Catalytic application of transition metal substituted Keggin type polyoxometalates in the oximation of aldehydes and ketones

Polyoxometalates (POMs) have received much attention owing to their enormous structural versatility and their present and potential applications. These superior properties allow the POMs-based catalysts to be designed at the atomic and molecular levels. Among a wide variety of heteropolyoxometalates, Keggin structures are the most stable and more easily available [1]. The modification of parent POMs are likely to help in development of new generation catalysts with enhanced properties of acidity, redoxpotential and stability. This can be basically done by incorporation of transition metal ions into the lacuna in parent POM structures (i.e., transition metal substitutedpolyoxometalates, TMSPOMs) [2]. Oxime is a chemical compound belonging to the imines, with the general formula R1R2C=NOH, where R1 is an organic side-chain and R2 may be hydrogen, forming an aldoxime, or another group, forming a ketoxime. Oximes have been widely used for the prepration of nitro, nitriles, amides and nitrones compounds. Also they present many properties such as antimicrobial, antioxidant, anti-depressive and antiviral agent. The usual method for preparation of oximes involves treatment of carbonyl compounds with hydroxylamine hydrochloride [3]. In this study, transition metal substituted Keggin type POMs (TMPOMs) with some of the first row transition metals, as tetrabutylammonium salt (TBA)x[PW11MO39].nH2O (M=Cr, Mn, Fe, Co, Ni, Cu, Zn) have been used in oximation of various aldehydes and ketones in the presence of hydroxylamine hydrochloride at room temperature. The reaction conditions such as kind and amount of TMSPOMs catalyst, amount of hydroxylamine hydrochloride, and type of solvent was initially optimized. Our results showed that among different TMSPOMs, (TBA)4[PW11FeO39].nH2O and (TBA)5[PW11ZnO39].nH2O have the best activity in oximation of aldehydes and ketones in ethanol. High conversion, short reaction times, room temperature and stability of the catalysts show a new synthetic application for TMPOMs in organic synthesis.