Electrochemical oxidation of 3-substituted indoles.

2-Oxindoles are an abundant heteroaromatic motif in natural products and pharmaceuticals. An appealing method for accessing 2-oxindoles is by oxidation of the corresponding indole, a transformation currently executed using stoichiometric quantities of unsafe chemical oxidants that can also form unwanted side-products. Herein, we report that 3-substituted indoles undergo a logistically straightforward, electrochemical oxidation to the corresponding 2-oxindole in the presence of potassium bromide (>20 examples), with only traces of the oxidative dimer detected. Cyclic voltammetry and control studies infer that the reaction proceeds by electrochemical generation of elemental bromine (Br) that upon reaction with indole, followed by hydrolysis, delivers the 2-oxindole. This procedure is an appealing alternative to existing methods used to access 2-oxindoles by oxidation of the parent indole.