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Indian Journal of Chemistry Vol. 44B, November 2005, pp. 2383-2386 Note CaCl2.2H2O assisted oxidation of alcohols with (NH4)2Cr2O7 F Shirini*a, M A Zolfigol b & A Safaria a Department of Chemistry, Faculty of Science, Guilan University, Rasht, Iran b Department of Chemistry, College of Science, Bu-Ali Sina University, Hamadan, Iran E-mail: [email protected] Received 15 September 2004; accepted (revised) 19 April 2005 CaCl2.2H2O has been found to be an efficient reagent for the acceleration of the oxidation of alcohols to their corresponding carbonyl compounds with (NH4)2Cr2O7 in solution and under solvent free conditions. Keywords: Calcium chloride oxidation, alcohols, carbonyl compounds IPC: Int.Cl.7 C 07 C 39/00 The oxidation of alcohols to their corresponding carbonyl compounds1,2 is one of the most important reactions in organic synthesis since the products are quite useful synthetic intermediates especially for the construction of carbon skeletons. There are many useful methods for this transformation of which manganese dioxide3, activated dimethylsulfoxides4-5, n-butylphenylphosphonium dichromate7, pyridinium chlorochromate8,9, ferric dichromate10, polyvinylpyridine-supported ferric dichromate11 and hypervalent iodines12-15 are examples . Although these methods are useful in organic synthesis, each of them suffers from one or more of the following disadvantages; lack of selectivity, long reaction time, strong protic and aqueous conditions, tedious work-up, producing bad-smelling co-products and explosivity on impact or on heating. Thus, a milder, more selective and inexpensive reagent is still in demand. In continuation of our ongoing research program directed towards the development of new oxidizing agents and systems16-18, herein we wish to report a convenient, efficient and simple method for the oxidation of alcohols to their corresponding carbonyl compounds in solution and under solvent free conditions. Results and Discussion Oxidation of different types of alcohols was investigated in the absence of solvent by ammonium dichromate in the presence of CaCl2.2H2O (Table I, Scheme I). The reaction is simply performed by stirring a mixture of alcohol, (NH4)2Cr2O7 and CaCl2.2H2O in an oil-bath (60°C) for the appropriate time (Table I). Alcohols were oxidized efficiently and the corresponding carbonyl compounds were isolated in good to high yields. No traces of carboxylic acids or other by-products were detected in all the cases studied. The only isolated products were the expected carbonyl compounds and, in the cases where the reaction had not gone to completion, the starting material was recovered. A or B R1R 2CHOH R1,R2 = Aryl, alkyl R1R 2CO A: (NH4)2Cr2O7/CaCl2.2H2O, solvent free, 60 OC B: (NH4)2Cr2O7/CaCl2.2H2O, CH3CN, reflux Scheme I In order to compare the obtained results with those obtained in solution, we studied the oxidation reaction in refluxing CH3CN. As shown in the Table I, in most cases, there are appreciable differences between the results obtained in solution and those in solvent free conditions. In conclusion, by omitting the solvent, in addition to ease of the work-up procedure, the reaction time was reduced and the need for solvent is avoided. This method is not suitable for the oxidation of allylic alcohols (Table I, entry 16). It must be emphasized that the oxidation did not proceed using (NH4)2Cr2O7 alone, even after prolonged heating. Although the mechanism of the reaction is not clear, but the presence of the water in the structure of CaCl2.2H2O is very important so that the rate of the reaction is diminished when CaCl2 is used instead of CaCl2.2H2O (Table I, entry 17). On the basis of these results we thought that the effect of CaCl2.8H2O on the acceleration of the reaction may only be related to the presence of H2O, considering