Acta Chim. Slov. 2003, 50, 563-568. 563 SILICA SULFURIC ACID/KBrOsAVET Si02 AS AN EFFICIENT HETEROGENEOUS SYSTEM FOR THE OXIDATION OF ALCOHOLS UNDER MILD CONDITIONS BiBi Fatemeh Mirjalili,a Mohammad Ali Zolfigol,* Abdolhamid Bamoniri,c Zahra Zaghaghi,a and Azizeh Hazara Department oj Lnemistry, Lotlege oj Science, laza University, laza. F.(J. Box. H9195-/41, Iran. b f r^i r* 1 / tjt j ^/^ji Department oj Lnemistry, Lotlege oj Science, Bu-Ali Sina University, tiamaaan. Lip Loae 651/4, P.O. Box 4135, Iran, E-mail: Zolfi@basu.ac.ir. C r /-11 /-i 1 / 7 7 ^/^JZČI Department oj Lnemistry, Lotlege oj Science, Kasnan University, Kasnan, Lip Loae j 116/, Iran. Received 07-11-2002 Abstract A combination of silica sulfuric acid and potassium bromate in the presence of wet Si02 were used as an effective oxidizing agent for the oxidation of alcohols to their corresponding aldehyde or ketone derivatives in dichloromethane, acetonitrile or toluene with good yields. Introduction For oxidation of organic functionalities, one turns often to high-valent metal oxides or their mineral salts. Classical reagents of this type are manganese dioxide (MnO^), potassium permangenate (KMnO/i), chromium trioxide (CrOs), potassium chromate (K2Cr04), and potassium dichromate (K^C^O?). These are ali frequently-used reagents, either in the laboratory or in industry, and yet they are beset with multiple liability. For satisfactory and reproducible results, these oxidants demand rigorous control of the experimental conditions. The other drawbacks against such oxidants and their use in multistage organic synthesis, in spite of their power are also their lack of selectivity, strong protic and aqueous conditions, low yields of the products, and tedious work-up. For instance, overoxidation of aldehydes to carboxylic acids is often unavoidable side reaction. Furthermore, the elevated reflux temperatures required by some oxidation procedures will favor inopportune secondary reactions. Likewise, the presence of strong acids or bases, which are required adjuncts as catalysts for some reactions, often leads to detrimental side reactions. As an example, the oxidation of primary alcohols to B.F. Mirjalili, M.A. Zolfigol, A. Bamoniri, Z. Zaghaghi, A. Hazar: Silica Sulfuric Acid/KBr03/Wet Si02... 564 Acta Chim. Slov. 2003, 50, 563-568. aldehydes by a chromium(VI) salt in sulfuric acid is often accompanied by the formation of an hemiacetal between the resulting aldehyde and the alcohol substrate, following by the ready oxidation of this intermediate to an ester. Along this line, we have introduced potentially useful oxidants for selective oxidation and oxidative deprotection of different functional groups. Therefore, we decided to choice a new reagent or reagent systems to overcome the above limitations. On the other hand, any reduction in the amount of sulfuric acid needed and/or any simplification in handling procedures is required for risk reduction, economic advantage and environment protection. In addition, there is current research and general interest in heterogeneous systems because of the importance such systems have in industry and in developing technologies. In continuation of our studies on the application of inorganic acidic salts and silica chloride, we found that silica gel reacts with chlorosulfonic acid to give silica sulfuric acid (I). It is interesting to note that the reaction is easy and clean without any work-up procedure because HC1 gas is evolved from the reaction vessel immediately. Therefore, we also find that silica sulfuric acid (I) is an excellent candidate to replace sulfuric acid in organic reactions without any limitations such as sulfonation of activated aromatic rings and destruction of acid sensitive functional groups. " Since the heterogeneous reagent systems have many advantages such as simple experimental procedures, mild