Oxidation of Aromatic Aldehydes to Esters: A Sulfate Radical Redox System

A mild oxidative esterification of various aromatic aldehydes by sulfate radical redox system was presented. In the reaction pathway exploration, the transiency of MeOSO₃^- was disclosed, which was generated from esterification between the in situ generated HSO₄^- and MeOH, a rate-limiting step in the process. More importantly, the selectivity-controlling step was represented by the subsequent nucleophilic displacement between MeOSO₃^- and aldehydes. The ionic oxidant 1a ((NH₄)₂S₂O₈) with more N-H numbers in the cation, as compared with 1c ((n-Bu₄N)₂S₂O₈) and 1d ((PyH)₂S₂O₈), has better performance in the oxidative esterification of aldehydes.