Iron-substituted polyoxotungstates as catalysts in the oxidation of indane and tetralin with hydrogen peroxide

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<ul><li><p>at</p><p>. Sava</p><p>Applied Catalysis A: General 366 (2009) 275281</p><p>pha</p><p>lts</p><p>P, S</p><p>Contents lists available at ScienceDirect</p><p>Applied Catalys</p><p>.e l1. Introduction</p><p>Oxidation is one of the most fundamental transformations inorganic chemistry. The conversion of hydrocarbons into oxyge-nated products has been broadly investigated over the last years,since the resulting products are valuable intermediates in organicsynthesis and some of these products are used in the constructionof larger molecules [14]. Indane ring, in particular, is present insystems with important biological and medicinal applications[5,6]. Selective oxidation of tetralin produces mainly 1-tetralone,an important source of synthetic precursors and reactive inter-mediates for a wide range of products, including pharmaceuticals,dyes and agrochemicals [7,8]. 1-Tetralone is important commer-cially as the starting material for 1-naphthol manufacture [9].</p><p>Stoichiometric oxidation reactions usually require excessiveamounts of strong oxidants like manganese dioxide, chromic acid,potassium dichromate or selenium dioxide and produce largeamounts of toxic waste when applied on an industrial process [10].Thus, the use of environmentally benign catalysts and oxidants isan urgent and promising area of research. In recent years, indane or</p><p>tetralin oxidation has been studied using tert-butylhydroperoxide,sodium periodate, hydrogen peroxide or molecular oxygen asoxidants in the presence of several catalysts, either in homo-geneous or heterogeneous systems [9,1131]. Transition metal-substituted polyoxotungstates are an extraordinarily versatileclass of complexes with high catalytic activity on a variety oforganic reactions including hydroxylation, epoxidation, oxidativedehydrogenation and oxidative cleavage processes, as demon-strated in our earlierwork [3239].We report here the oxidation ofindane and tetralin with H2O2 catalysed by iron(III)-substitutedpolyoxotungstates (Schemes 1 and 2). The use of aqueous H2O2 inthe oxidation of organic substrates is very attractive from the pointof view of synthetic organic chemistry, since aqueous H2O2 is anenvironmentally clean and easy to handle reagent [40,41]. As far aswe know, there are no references to the use of iron-substitutedpolyoxotungstates in the oxidation of these arenes.</p><p>2. Experimental</p><p>2.1. Reagents and synthetic procedures</p><p>Acetonitrile (Panreac), 30% (w/w) hydrogen peroxide (Riedel-de-Haen), indane (Aldrich), tetralin (Aldrich), 1H-indene (Fluka), 1-indanol (Fluka), 1-indanone (Fluka), 1-tetralol (Fluka), 1-tetralone</p><p>Available online 23 July 2009</p><p>Keywords:</p><p>Polyoxometalates</p><p>Polyoxotungstates</p><p>Oxidation</p><p>Hydrogen peroxide</p><p>Indane</p><p>Tetralin</p><p>monooxygenation and dioxygenation products. Indane oxidation reactions produce also dehydrogena-</p><p>tion and hydroperoxidation products. As a result, 1H-indene and indane hydroperoxide are formed.</p><p>Interestingly, tetralin gives rise to the cleavage of carboncarbon bond, producing 4-(2-hydroxyphe-</p><p>nyl)butanal. In the present conditions, this aldehyde is probably arising from tetralin hydroperoxide.</p><p>Depending on the reaction conditions,moderate selectivities for the corresponding ketones are obtained,</p><p>affording conversions as high as 59% and 34% for indane and tetralin, respectively. In order to understand</p><p>the reactions pathway, the oxidation of 1-indanol, 1-indanone, 1H-indene, 1-tetralol and 1-tetralone is</p><p>also carried out with an iron(III)-substituted polyoxotungstate as catalyst and H2O2 as oxidant. The</p><p>results show that 1-indanol and 1-tetralol give an important contribution for the formation of the</p><p>corresponding ketones. As far aswe know, the use of iron-substituted polyoxotungstates in the oxidation</p><p>of these arenes is presented for the rst time.</p><p> 2009 Elsevier B.V. All rights reserved.</p><p>* Corresponding author. Tel.: +351 234370734; fax: +351 234370084.</p><p>E-mail address: anacavaleiro@ua.pt (Ana M.V. Cavaleiro).