reaction conditions and minimization of chemical wastes as compared to their liquid phase counterparts. " Our goal, in undertaking this line of work, was two-fold: (a) to overcome the limitations and drawbacks of the reported methods such as: tedious work-up, strongly acidic media (H0~ -8), and safety problems (storage, handling, using and also presence of toxic transition metal cations such as Cr+ , Hg+ , Cu+ within molecular structure of the reagents), (b) moreover, high-yielding production of carbonyl compounds using a novel combination of reagents are our main interest. The above facts, encouraged us to seek a completely heterogeneous system for the oxidation of various alcohols, and we have investigated a number of different reaction conditions based upon the in situ generation of HBr03 at the surface of Si02 in low concentration by inorganic acidic resin i.e. silica sulfuric acid and potassium bromate. B.F. Mirjalili, M.A. Zolfigol, A. Bamoniri, Z. Zaghaghi, A. Hazar: Silica Sulfuric Acid/KBr03/Wet Si02... Acta Chim. Slov. 2003, 50, 563-568. 565 Results and discussion In this article we would like to report a simple and convenient method for the effective conversion of alcohols (1) to their corresponding aldehyde or ketone derivatives (2) under heterogeneous conditions (Scheme 1). Si02rOSC>3H I Different types of alcohols (1) were subjected to oxidation reaction in the presence of silica sulfuric acid (I), KB1O3 (II), and wet Si02 in dichloromethane, acetonitrile or toluene. The oxidation reactions were performed under completely heterogeneous conditions with good to excellent yields (Table). It was also observed that the oxidation of primary alcohols (1) gives only aldehydes. The present oxidation reaction can be readily carried out only by placing silica sulfuric acid (I), KBrC>3 (II), and wet Si02 in dichloromethane, acetonitrile or toluene in a reaction vessel with efficiently stirring the resulting heterogeneous mixture under reflux conditions. The results and reaction conditions are shown in the Table. H OH \ / C / \ R1 R2 I, II wet Si02 O II C / \ R1 R2 12 Scheme 1 In order to show the chemoselectivity of the method we have carried out the successful oxidation of benzyl alcohol in the presence of cyclohexanol (Scheme 2 and Entry 11). CH2OH OH Si02^-OS03H (I) KBr03 (II) solvent, reflux Hs .O C 90% O 0% Scheme 2 B.F. Mirjalili, M.A. Zolfigol, A. Bamoniri, Z. Zaghaghi, A. Hazar: Silica Sulfuric Acid/KBr03/Wet Si02... 566 Acta Chim. Slov. 2003, 50, 563-568. Table. Oxidation of various alcohols (2.5 mmol) to aldehyde and ketons by silica sulfuric acid (I) (0.2 g), KBr03 (II) (1 mmol, 0.16 g), and wet SiO2(60% w/w, 0.2 g) in organic solvent under heterogeneous condition. Entry Substrate (f CH2OH OH CH2OH OCH3 X*^Nv/'OH H OH ^OH Product ,-CHO OCH3 Solvent Conditions CH2C12 CH2C12 CH3CN RT. Reflux Reflux Time Yield (min) (%) 2 (60% w/w, 0.2 g) in dichloromethane (5 mL) was stirred at room temperature for 30 minutes. The heterogeneous mixture was filtered and washed with Na2S203 solution (10%, 2x20 mL). The organic phase was collected. Anhydrous Na2S04 (2 g) was added to the solution and filtered after 15 minutes. Dichloromethane was removed. The solid was crystallized from «-hexane. The /?-benzoquinone was obtained in good yield (Table). Acknowledgements Financial support for this work by the research affairs, Yazd University, Yazd, Iran and also Bu-Ali Sina University, Hamadan, Iran are gratefully acknowledged. 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Povzetek Za oksidacijo alkoholov v ustrezne aldehide in ketone smo uporabili kombinacijo žveplene kisline na silikagelu, kalijevega bromata in vlažnega SiO2. Reakcije so potekle v acetonitrilu, diklorometanu ali toluenu z dobrimi izkoristki. B.F. Mirjalili, M.A. Zolfigol, A. Bamoniri, Z. Zaghaghi, A. Hazar: Silica Sulfuric Acid/KBr03/Wet Si02...