Reaction of electron rich heterocycles with electrophiles. Formylation of indoles via thiomethylation

The formylation of indoles 1 by dithiocarbenium‐ions 2 is discussed. The reaction proceeds to thioacetals 4 under mild conditions. Longer reaction times yield bisindolylmonomethylcyanines 5–6. Generally, the reaction is inhibited by the presence of electron‐withdrawing groups at the pyrrol nucleus of indole 1, e.g. an acetyl group.