Studies on [PtCl2 ]- or [AuCl]-catalyzed cyclization of 1-(Indol-2-yl)-2,3-allenols: the effects of water/steric hindrance and 1,2-migration selectivity.

The [PtCl2 ]- or [AuCl]-catalyzed reaction of 1-(indol-2-yl)-2,3-allenols occurred smoothly at room temperature to afford a series of poly-substituted carbazoles efficiently. Compared with the [PtCl2 ]-catalyzed process, the [AuCl]-catalyzed reaction represents a significant advance in terms of the scope and the selectivity. Selective 1,2-alkyl or aryl migration of the gold carbene intermediate was observed: compared with the methyl group, the isopropyl, cyclopropyl, cyclobutyl, and cyclohexyl groups migrate exclusively; the cyclopropyl group shifts selectively over the ethyl group; the 1,2-migration of a non-methyl linear alkyl is faster than methyl group; the phenyl group migrates exclusively over methyl or ethyl group. DFT calculations show that water makes the elimination of H2 O facile requiring a much lower energy and validates the migratory preferences of different alkyl or phenyl groups observed.