Iridium- and ruthenium-catalysed synthesis of 2,3-disubstituted indoles from anilines and vicinal diols

A straightforward and atom-economical method is described for the synthesis of 2,3-disubstituted indoles. Anilines and 1,2-diols are condensed under neat conditions with catalytic amounts of either [Cp*IrCl2](2)/MsOH or RuCl3 center dot xH(2)O/phosphine (phosphine = PPh3 or xantphos). The reaction does not require any stoichiometric additives and only produces water and dihydrogen as byproducts.

Anilines containing methyl, methoxy, chloro and fluoro substituents can participate in the cyclocondensation. Meta-substituted anilines give good regioselectivity for 6-substituted indoles, while unsymmetrical diols afford excellent regioselectivity for the indole isomer with an aryl or large alkyl group in the 2-position.

The mechanism for the cyclocondensation presumably involves initial formation of the alpha-hydroxyketone from the diol. The ketone subsequently reacts with aniline to generate the alpha-hydroxyimine which rearranges to the corresponding alpha-aminoketone. Acid-or metal-catalysed electrophilic ring-closure with the release of water then furnishes the indole product.