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Indian Journal of Chemistry Vol. 44B, July 2005, pp. 1524-1526 Note CrO3/Al2O3: Rapid oxidation of alcohols to carbonyl compounds in a solvent- free system A R Kiasat*, F Kazemi* & K Nourbakhsh Chemistry Department, College of Science, Shahid Chamran University, Ahvaz Iran 6137-4-3169, Iran, e-mail: [email protected] Received 24 March 2004; accepted (revised) 21 December 2004 Primary and secondary alcohols are efficiently oxidized to the corresponding carbonyl compounds using CrO3 supported onto alumina under solvent- free conditions. It has been found that the presence of small amount of t-butanol in the reaction media is essential. Over-oxidation of aldehydes to carboxylic acids and cleavage of carbon–carbon double bond is not observed by this method. IPC: Int.Cl.7 C 07 C 49/00 Oxidation of primary and secondary alcohols to the corresponding carbonyl compounds has attracted considerable attention during recent years1,2. The use of high oxidation state transition metals3 such as chromium (VI) reagents for alcohol oxidation is well known. Since the chromium residues are environmentally hazardous and are potentially dangerous (ignition or explosion) during product isolation and waste disposal,4 it would be advantageous to develop oxidizing methods which use chromium trioxide on some solid supports. CrO3 supported on resin,5 graphite,6 halosilane,7 silica gel,8 aluminum silicate,9 kieselguhr,10 and zirconium (IV) oxide11 have been previously reported as oxidizing agents in the presence of solvents. Recently, application of CrO3 supported on wet alumina,12 wet silica gel3 and zeolite HZSM-513 have been reported as oxidizing agents for oxidation of alcohols to carbonyl compounds under microwave irradiation in solvent-less system. It has been shown that in the absence of inorganic supports the oxidation reactions under microwave irradiation are sluggish and considerable amounts of starting materials are recovered unchanged. Very recently we reported14 that CrO3 in the presence of copper sulphate efficiently oxidized alcohols under free solvent conditions. We report conditions whereby various types of primary and secondary alcohols can be conveniently oxidized to the corresponding carbonyl compounds under mild reaction conditions by CrO3 supported onto Al2O3 in solvent-free conditions. Chromium trioxide supported on alumina was prepared by simply cogrinding alumina with CrO3 in the ratio 10:1 (w/w) in an agate mortar. In this simple and efficient method the alcohols were oxidized to the corresponding carbonyl compounds when the former was ground in a mortar with a pestle in the presence of supported CrO3 on Al2O3 and a few drops of tBuOH (Scheme I). The feasibility of the oxidation was first examined using benzyl alcohol as a model substrate. Thus, benzyl alcohol was thoroughly mixed with 2 equivalents of CrO3/Al2O3 and small amount of t-BuOH in an ambient air environment at room temperature and benzaldehyde was obtained in 81% isolated yield within 7 min. It is noteworthy that the oxidation did not proceed to completion even after prolonged hours of reaction when less than 2 equivalents of the oxidant were used. With the first successful result in hand, oxidation of other alcohols with CrO3/Al2O3 were carried out under same conditions. The results obtained are presented in Table I. As shown in Table I, primary and secondary saturated aliphatic, benzylic, and the heterocyclic alcohols were oxidized to the corresponding carbonyl compounds in good to excellent isolated yields in a very short time (510 min). Over-oxidation of primary benzylic alcohols to carboxylic acids, which is common in the some other reagents, was not observed. One of the interesting features of this reagent was its ability to convert hydrobenzoin to benzoin contrasting to the results obtained by many other reagents, which cleaved the carbon-carbon bond to give the corresponding aldehyde or acid. An α,β-unsaturated alcohol, 4-phenyl-but-3-en-2-ol was easily converted to 4-phenyl-but-3-en-2-one in 90% yield showing that carbon-carbon double bonds were not prone to cleavage in this method and no isomerization of double bonds were observed. R1 OH R2 CrO3 / Al2O3 Solvent free Grinding Scheme I R1 O R2 NOTES 1525 Table I ⎯ Conversion of alcohols to carbonyl compounds using CrO3 supported onto Al2O3a Entry Substance Product 