Metal‐Free Direct Esterification of Aldehydes Using Hydroxylamine‐ <i>O</i> ‐Sulfonic Acid

Abstract Ester synthesis has traditionally depended on expensive metal catalysts and harsh reaction conditions. Despite the widespread use of these methods, challenges remain in the efficient synthesis of substituted esters. Here, we present a new metal‐free esterification method, in which hydroxylamine‐ O ‐sulfonic acid (HOSA) reacts with aldehydes in the presence of alcohols to produce alkyl alkanoates or benzoates. The reaction proceeds via in situ generation of nitrile and imidate intermediates. This method affords esters of aliphatic, aromatic, polycyclic, and heteroaromatic aldehydes in good to excellent yields and tolerates a variety of functional groups