COBALT(II)-CATALYZED REACTION OF ALDEHYDES WITH ACETIC-ANHYDRIDE UNDER AN OXYGEN ATMOSPHERE - SCOPE AND MECHANISM

The reaction of aldehydes with acetic anhydride in the presence of catalytic cobalt(II) chloride under an oxygen atmosphere at ambient temperature is dependent upon the reaction medium. Aliphatic aldehydes react in acetonitrile to give 1,2-diones whereas the aromatic aldehydes are acylated to yield the corresponding acylals. On the other hand, carboxylic acids are obtained from aliphatic and aromatic aldehydes by conducting the reaction in dichloroethane or benzene. Cobalt(II) chloride in acetonitrile catalyzes the conversion of aliphatic aldehydes to the corresponding anhydrides in the absence of acetic anhydride whereas aromatic aldehydes remain largely unaffected under these conditions. A preliminary mechanistic study in three different solvents (i.e. acetonitrile, dichloroethane, and DMF) has revealed that in acetonitrile and in the presence of acetic anhydride, aliphatic aldehydes behave differently than aromatic aldehydes. Some trapping experiments using methyl acrylate and stilbene have been conducted to demonstrate the occurrence of an acyl cobalt and peroxyacyl cobalt intermediate during these reactions.