Facile Oxidation of Aldehydes to Carboxylic Acids with Chromium(V) Reagents

Recently we reported a convenient method of oxidation of alcohols to carbonyl compounds using chromium(V) reagents.¹ Although a variety of reagents are available for effecting this transformation, there are only a few reagents which have been successfully used for the oxidation of aldehydes to carboxyllc adds. Chromic acid, silver oxide and potassium permanganate are commonly employed for this purpose and reactions are performed in protic media under conditions which are not that mild.² The “non-aqueous” chromium(VI) reagent, pyridinium dichromate, recently reported by Corey³ oxidises alcohols and aldehydes to carboxylic acids in DMF at room temperature. Although Cr(V) species is postulated as an intermediate in all oxidations with Cr(VI), no systematic oxidation studies have been reported with these reagents. This note reports the results of some fruitful investigations on aldehyde → carboxylic acid conversion involving some “non-aqueous” chromium (V) complexes 1, 2, 3 and 5 under anhydrous conditions.