Evaluation of H ₂ O ₂ /HFIP as a convenient sulfoxidation reagent for boronate containing thioethers

It is recognized that boronic acid derivatives are often sensitive to common sulfur oxidizing reagents. In this paper the value of the H 2 O 2 /HFIP combination as a viable thioether (sulfide) monoxidation reagent is demonstrated, with the reagent system providing sulfoxide while in many cases leaving the boronate functionality unperturbed. Bpin bearing sulfides were converted to sulfoxide in 49%–86% yield, with occasional alcohol side products arising from oxidation at the boron-bearing carbon. Other boronate esters possessing chemical resilience comparable to Bpin were also amenable to the chemistry, with sulfoxides isolated in 66%–96% yield. Less robust boronic acid derivatives such as Bcat and BDan readily succumbed to alcohol conversion. Sulfides containing geminal bis(boronates) were cleanly converted to sulfoxide. The chemical yields and the convenience of this sulfoxidation procedure make this protocol attractive for the oxidation of selected boronate containing thioethers. Dedicated to the memory of Willie Leigh, a friend and mentor.