I2-mediated C3-formylation of indoles by tertiary amine and H2O

An I2-promoted 3-formylation of free (N-H) and N-substituted indoles with tetramethylethylenediamine (TMEDA) and H2O as the carbonyl source is achieved, providing 3-formylindole in moderate to excellent yields with good functional group tolerance. This procedure represents an exceedingly attractive alternative to the traditional formylation methods.