The rearrangement of N-(2-bromoallyl)-arylamines to 2-methylindoles

Abstract

N-(2-Bromoallyl)-arylamines cyclise readily to substituted 2-methylindoles when heated with methanolic or aqueous boron trifluoride. Methyl, hydroxy-, and halogeno-substituted 2-methylindoles can be prepared in very good yield by this procedure. The reaction was shown to be intramolecular and two possible mechanisms are discussed : (i) a Claisen rearrangement and (ii) a mechanism involving a concerted bromine elimination and 1,2-nitrogen shift to give an allylic carbonium ion, followed by ring closure. Experiments with carbon-14 labelled N-2-bromoallylaniline gave an isotope distribution in the product midway between that expected for either of these mechanisms.