New substitution reactions on indole promoted by the Cr(CO)₃ unit

Indole derivatives form complexes with the Cr(CO)₃ unit at the 6-membered ring. Addition of reactive carbon nucleophiles occurs at the coordinated ring with strong preference for C-4 and, in a few cases, for C-7. Oxidation of the intermediate cyclohexadienyl anionic complexes produces substituted indoles in good yield. Complexes of N-methylindoline and benzofuran also undergo the addition/oxidation sequence preferentially at C-4. Changes in the substituents at C-3 and the nitrogen atom in the indole complexes influences the regioselectivity so that either C-4 or C-7 substitution can be observed. The selectivity is correlated with EHT calculations on free indole and complexes indole.