The reaction of indoles with diborane

Reaction of excess diborane with N-unsubstituted indoles is postulated to yield a stable Nind-boron adduct. Further reaction of this adduct depends on the conditions employed. If the excess hydride is quenched with acetone in the absence of a proton source, no net reaction occurs and starting indole is recovered. Effective reduction of indoles to indolines under nonacidic conditions occurs, however, if the initial nitrogen-boron adduct is treated with sodium methoxide-methanol. Indole, and mono-, di-, and aromatic ring substituted derivatives containing an Nind-H are reduced in good yield; N-methylindoles are not reduced. When methanol-O-d is used as a proton source, specific incorporation of one carbon-bound deuterium atom at C3 of the indoline occurs. Reduction of 2,3-dimethylindole yields a 47:53 mixture of cis- and trans-2,3-dimethylindoline while reduction of tetrahydrocarbazoles gives only the corresponding cis-hexahydrocarbazoles. A mechanism consistent with these data is proposed. Indole is excluded as an intermediate in the diborane reduction of oxindole to indoline.