresponding ketones or aldehydes. Thus, these transformations constitutes useful methods to convert a carbonyl functionality to carbon carbon single bond or double bond.
References
[1] (a) N. M. G. Franssen, A. J. C. Walters, J. N. H. Reek, B. de Bruin, Catal. Sci. Technol. 2011, 1, 153-165. (b) E. Jellema, A. L. Jongerius, J. N. H. Reek, B. de Bruin, Chem. Soc. Rev. 2010, 39, 1706-1723.
[2] Y. Zhang, J. Wang, Eur. J. Org. Chem. 2011, 1015-1026.
[3] (a) J. Barluenga, C. Valdés, Angew. Chem. Int. Ed. 2011, 50, 7486-7500. (b) Z. Shao, H. Zhang, Chem. Soc. Rev. 2012, 41, 560-572. (c) Q. Xiao, Y. Zhang, J. Wang, Acc. Chem. Res. 2013, 46, 236-247.