M. Chakrabarty et al., Neat formic acid: An excellent N-formylating agent for carbazoles, 3-alkylindoles, diphenylamine and moderately weak nucleophilic anilines, SYN COMMUN, 30(2), 2000, pp. 187-200
Neat formic acid alone efficiently N-formylates carbazoles, 3-alkylindoles,
diphenylamine and even moderately weak nucleophilic anilines to furnish th
e corresponding N-formyl derivatives in 72-87% yields.