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.