Ammonium Chlorochromate Adsorbed on Montmorillonite K-10: Selective Oxidation of Alcohols under Solvent-free Conditions$ Majid M. Heravi,* Reza Kiakojoori and K. Tabar Hydar Chemistry and Chemical Engineering Research Center of Iran, P.O. Box 14335-186, Tehran, Iran A simple and selective oxidation of alcohols to carbonyl compounds on clay supported ammonium chlorochromate under solventless conditions is expedited by microwave irradiation. Prompted by stringent environment protection laws in recent years, there has been increasing emphasis on the use and design of environmentally friendly solid acid±base cata- lysts to reduce the amount of toxic waste and by-products arising from the chemical processes.1 The oxidation of alcoholic groups to carbonyl functional- ities continues to receive attention from chemists in search of newer and selective methods of oxidation.2 Chromium-based reagents have extensively been used in organic synthesis.3 The utility of chromium(VI) reagents in oxidative transformation is compromised due to their inherent toxicity, cumbersome preparation and potential danger (ignition or explosion) in terms of product isolation and waste disposal.Introduction of reagents4 on solid sup- ports has circumvented some of these problems and pro- vided an attractive alternative in organic synthesis in view of the selectivity and associated ease of manipulation. Therefore it is not surprising that a large number of chro- mium-based oxidants impregnated on solid supports have been explored.5 These supported reagents in solvents oxidize primary and secondary alcohols.6 Organic solvents are not only expensive, but are often �ammable, toxic and environmentally hazardous.Consequently, there is scope for the development of rapid and solventless methods that have manipulative advantages over heterogeneous reactions. In continuation of our investi- gations on organic reactions in solventless systems,7,8 we now report a facile and selective oxidation of alcohols to carbonyl compounds using ammonium chlorochromate adsorbed on montmorillonite K-10 under solvent-free conditions that is accelerated in most cases by exposure to microwaves.The reagent is easily prepared by addition of a weighed amount of montmorillonite K-10 to a solution of am- monium chlorochromate9 in water and rotary evaporating to dryness. The reaction is conducted by mixing ®nely ground supported reagent with neat alcohols.In the absence of the clay the reactions are slow and considerable amounts of alcohols are recovered unchanged at room temperature or even upon microwave irradiation for an extended period. As an example the reaction of 5-methyl-2-nitrobenzyl alcohol with ammonium chlorochromate results in the formation of only 30% of 5-methyl-2-nitrobenzaldehyde, whereas the yield increases to almost to quantitative in the case of the clay-supported reagent. In a few cases, the reac- tions are completed upon simple mixing; gentle warming by microwaves accelerates some others (Table 1).The reactions are relatively clean with no tar formation typical of many CrO3 reactions. Primary alcohols were oxidized to the corresponding alde- hydes and an overoxidation of aldehydes to the correspond- ing acid derivative was not observed even after prolonged irradiation and with excess of supported chromium reagent. On the other hand the oxidations of aryl-substituted unsatu- rated alcohols were less than satisfactory. The carbon± carbon double bond was partially cleaved under the above conditions.Cinnamaldehyde for example was obtained in only 61% yield along with 39% of benzaldehyde in the oxidation of cinnamyl alcohol. In conclusion, oxidation with ammonium chlorochromate supported on montmorillonite K-10 under solvent-free con- ditions is a rapid, manipulatively simple and selective proto- col which avoids the drastic conditions usually employed.Experimental