A New Insight into the Oxidation of Cyclododecane with Hydrogen Peroxide in the Presence of Iron-Substituted Polyoxotungstates

Abstract

The catalytic homogeneous oxidation of cyclododecane with hydrogen peroxide in the presence of tetrabutylammonium salts of iron-substituted Keggin-type polyoxotungstates of general formula (TBA) 4 H z XW 11 Fe(H 2 O)O 39 ·nH 2 O (where X = P, Si, B, and z = 0-2) is described. In the reaction conditions reported, the corresponding alcohol, ketone, and hydroperoxide are obtained as the main reaction products. The catalytic activity of the anions with phosphorous, silicon, and boron is compared in different reaction conditions. These catalytic oxidation reactions seem to be radical processes, since they are totally inhibited in the presence of I 2 , a well-known radical scavenger.Selective oxidation of saturated hydrocarbons is one of the most challenging subjects in oxidation chemistry and many catalytic systems for these transformations have been extensively investigated over the last decades. 1-5 Among others, some systems have been described in which the well-known transition-metal-monosubstituted Keggin-type polyoxometalates, 6 like a-[XW 11 M(H 2 O)O 39 ] n-(X = P, Si, M = first-row transitionmetal ion), have been employed as catalysts or catalyst precursors for several types of alkane oxidative reactions, using different oxidants (e.g. PhIO, O 2 , TBHP) and diverse reaction conditions. 7-10 Alcohols and ketones have been the prominent reaction products obtained in these studies.The use of aqueous H 2 O 2 in the oxidation of organic substrates is very attractive from the point of view of synthetic organic chemistry, since aqueous H 2 O 2 is considered an inexpensive, environmentally clean and easy to handle reagent. 11,12 To our knowledge, the first report concerning the oxidation of alkanes with H 2 O 2 in the presence of a transition-metal-substituted polyoxotungstate was published by Mizuno et al., 13 who used {g-SiW 10 [Fe(OH 2 )] 2 O 38 } 6-in the catalytic oxidation of cyclohexane, n-hexane, n-pentane, and adamantane. Our studies on cyclohexane and cyclooctane oxidation with hydrogen peroxide in acetonitrile have shown that considerable amounts of the corresponding alkyl hydroperoxides could be formed in the presence of several iron(III)-substituted heteropolytungstates. 14-19 The formation of alkyl hydroperoxides may be explored in several ways, as they may be selectively decomposed to ketone(aldehyde)-alcohol mixtures 20,21 or reduced to the corresponding alcohols.The oxidation of cyclododecane is important from the industrial point of view as it provides precursors for the synthesis of nylon-12 polymers. 22 This reaction is usually carried out using O 2 and Co(II) acetate in the presence of boric acid until attaining 30-35% conversion, originating 80% of cyclododecanol, 10% of cyclododecanone, and 10% of byproducts. The catalytic dehydrogenation of the cyclododecanol-cyclododecanone mixture at 220 °C originates pure cyclododecanone. 23 Despite its significance, the catalytic oxidation of cyclododecane is much less studied than related processes concerning cyclohexane. Cyclododecane is much less reacti...

Manganese Mono-Substituted Borotungstate: Characterization and Catalytic Application

“…The Keggin anions and their derivatives have been used as catalysts for various types of oxidative reactions, like the epoxidation of alkenes and the hydroxylation of alkanes [3][4][5]. In particular, transition metal mono-substituted Keggin-type heteropolytungstates, [XW 11 M(H 2 O)O 39 ] (n-m)-(X = P, Si, B), in which a transition metal cation, M n+ , is co-ordinated to the binding sites of lacunary heteropolyanion α-[XW 11 O 39 ] n-, have generated great interest as oxidative catalysts [6][7][8][9][10][11][12][13]. Even so, oxidative catalysis in the presence of Keggin-type mono-substituted borotungstates has not been widely studied and only a few reports were found.…”