DIBORANE AS A REDUCING AGENT - VI. THE NOVEL REDUCTION OF INDOLE-1-CARBOXALDEHYDES TO 1-METHYL-INDOLES, DI(INDOLYLMETHYL)ETHERS AND INDOLYLMETHYL INDOLINES

Article abstract of DOI:10.1016/S0040-4020(01)98809-X

Reduction of the indole-1-carboxaldehydes (1a-1f) with borane/THF gives the 1-methylindoles (4) in 42-91 percent yields together with the di(indolylmethyl)ethers (8), the indolylmethyl indolines (7), the unsymmetric ether (10) and the indolenine (11) as the minor products, except 7a.This appears to be the first report on the formation of symmetric ethers in the borane/THF reduction of an oxygen function.The formation of 7a and 7b from 1a and 1b implies that electrophilic substitution takes place primarily at position 3 of 3-substituted indoles. 1c-1f did not form the corresponding 7 probably because of steric hindrance.These results are discussed in relation to the mechanisms of borane/THF reduction, origin of the different products and electrophilic substitution in 3-substituted indoles.