Unusual Regio‐ and Chemoselectivity in Oxidation of Pyrroles and Indoles Enabled by a Thianthrenium Salt Intermediate

Abstract

A dearomative oxidation of pyrroles to Δ3‐pyrrol‐2‐ones is described, which employs a sulfoxide as oxidant, in conjunction with a carboxylic acid anhydride and a Brønsted acid additive. 3‐Substituted pyrroles undergo regioselective oxidation to give the product isomer in which oxygen has been introduced at the more sterically hindered position. This counterintuitive regioselectivity is rationalized by a proposed mechanism that proceeds by initial thianthrenium introduction at the less‐hindered pyrrole α‐position, followed by distal attack of an oxygen nucleophile and subsequent elimination of thianthrene. Furthermore, the same reaction conditions are also able to effect a chemoselective oxidation of indoles to indolin‐3‐ones and additionally of indolin‐3‐ones to 2‐hydroxyindolin‐3‐ones. Here again, the observed regio‐ and chemoselectivities are rationalized through the intermediacy of a thianthrenium salt.