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DOI: 10.1055/sos-SD-026-00532

Landais, Y.; Vincent, J. M.Science of Synthesis, (200526650.

Cobalt(II) chloride catalyzed coupling of acetic anhydride in acetonitrile with aliphatic aldehydes 13 provides a mixture of symmetrical and unsymmetrical 1,2-diketones 14 and 15 (Scheme 5).[‌17‌] Although the symmetrical 1,2-diketone 14 is obtained in modest yield (after distillation), this method can be useful as it is simple and involves readily available and inexpensive starting materials. A small amount of carboxylic acid resulting from the oxidation of the aldehyde is also observed. Under the same conditions, aryl aldehydes are converted into the corresponding diacyl acetals. The nature of the solvent has a profound influence on the outcome of the process. When the reaction is performed in acetonitrile under an oxygen atmosphere, 1,2-diketones are obtained in fair yield, however, the reaction in dimethylformamide affords only carboxylic acids. It is also noteworthy that in the absence of acetic anhydride, cobalt(II) chloride in acetonitrile and oxygen converts the aliphatic aldehydes into the corresponding anhydrides in moderate yields. Aryl-substituted 1,2-diketones can be obtained by coupling the corresponding aldehydes in the presence of thiamine (vitamin B1) and sodium cyanide.[‌18‌] Although this method, which is known as the benzoin condensation, generally provides α-hydroxy ketones (acyloins), thiamine also catalyzes the oxidation of acyloins into 1,2-diketones. Long reaction times are needed and moderate to good yields of symmetrical diaryl-substituted 1,2-diketones are obtained using this method.[‌18‌]

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