Selective oxidation of natural products catalyzed by chromium complexes

Abstract

Five chromium complexes (chromium (III) palmitate, chromium (III) stearate, chromium (III) behenate, chromium (III) acetylacetonate and chromium (III) salen-Cl) were screened as a suitable catalyst for oxidation of model substrates (1-hexanol, cyclohexanol and cyclohexene) to the corresponding carbonyl compounds. The oxidation of cyclohexene and cyclohexanol with tert-butyl hydroperoxide mediated by chromium (III) stearate, afforded the respective cyclohexanone in good yield and high chemoselectivity, while 1-hexanol oxidation failed to produce a carbonyl compound. The study of chemo- and regioselectivity of oxidation of primary alcohols in the presence of secondary ones showed that the major recovered product was ketone. In addition, the oxidation of l-phenylethane-l,2-diol gave a-hydroxy ketone without contamination of either a dicarbonyl compound or ketone acid. The oxidation of various natural products containing hydroxyl group (menthol, 1-phenylethanol, cholestanol and cinnamyl alcohol) was studied under developed conditions over chromium (III) stearate using tert-butyl hydroperoxide gave moderate to excellent yields of carbonyl products. Moreover, the oxidation of natural products containing C=C moiety (α-pinene, α-ionone and β-ionone) provided enone products in fair yield with good selectivity particularly the oxidation of α- and y-terpinenes afforded aromatic products in high yield.