ZnBr2-mediated synthesis of indoles in a ball mill by intramolecular hydroamination of 2-alkynylanilines

A method for the intermolecular hydroamination of 2-alkynylanilines in a planetary ball mill is described, which avoids the use of solvents during the reaction. Different additives were tested, whereby ZnBr2 in stoichiometric amounts showed the best performance in terms of yield and selectivity. Furthermore, the materials used for milling balls and beakers as well as the milling time and the rotation frequency were changed in order to achieve the best reaction conditions. As a result, two methods for the synthesis of 2-substituted 1H-indoles are described with their respective advantages and disadvantages. Both methods were used to convert different alkyl and aryl containing 2-ethynylanilines.