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1-heptanol 1-octanol benzyl alcohol 4-methoxybenzyl alcohol 4-chlorobenzyl alcohol 2-hydroxybenzyl alcohol 3-nitrobenzyl alcohol cyclohexanol cycloheptanol 2-isopropyl-4-methoxycyclohexanol 4-phenylcyclohexanol 1-phenylethanol benzhydrol 4-phenyl-but-3-en-2-ol hydrobenzoin heptanal octanal benzaldehyde 4-methoxybenzaldehyde 4-chlorobenzaldehyde 2-hydroxy benzaldehyde 3-nitrobenzaldehyde cyclohexanone cycloheptanone 2-isopropyl-4-methoxycyclohexanone 4-phenylcyclohexanone 1-phenylethanal benzophenone 4-phenyl-but-3-en-2-one benzoin Time (min) Yieldb,c (%) 6 6 7 5 5 7 10 7 7 7 8 5 7 7 8 68 73 81 85 91 71 93 67 92 95 87 79 89 90 87 a Molar ratio of reagent to substrate was 2:1. b Yields refer to pure isolated products. c products were characterized by comparison of their physical data, IR, 1H NMR spectra with authentic samples17. The promoting effect of Al2O3 was confirmed by comparing our results with those previously reported by Ji-Dong Lou et al15, who applied CrO3 in the absence of inorganic support for the oxidation of primary alcohols to the corresponding aldehydes in solvent-free conditions at room temperature. The oxidation of benzyl alcohol with CrO3 was completed after 3 hrs, while with CrO3/Al2O3, the time required for completion of oxidation reaction was 7 min. The rapid and selective formation of products demonstrates the efficiency of this new method. The structures of all the products were settled from their analytical and spectral (IR, 1H NMR) data and by direct comparison with authentic samples. In summary, oxidation of alcoholic groups with CrO3/Al2O3 in solvent-less system has advantages over previously reported methods as it provides a facile, useful, and inexpensive technique addition to the existing method of oxidations. The advantages of this environmentally friendly protocol are mild reaction condition, simple work-up procedure and short reaction time. Experimental Section All oxidation products were known compounds and were identified by comparison of their physical and spectral data with those of authentic samples.17 Alcohols, CrO3, and Al2O3 were purchased from Fluka and Merck. The purity of the products and reaction monitoring were accomplished by TLC on silica gel polygram SILG/ UV 254 plates. General procedure for the oxidation of alcohols by CrO3/Al2O3. A mortar was charged with Al2O3 (2 g) and CrO3 (0.2 g, 2 mmol) and the mixture was ground with a pestle for 1 min. Then alcohols (1 mmol) and about seven drops of t-butanol were added to the mixture. The reaction mixture was ground for the time specified in Table I. The progress of reaction was monitored by TLC using CCl4-ether (4:1). The reaction mixture was poured into a mixture of ether (20 ml) and water (10 ml). The organic layer was passed through a small bed of alumina and evaporated to dryness to give the pure carbonyl compounds (Table I) (Caution: CrO3 is a highly toxic agent. All chromium (VI) reagents must be handled with care16). Acknowledgement Authors acknowledge the partial support of this work by Chamran University Research Council. References 1 Larock R C, Comprehensive Organic Transformations, (VCH, Weinheim), 1989. 2 Heravi M M, Farhangi N, Beheshtiha Y S, Assadolah K, Ghassemzadeh M & Tabarhydar K, Phosphorus, Sulfur and Silicon, 177, 2002, 2883 (and references cited therein). 3 Eisenbraun E, Org Synth Coll, 5, 1973, 310. 4 Heravi M M, Kiakojoori R & TabarHydar K, J Chem Res, 1998, 658. 1526 INDIAN J. CHEM., SEC B, JULY 2005 5 Cainelli G, Cardillo G, Orena M & Sandri S, J Am Chem Soc, 98, 1976, 6737. 6 Lalancette J M, Rollin G & Dumas P, Can J Chem, 50, 1972, 3058. 7 Aizpuruan J M & Palomo C, Tetrahedron Lett, 24, 1983, 4367. 8 Santaniello E, Ponti F & Manzocchi A, Synthesis, 1978, 534. 9 Lou J D & Wu Y Y, Synth Commun, 17, 1987, 1717. 10 Lou J D, Synth Commun, 19, 1989, 1841. 11 Nakamura H & Matsuhashi H, Bull Chem Soc Jpn, 68, 1995, 997. 12 Varmar R S & Saini R K, Tetrahedron Lett, 39, 1998, 1481. 13 Heravi M M, Ajami D, TabarHydar K & Ghassemzadeh M, J Chem Res, 1999, 334. 14 Kiasat A R, Kazemi F & Nourbakhsh K, Phosphorus, Sulfur and Silicon related, 2004 457. 15 Lou J D & Xu Z N, Tetrahedron Lett, 43, 2002, 6095 and references cited therein). 16 Cupo D Y & Wetterhahn K E, Cancer Res, 45, 1985, 1146. 17 Shriner R L, Fuson R C, Curtin D Y & Morrill T C, The Systematic Identification of Organic Compounds, 6th edn. 1980.