All products are known compounds and their physical data were essentially identical with those of authentic samples. Microwave irradiations were carried out in a National oven, Model 5250 at 900 W. J. Chem. Research (S), 1998, 656±657$ Table 1 Oxidation of alcohols to carbonyl compounds using ammonium chlorochromate adsorbed on montmorillonite K-10 Entry Alcohol Product Yield (%)a 1 Benzyl Benzaldehyde 92 (85) 2 4-Methylbenzyl 4-Methylbenzaldehyde 93 (85) 3b 5-Methyl-2-nitrobenzyl 5-Methyl-2-nitrobenzaldehyde 90 (81) 4b 4-Nitrobenzyl 4-Nitrobenzaldehyde 95 (83) 5b Salicyl Salicylaldehyde 95 (82) 6 a-Phenylethyl Acetophenone 94 (88) 7 Benzhydrol Benzophenone 90 (81) 8b Benzoin Benzil 90 (80) 10b Cyclohexanol Cyclohexanone 88 (72) 11b 2-Methylcyclohexanol 2-Methylcyclohexanone 85 (75) 12b (±)-Menthol Menthone 85 (74) aYields are based on GLC analysis; figures in parentheses refer to isolated yields.bReaction completed under microwave irradiation.Preparation of Ammonium Chlorochromate/Montmorillonite K-10. �To a solution of chromium trioxide (40 g, 0.4 mol) in water (100 mL) was added ammonium chloride (21.4 g, 0.4 mol) within 15 min at 40 8C. The mixture was cooled until a yellow-orange solid $This is a Short Paper as de®ned in the Instructions for Authors, Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there- fore no corresponding material in J. Chem. Research (M). *To receive any correspondence. 656 J. CHEM. RESEARCH (S), 1998formed. Reheating to 40 8C gave a solution. Montmorillonite K-10 (200 g) was then added with stirring at 40 8C. After evaporation in a rotary evaporator, the orange solid was dried in vacuum for 2 h at 70 8C. It can be kept for several months in air at room tempera- ture without losing its activity. Oxidation of Alcohols in the Solventless System.DThe above reagent (1.7 g, 2.6 mmol) was added to an appropriate neat alcohol (1.3 mmol).This mixture was thoroughly mixed using a pestle and mortar. An exothermic reaction ensued with darkening of the orange reagent and was complete almost immediately as conRrmed by TLC (hexane�}AcOEt, 8: 2). The product was extracted into CH2Cl2 and passed through a small bed of silica gel (1 cm) to a€ord the corresponding pure carbonyl compounds (Table 1). Oxidation of Alcohols under Microwave Irradiation and Solvent- free Conditions (General Procedure).DThe above reagent (1.7 g, 2.6 mmol) was added to an appropriate neat alcohol using a beaker and mixed thoroughly by a spatula. The reaction mixture was placed in a beaker inside a microwave oven and irradiated for 1 min.On completion of the reaction (TLC; hexane�}ethyl acetate, 8: 2) the crude product was directly charged onto a silica gel column. Elution with hexane�}ethyl acetate a€orded the pure carbonyl compound (Table 1). Received, 8th June 1998; Accepted, 29th June 1998 Paper E/8/04309D References 1 C.B. Khouv, C. B. Dartl, J. A. Lalenger and M. E. Davis, J. Catal., 1994, 149, 195. 2 S. V. Pitre, M. V. R. Reddy and Y. D. Vankar, J. Chem. Res. (S), 1997, 462; R. S. Varma, R. K. Sami and R. Dahiya, J. Chem. Res. (S), 1998, 120. 3 A. J. Fatiadi, in Organic Synthesis by Oxidation with Metal Compounds, ed. C. R. H. I. de Jonge Miys, Plenum, New York, 1986, pp. 119�}260. 4 Preparative Chemistry Using Supported Reagents, ed. D. Laszlo, Academic Press, San Diego, 1987; A. Mckillop and D. W. Young, Synthesis, 1979, 401, 481. 5 T. Brunelet, C. Jouitteau and G. Gellbard, J. Org. Chem., 1986, 51, 4016; J. W. Suggs and L. Yluarte, Tetrahedron Lett., 1986, 27, 437. 6 Y. Shia Cheng, W. L. Liu and Shia H. Chen, Synthesis, 1986, 223. 7 M. M. Heravi, K. Aghapoor, M. A. Nooshabadi and M. M. Mojtahidi, Monatsh. Chem., 1997, 128, 1143. 8 K. Aghapoor, M. M. Heravi and M. A. Nooshabadi, Indian J. Chem., Sect. B, 1998, 37, 84. 9 G. Sh. Zharg, Q. Z. Shi, M. F. Chem and K. Cai, Synth. Commun., 1997, 27, 3691. J. CHEM. RESEARCH (