Cp*Rh catalyst

In 2008, Fagnou et al. reported that acetanilide couples with alkynes in the presence of a Cp Rh catalyst and a copper salt oxidant through ortho C—H bond cleavage to produce Al-acetylindole derivatives [Eq. (a) in Scheme 25.36] [28a]. In contrast, the authors found that benzanilide, which possesses two types of cleavable ortho C—H bonds on anilino and benzoyl moieties, undergoes annulation accompanied by the selective cleavage of the benzoyl C—H bond to give isoquinolinone derivatives [Eq. b] [20]. 

The nucleophilicity of an organocobalt(III) species was utilized by Hummel and Ellman to achieve condensation of an azobenzene and an aldehyde to afford an A/ -aryl-2//-indazole derivative (Scheme 10.13) [35]. A combination of a cationic Cp Co complex bearing a noncoordinating B(CgF5)4 anion and acetic acid promotes direct addition of the azobenzene to the aldehyde through azo-directed ortho-metalation and subsequent insertion of the C=0 bond into the aryl-Co bond. While these catalytic steps are likely reversible, intramolecular dehydrative condensation of the azo and the alcohol moieties in the aldehyde adduct serves as a driving force for the formation of the indazole product. The reaction shows a substrate scope comparable to that of the same reaction catalyzed by a Cp Rh catalyst [36] and is applicable to both symmetrical and unsymmetrical azobenzene derivatives and to aromatic, heteroaromatic, and aliphatic aldehydes. 

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