Cathodic reduction of N-(2-iodophenyl)-N-alkylcinnamides: a novel sequential electrochemical radical cyclisation and hydroxylation

In recent years, intramolecular aryl radical cyclisation has emerged as a useful route for the synthesis of benzannulated heterocycles and carbocycles. The aryl radicals are generated in situ from aryl halides (iodides or bromides) with tributylstannyl hydride-AIBN, SmI2, Co(I) or under photochemical conditions. The present work envisages the generation of aryl radicals by cathodic reduction of the carbon-iodine bond of N-(2-iodophenyl)-N-alkylcinnamides and their intramolecular cyclisation. The cathodic reduction of N-(2-iodophenyl)-N-alkylcinnamides under deaerated conditions in DMF gave 1-alkyl-3-benzylindolin-2-ones regioselectively and in the presence of oxygen yielded surprisingly 1-alkyl-3-hydroxy-3-benzylindolin-2-ones. Both these products were formed by a 5-exo-trig process in good yields. A mechanism for the formation of the products has been proposed through the use of cyclic voltammetry, coulometry and controlled-potential electrolysis as well as deuterium labelling.