Economic synthesis of quinaldinium fluorochromate(VI), (QnFC), and solvent-free periodic acid oxidation of alcohols catalyzed by QnFC

Turkish Journal of Chemistry http://journals.tubitak.gov.tr/chem/ Research Article Turk J Chem (2014) 38: 63 – 69 c TÜBİTAK ⃝ doi:10.3906/kim-1303-12 Economic synthesis of quinaldinium fluorochromate(VI), (QnFC), and solvent-free periodic acid oxidation of alcohols catalyzed by QnFC Melek CANBULAT ÖZDEMİR∗, Hatice Beytiye ÖZGÜN Department of Chemistry, Faculty of Science, Gazi University, Ankara, Turkey Received: 06.03.2013 • Accepted: 18.06.2013 • Published Online: 16.12.2013 • Printed: 20.01.2014 Abstract: A 1:1:1 stoichiometric reaction between CrO 3 , aqueous HF, and quinaldine affords orange crystalline quinaldinium fluorochromate(VI) (QnFC) (C 10 H 9 NH[CrO 3 F]) in 99.4% isolated yield. A highly efficient, simple, chemoselective, and environmentally benign procedure for QnFC (3 mol%) catalyzed oxidation of primary and secondary alcohols to aldehydes and ketones using 1.1 equiv of H 5 IO 6 under solvent-free conditions is described. Key words: Periodic acid, solvent-free, oxidation of alcohols, quinaldinium fluorochromate(VI) 1. Introduction Oxidation reactions are of great interest in fine chemistry, at both the laboratory and the industrial scale. 1,2 The oxidation of alcohols to aldehydes and ketones is a fundamental process in organic synthesis. 3−5 The traditional methods for this purpose usually employ stoichiometric quantities of inorganic reagents such as chromate and permanganate. These methods are quite useful in laboratory-scale reactions but usually generate significant amounts of inorganic waste that damage the environment seriously in large-scale reactions. 6,7 Oxochromium(VI)amine complexes such as pyridinium chlorochromate (PCC) and pyridinium fluorochromate (PFC) are extensively used in the oxidation of alcohols owing to their commendable performance under mild conditions, 8−11 but the requirement of at least a stoichiometric amount of the oxidant to complete the oxidation is a disadvantage due to the high toxicity of chromium reagents. The present concern about the toxicity and environmental implications of oxochromium(VI) has provided encouragement for the study and use of catalytic oxochromium(VI) reagents in conjunction with stoichiometric co-oxidants, particularly when applied to the large-scale preparations found in industries where the disposal of byproducts is a constant problem. Periodic acid (H 5 IO 6 ) has been used as co-oxidant in several mild and selective oxidation reactions of alcohols catalyzed by oxochromium(VI) reagents. 12,13 These oxidation reactions are often carried out in acetonitrile and sometimes at elevated temperatures. Problems are thus encountered for the complete removal of the solvent. Of particular significance would be the establishment of solvent-free processes, which are not only of interest from an ecological point of view but avoiding the use of organic solvents during the reactions in organic synthesis leads to clean, efficient, and economical technology. To the best of our knowledge, no chromium(VI) catalyzed oxidation reactions of alcohols have been carried out under solvent-free conditions in which periodic acid is used as the terminal oxidant. An attractive ∗ Correspondence: 63 CANBULAT ÖZDEMİR and ÖZGÜN/Turk J Chem alternative is to carry out these oxidation reactions under solvent-free conditions. We report herein a facile and efficient oxidation of primary and secondary alcohols to aldehydes and ketones using only 3 mol % QnFC and 1.1 equiv of H 5 IO 6 , under solvent-free conditions at room temperature (Scheme 1). O OH H5IO6 (1.1 equiv.), QnFC ( 3 mol %) solvent-free, r.t R1 R1 R2 R2 R1, R2 = Alkyl, aryl and H Scheme 1. QnFC catalyzed oxidation of alcohols to the corresponding aldehydes and ketones with periodic acid under solvent-free conditions. 2. Results and discussion In continuation of our ongoing program to develop the use of solvent-free systems for environmentally benign synthetic protocols, 14−16 we examined the use of this procedure for periodic acid oxidation of alcohols catalyzed by QnFC. Although we have previously reported QnFC as a versatile reagent for various oxidative transformations, 17 the need to use an excess or at least a stoichiometric amount of QnFC to perform the oxidations is a drawback, due to all the known disadvantages of chromium-based compounds. It was first considered worthwhile to try the atom efficient synthesis of QnFC. Apart from our previous procedure, minimal amounts of water have been used in order to enable waste minimization and prevent the loss of QnFC due to its solubility in water. Thus, a 1:1:1 stoichiometric reaction among CrO 3 , aqueous HF and quinaldine affords orange crystalline quinaldinium fluorochromate(VI) in 99.4% isolated yield. QnFC thus obtained is substantially pure and crystalline and so no recrystallization is needed. QnFC melts at 146–148 ◦ C and the results of analysis and characterization data compare very well with those reported earlier. 17 All the oxidation reactions were carried out at room temperature under solvent-free conditions. In a control experiment, benzyl alcohol was converted into benzaldehyde with 1.1 equiv. H 5 IO 6 in 15% yield after 180 min in the absence of QnFC. In order to find the catalytic amount of QnFC required for maximum yield, the oxidation reactions of benzyl alcohol were performed by using 1.1 equiv. of H 5 IO 6 . The use of 3 mol % QnFC maximized the yield of benzaldehyde. An increase in QnFC amount over 3 mol % does not produce more conversion. This is illustrated in Figure 1. Hence 3 mol % of QnFC was maintained during the oxidation of all the alcohols. 110 Yield % 90 70 50 30 0 0.5 1 1,5 2 2.5 3 3.5 4 4.5 Mol % QnFC Figure 1. Effect of the catalyst amount on the oxidation of benzyl alcohol using 1.1 equiv. H 5 IO 6 and benzyl alcohol for 2 min at room temperature under solvent-free conditions. 64 CANBULAT ÖZDEMİR and ÖZGÜN/Turk J Chem Having optimized the reaction conditions using benzyl alcohol as a model substrate, the oxidation of various aromatic and aliphatic alcohols was then examined to establish the generality of the method (Table 1). Both electron-poor (entries 3 and 4) as well as electron-rich benzylic primary alcohols (entries 1 and 2) were oxidized smoothly to give aldehydes in a short time without overoxidation to carboxylic acids. Aliphatic primary and secondary alcohols (entries 5–8) were oxidized to give the corresponding carbonyl compounds in high to excellent yields. Notable is that the oxidation of 1-octanol (entry 5) gave octanal in 84% yield, unlike PCC/H 5 IO 6 oxidation, which gave a complex mixture. 13 Table 1. Comparison of solvent-free oxidations by QnFC/H 5 IO 6 and by QnFC. QnFC/H 5IO 6 QnFC Time (min) Yield (%) b Time (min) Yield (%) b 1 2 93 180 86 237 237 2 2 95 120 90 234 233 4 97 120 96 265 265 5 86 240 87 320 320 4 84 120 85 105 106 5 85 300 78 161 162 7 5 90 120 91 (...truncated)