that the presence of the other part of the molecule is not important. Lack of the progress of the reaction in wet acetonitrile disproved this hypothesis. INDIAN J. CHEM., SEC B, NOVEMBER 2005 2384 Table I ⎯ Oxidation of alcohols to their corresponding carbonyl compoundsa Entry Substrate 1 2 Product CH2OH CHO CH2OH CHO Cl 3 CH2OH Cl CHO CH2OH CHO Br 5 CH2OH 6 CHO CH2OH CHO CH2OH O2N CHO t-Bu CH2OH t-Bu CHO CH2OH CHO Me 10 11 PhO CH2OH PhO CHO MeO CH2OH MeO CHO OH N N OH OH CHO O 16 OH a 90 0.12 99 2.2 90 0.08 92 4 85 0.12 95 3.75 82 0.17 85 1.67 90 1 60 3.75 70 0.42 75 1.5 90 0.17 95 3.3 95 0.08 92 1.5 80 0.83 92 1.5 85 0.25 80 3.3 90 0.42 95 2.5 70 1.67 75 1 50 0.5 95 3 70 1.8 95 1 ⎯c 1 ⎯c 2 30 d 2 40 d CHO 14 17 3.2 O OH 15 95 Me 12 13 0.17 O2N O2N 9 92 NO2 O2N 8 1.75 Br NO2 7 Solvent free oxidation Time/hr Yield (%) b Cl Cl 4 Oxidation in CH3CN Time/hr Yield (%)b CH2OH CHO CHO Products were characterized by their physical constants, comparison with authentic samples and IR and NMR spectroscopy; b Isolated yield; c Mixture of products; d Oxidation in the presence of CaCl2. NOTES 2385 Table II ⎯ Comparison of some of the results obtained by the oxidation of alcohols using (NH4)2Cr2O7 in the presence of CaCl2.2H2O under solvent free conditions at 60°C, with some of those reported by K2Cr2O7 (ref.19), MnO2 (ref. 20) and 4-aminopyridinium chlorochromate supported on silica gel (ref. 21). Entry Substrate (NH4)2Cr2O7 1 2 CH2OH MeO CH2OH 3 Time (hr) (Yield %) K2Cr2O7 MnO2 4-Aminopyridinium chlorochromate (0.17)(95) (4)(80) (48)(77) (2)(97) (0.25)(80) (4)(90) (48)(83) (1.5)(100) (0.42)(95) (3)(95) (24)(87) (3)(95) OH In ordet to show the efficiency of the present method, some of our results have compared with some of those reported in the literature19-21 (Table II). In summery, the CaCl2.2H2O was found to be an efficient reagent for the promotion of the oxidation of alcohols to their corresponding carbonyl compounds using (NH4)2Cr2O7. This new procedure is very simple and cleanly affords the desired products in good to high yields. (NH4)2Cr2O7 (0.756 g, 3 mmoles) and CaCl2.2H2O (0.147 g, 1 mmole) in CH3CN (3 mL) was stirred magnetically under reflux condition for the specified time (Table I). The progress of the reaction was monitored by TLC. After completion of the reaction the mixture was filtered and the solid residue was washed with CH3CN (5 mL). Evaporation of the solvent followed by column chromatograghy on silica gel gave the corresponding carbonyl compounds in good to high yield. Experimental Section General. Chemicals were purchased from Merck, Fluka, BDH and Aldrich Chemical Companies. Products were separated and purified by different chromatography techniques, and were identified by the comparison of their m.p., b.p., IR, NMR and refractive index with those reported for the authentic samples. All yields refer to the isolated products. The purity determination of the substrates and reaction monitoring were accomplished by TLC on silica gel polygram SILG/UV 254 plates. Column chromatography was carried out on Merck kisselgel 60H. General procedure for the oxidation of alcohols under solvent free conditions. A mixture of alcohol (1 mmole), (NH4)2Cr2O7 (0.756 g, 3 mmoles) and CaCl2.2H2O (0.147 g, 1 mmole) was heated in an oilbath (60°C) for the specified time (Table I). The progress of the reaction was monitored by TLC. After completion of the reaction the mixture was triturated with CH3CN (5 mL) and filtered. Evaporation of the solvent followed by column chromatograghy on silica gel gave the corresponding carbonyl compounds in good to high yield. General procedure for the oxidation of alcohols in CH3CN. A suspension of alcohol (1 mmole), Acknowledgement Authors gratefully acknowledge the financial support from Research Council of Guilan University. 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