Insertion reactions carbazoles synthesis

The la type classification also applies to a group of reactions attributable to nitrene or related (nitrenoid) intermediates. Although not as prominent as carbonyl condensation reactions, these reactions are especially useful for carbazole synthesis. These are formally insertion reactions but mechanistic scrutiny frequently reveals that the reactions are more complicated than a one step insertion process (Scheme 29). 

Formal insertions into aromatic C-H bonds often proceed by another mechanism. This is also true in intramolecular cases, and the formation of carbazole from biphen-2-ylnitrene, a weU-studied example, probably occurs by an electrocycHc reaction followed by 1,5-hydrogen shift The high yielding synthesis of indoles from azidocinnamates proceeds similarly by cycHzation of a vinyl nitrene (Scheme 6.32). 

In 2013, Takemoto and co-workers developed a palladium-catalyzed isocyanide insertion and alkyne functionalization for the synthesis of 2-arylindole derivatives, including tetracyclic carbazoles (Scheme 2.105). In this process, the formation of two C-C bonds via isocyanide insertion was achieved, and isocyanide was effectively used by incorporating both the C and N atoms as components of the indole skeleton. Regarding the reaction mechanism, the reaction started with the oxidative addition of Pd(0) to aryl... 

The photolysis of 2-azidobiphenyl 64 or similar aryl nitrene precursors yields carbazole 66 by insertion of the singlet nitrene 65 into the ortho C-H bond of the neighboring benzene ring. This efficient reaction may be extended to the synthesis of other heterocycles. 

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