Other Thermal Benzannulations

A complementary access to extended indole and carbazole systems is based on the thermal intramolecular benzannulation of ortfio-al kyn ylan i 1 i nocarbene complexes or on a photo-induced benzannulation of phenylpyrrolylcarbene complexes. The first example involves an intramolecular access to the carbazole skeleton. Refluxing a solution of orflio-alkyn yl phen yl amino carbene complex 109 in acetonitrile gave a 63 % yield of benzocarbazole 110. Less strongly coordinating solvents (TBME, THF, or di-n-butyl ether) or other substituents less bulky than 2,4,6-trimethylphenyl (for example, phenyl or 4-methylphenyl) led to a considerably reduced yield (Scheme 39) [83]. 

The benzaimulation is typically performed under mild thermal conditions in the temperature range of 45 to 60°C and gives yields up to 92%. Alternative reaction procedures include a photoinduced decarbonylation as the initial step which allows benzannulations under low-temperature conditions. However, because of the lower selectivity due to the presence of other photosensitive intermediates, photoinduced benzannulations gained synthetic importance only in cases where the thermal protocol fails. [51 ] In a few examples the benzannulation was promoted by high intensity ultrasound or was carried out under dry state absorption conditions. [52] Although these alternative techniques may lead to... 

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