The bromination and chlorination of 2,3-dialkylindoles. Isolation of 3-bromo- and 3-chloro-2,3-dialkylindolenines and acid catalyzed conversion to 3-methoxyindolenines

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Abstract
The bromination of 2,3-dimethylindole in acetic acid followed by hydrolysis has been shown to yield the known dimer 16 of 3-hydroxy-2,3-dimethylindolenine 15 and not 3-hydroxymethyl-2-methylindole 12 as reported previously by others. Bromination of 2,3-dimethylindole in the presence of triethylamine has yielded 3-bromo-2,3-dimethylindolenine 17 as a crystalline solid which is stable at room temperature in a nitrogen atmosphere in the presence of triethylamine but decomposes vigorously in the absence of base. Under mildly acidic conditions 17 reacts rapidly with methanol to yield 3-methoxy-2,3-dimethylindolenine 3 in good yield. With tert-butyl hypochlorite in the presence of triethylamine, 2,3-dimethylindole gives 3-chloro-2,3-dimethylindolenine 2 which has chemical properties similar to 17 reacting readily with methanol to give 3. Similarly tetrahydrocarbazole was converted to the analogous chloroindolenine 5 and bromoindolenine 18 both of which readily underwent methanolysis to give 6 in good yield. A mechanism for the acid catalyzed methanolysis of 3-chloro- and 3-bromoindolenines is proposed.

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