Carboxylic acids, reaction with diarylamines

The cyclodehydrohalogenation of 2-halo-Ar,AT-diarylamines is analogous to the classical Heck reaction [114-116] and represents a palladium(0)-catalyzed process (Scheme 28). Cyclization of the diarylamine 78 with a palladium(O) catalyst, generated in situ by reduction of palladium(II) with triethylamine, affords carbazole-1-carboxylic acid 79 in 73% yield [122]. 

Ring closure y to a heteroatom is also a rather uncommon [5 + 1] procedure although there are some important exceptions. The most widely investigated is the Bernthsen acridine synthesis in which a diarylamine is condensed with a carboxylic acid in the presence of a Lewis acid (equation 73). More recently, it has been shown that acylanilines react with the Vilsmeier-Haack reagent to give quinolines in good yield (e.g. equation 74) and the mechanism of the reaction has been elucidated. A final example of [5 +1] ring closure y to a heteroatom which is of occasional use is the pyrazine synthesis outlined in equation (75). 

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