Attempted reduction of N-formyl-3-methylindole (1a) and 2,3-dimethyl-N-formylindole (1b) with complex metal hydrides leads to skatole and 2,3-dimethylindole, respectively. Diborane reduction of indole derivatives has furnished interesting results.1,2 Further studies with this reagent are now reported. Diborane reduction of 1a gives 1,3-dimethylindole (4a, 49.4%) and 1,3-dimethyl-3-(3′-methylindolyl-1′-methyl)indoline (6a, 35.8%). Similarly, reduction of 1b with diorane affords 1,2,3-trimethylindole (4b, 52.4%) and two diastereoisomeric 1,2,3-trimethyl-3- (2′,3′-dimethylindolyl-1′-methyl)indolines (6b, 36.7%, and 6c, 6.8%). This appears to be the first report on the successful reduction of an N-acylindole to the corresponding N-alkyl derivative. The formation of the indolylmethylindolines (6) in the diborane reduction of N-formylindoles (1) implies that electrophilic substitution takes place primarily at the 3 position of 3-substituted indoles. These results are discussed in the light of the mechanisms of diborane reduction and electrophilic substitution in 3-substituted indoles. © 1975, American Chemical Society. All rights reserved.