Phosphonium ion tagged chiral phosphoric acids and their application in Friedel-Crafts reactions of indoles

The attachment of phosphonium ion phase tags to chiral binapthyl-based phosphoric acid catalysts, and the use of these materials in a range of organocatalytic asymmetric Friedel-Crafts reactions of indoles has been studied. Placement of the tags at the 3 and 3′ positions of the phosphoric acid, so that they could serve as steric blocking groups, failed to produce an active catalyst. However, moving the phosphonium ion groups to the 6 and 6′ positions produced an efficient and enantioselective catalyst. Aided by the presence of the phase tags, the chiral catalyst was easily removed at the end of the reactions, and could be reused several times, albeit with somewhat decreased efficiency and enantioselectivity.

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