Abstract
Copper(II) nitrate supported on faujasite zeolites such as H-NaX is employed as solid acid catalysts for the clean and less hazardous catalytic oxidative cleavage of C=N bond under mild conditions. The reactions proceed very smoothly, and the yields are excellent.
1. Introduction
Increasing awareness of environmental hazards forces chemists to look for “eco-friendly” reaction conditions. In this connection, the use of heterogeneous catalysts involving solid reagents supported on high surface area materials, obtained by introduction of the reagent onto or into an organic polymeric or an inorganic porous or layered support material meets the fundamental challenges in the protection of the environment. These supported reagents have advantages such as easy handling, good dispersion of active sites leading to improved reactivity, safer and milder reaction conditions and minimal pollution [1, 2]. Oximes, hydrazones, and semicarbazones are useful as preferred derivatives for the identification and characterization of carbonyl compounds [3]. Several reagents have been reported for the regeneration of the carbonyl groups from the mentioned derivatives [4–9]. Although some of the methods involve mild reaction conditions, most of them require strong acidic media, long reaction time, a strong oxidizing agent (which causes over oxidation), and expensive and not readily available reagents.
Anhydrous metallic nitrates with a bidentate covalent coordination and with the available lower intermediate oxidation state for the metal, find maximum reactivity, and wider applicability [10, 11]. Their use, however, in organic syntheses is limited by solubility problems.
Laszlo et al. have used the K10-montmorillonite clay-supported copper(II) nitrate (claycop) and iron(III) nitrate (clayfen) for many of the organic reactions like oxidation of alcohols, oxidative coupling of thiols, hydrolytic cleavage of imine derivatives of carbonyl compounds, cleavage of tosylhydrazones, phenyl hydrazones, 2,4-dinitrophenylhydrazones and semicarbazones [12]. K10-montmorillonite supported Thallium nitrate is used for the oxidative rearrangement of alkyl aryl ketones into alkyl aryl carboxylates [13]. Pyridinium chlorochromate [14], potassium dichromate [15] and sodium metaperiodate [16] are some alumina-supported oxidants used for the chemoselective oxidation of alcohols and sulfides. In this paper we report a general method for the oxidative cleavage of C=N to their carbonyl derivatives in excellent yields without using any microwave or ultrasonic irradiation. To the best of our knowledge, this is the first report using a zeolite-supported cupric nitrate as the reagent for the regeneration of carbonyl group from oximes, hydrazones and semicarbazones. Moreover, copper-salts are inexpensive, easy to handle and environmentally friendly.
2. Experimental
2.1. Preparation of Zeolite H-Nax Supported Copper(II) Nitrate
To a solution of (0.483 g, 2 mmoL) in acetone (15 mL), activated zeolite H-NaX (1 g) (obtained by activating -NaX form of zeolite which has been prepared by partial exchange of sodium with ammonium salt in commercially available NaX) was added at once with stirring over a magnetic stirrer for 2 hours. Then the solvent was removed in a rotary evaporator. The blue powder formed was dried further at 130°C under reduced pressure. 1 g of H-NaX zeolite supported copper(II) nitrate reagent contains about 0.326 g of (1.35 mmoL).
2.2. General Procedure for the Oxidative Cleavage of C=N
0.05 g of substrate was ground with 0.25 g of activated supported reagent using mortar and pestle and refluxed using 10 mL of dichloromethane as solvent for specified time. The reaction mass was cooled to room temperature, filtered the catalyst and washed with dichloromethane twice. The filtrate was washed with distilled water thrice. After drying over anhydrous sodium sulphate, the solvent was evaporated to give the product. Percentage conversion of deprotection was checked by GC analysis.
3. Results and Discussions
The results illustrated in Table 1 indicate that the reaction is successful for a variety of aliphatic and aromatic oximes, phenylhydrazones, p-nitrophenylhydrazones and semicarbazones (Scheme 1). All these carbonyl derivatives were converted back to their corresponding aldehydes and ketones in dichloromethane as the optimal solvent, among the tested solvents including: methanol, ethanol, and acetonitrile taking benzaldehyde oxime as a representative example where the yields are found to be 79%, 81%, 72%, respectively. The reaction is found to be general with compounds having variety of functional groups like chloro, nitro, phenolic hydroxyl, and methoxy groups.
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As evident from the results, aldehyde derivatives were generally deprotected relatively faster than keto derivatives. It was also interesting to note that by controlling the amounts of the reagent, it was possible to avoid further oxidation of the liberated aldehydes to the corresponding carboxylic acids (entries 1–6), while we have demonstrated a facile aromatic nitration reaction with cupric nitrate in the presence of solid support [17], we did not observe any nitration of aromatic substrates during the cleavage reaction when we used optimum ratio of substrates and oxidants. Prolonged reaction time as well as excess of supported reagent in the case of aldehydes leads to further oxidation to the respective acids. The reaction failed to produce the ketones without cupric nitrate or H-NaX zeolite.
The superiority of H-NaX supported cupric nitrate as catalyst has been proved by the inefficiency in the regeneration of carbonyl groups from benzaldehyde semicarbazones by simple NaX supported or with unsupported , revealing the involvement of acidic sites present in the solid support. The optimum ratio of substrate to oxidant () was determined for complete conversion of oximes, semicarbazones and phenyl hydrazones to the corresponding carbonyl compounds. The recovered catalyst was verified for three times to catalyze the deprotection of benzaldehyde oxime. The efficiency of catalyst decreases considerably during the successive reusability tests.
Regarding the mechanism of the deprotection, it is proposed that the diffusion of the cupric ions into the zeolite H-NaX surface lattices and the subsequent formation of finally lead to the generation of and ions. The presence of these ions has been confirmed by comparison of IR spectra of and -loaded zeolite, which shows strong peaks around – due to symmetric stretching of nitrate ion, due to asymmetric stretching of nitrate ion, due to in-plane deformation of nitrate ion and due to out of-plane deformation of nitrate ion, and peak around – due to . The nitrosonium ion may act as an electrophile, giving the corresponding carbonyl compounds. Based on the observations, the proposed mechanism for cleavage of the carbon–nitrogen double bond in oximes is presented in Scheme 2.
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4. Conclusions
In conclusion, from commercially available NaX zeolite, a facile heterogeneous catalytic method, involving the more acidic form, namely, H-NaX with cupric nitrate has been employed for oxidative cleavage of C=N for the regeneration of carbonyl compound. It will be obvious that advantages of heterogeneous catalysis in terms of easy separation; and consistent yields are noteworthy. The operational simplicity, selectivity and cheapness, and good yields in very short times make this procedure a useful, attractive alternative to previously available methods.
Acknowledgment
K. Sivakumar thanks Jawaharlal Nehru Memorial Fund (JNMF) for the award of fellowship.