</p><p>0926-860X/$ see front matter 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.apcata.2009.07.022Iron-substituted polyoxotungstates as cand tetralin with hydrogen peroxide</p><p>Ana C. Estrada a, Mario M.Q. Simoes b, Isabel C.M.SArtur M.S. Silva b, Jose A.S. Cavaleiro b, Ana M.V. CaDepartment of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, PortugalbDepartment of Chemistry, QOPNA, University of Aveiro, 3810-193 Aveiro, Portugal</p><p>A R T I C L E I N F O</p><p>Article history:</p><p>Received 24 April 2009</p><p>Received in revised form 3 July 2009</p><p>Accepted 9 July 2009</p><p>A B S T R A C T</p><p>The homogeneous liquid</p><p>tetrabutylammonium sa</p><p>[XW11Fe(H2O)O39]n, X =</p><p>journa l homepage: wwwalysts in the oxidation of indane</p><p>antos b, M. Graca P.M.S. Neves b,leiro a,*</p><p>se oxidation of indane and tetralin with hydrogen peroxide catalysed by</p><p>of iron(III)-substituted polyoxotungstates of general formula</p><p>i or B is described. The system presented here gives rise to benzylic</p><p>is A: General</p><p>sev ier .com/ locate /apcata</p></li><li><p>(Aldrich) and ceric sulphate (Aldrich) were used as received. Allother solvents used herein were obtained from commercialsources and used as received or distilled and dried using standardprocedures. The following tetrabutylammonium (TBA) salts of thepolyoxometalates used were prepared by described procedures:(TBA)4[PW11Fe(H2O)O39]2H2O [32], (TBA)4H[SiW11Fe(H2O)O39][42], and (TBA)4H2[BW11Fe(H2O)O39]H2O [33]. The obtainedcompounds were characterized by elemental analysis, thermo-gravimetry, and infrared spectroscopy.</p><p>2.2. General oxidation procedure</p><p>The typical procedure was as follows: to the substrate(1.0 mmol) and the catalyst (1.5 or 3.0 mmol) in 3.0 mL ofacetonitrile, aqueous 30% (w/w) H2O2 (2.0, 4.0 or 9.8 mmol) wasadded. The mixture was stirred at reux and aliquots werewithdrawn from the reactionmixture and injected into the GCFIDor GCMS (1.0 mL) using 1-hexanol as internal standard. Blankreactions were also carried out for both substrates. Yields andconversions were determined by GC. Unused H2O2 and hydroper-oxides produced were quantied by the titration of aliquots with</p><p>tetramethylsilane (TMS) as internal reference. Preparative thinlayer chromatographywas performed on silica gel (Merck silica gel60 GF254).</p><p>At the end of the reactions with [PW11Fe(H2O)O39]4, a drop of</p><p>the reaction mixture was dried on a KBr pellet and the infraredspectrummeasured in order to assess the stability of the catalysts.</p><p>2.4. Product chromatographic separation and characterization</p><p>Products (1ac) and (2a and b) were identied by comparingtheir mass spectra with the information available from the GCMSdatabase and also by GC co-injection of commercially availablestandards. The identity of compounds (1d) and (2d) was conrmedby comparing its mass spectra with the information available from</p><p>Scheme 1.</p><p>tu</p><p>(</p><p>3</p><p>3</p><p>1</p><p>1</p><p>3</p><p>3</p><p>1</p><p>1</p><p>2</p><p>3</p><p>103 31 11 9 39 8 33</p><p>176 53 11 10 51 7 21</p><p> 3 36 20 37 6 0</p><p>aqueous 30% (w/w) H2O2 (2.0 or 4.0 mmol) are stirred in 3.0 mL of CH3CN at reux.</p><p>ount of catalyst used.</p><p>A.C. Estrada et al. / Applied Catalysis A: General 366 (2009) 275281276ceric sulphate 0.1 M using ferroin as indicator [43]. The amount ofH2O2was determined considering the yields of the hydroperoxidesdetermined by GCMS.</p><p>2.3. Instruments and methods</p><p>GCFID and GCMS analyses were performed using a Varian3900 apparatus and a Finnigan Trace GC/MS (Thermo Quest CEInstruments), respectively, equippedwith fused silica capillary DB-5 type columns (30 m 0.25 mm i.d.; 0.25mm lm thickness)using helium as the carrier gas (35 cm/s). The gas chromatographicconditions were as follows: column initial temperature (70 8C,1 min); temperature rate (18 8C/min); column nal temperature(260 8C); injector temperature (260 8C); detector temperature(270 8C). Retention time (min): 1-hexanol (I.S.) = 3.3; indane(1) = 3.9; 1H-indene (1a) = 4.2; 1-indanol (1b) = 5.8; 1-indanone(1c) = 6.2; 1,3-dihydroxyindane (1d) = 6.8; 1-hydroperoxyindane(1e) = 7.0; tetralin (2) = 4.9; 1-tetralol (2a) = 6.7; 1-tetralone(2b) = 6.9; 4-(2-hydroxyphenyl)butanal (2c) = 7.8; 1,4-dihydrox-ytetralin (2d) = 7.9.</p><p>1H and 13C NMR spectrawere recorded in CDCl3 solutions, usinga Bruker Avance 300 at 300.13 MHz and 75.47 MHz, respectively.The chemical shifts are expressed in d (ppm) values relatively to</p><p>Table 1Oxidation of indane with hydrogen peroxide catalysed by Fe(III)-substituted polyoxo</p><p>Entry Catalyst Sub/Cat H2O2/Sub Consumed</p><p>H2O2 (mmol)</p><p>1 PW11Fe 667 2.0 0.75</p><p>2 4.0 1.25</p><p>3 333 2.0 1.69</p><p>4 4.0 3.00</p><p>5 SiW11Fe 667 2.0 0.81</p><p>6 4.0 1.66</p><p>7 333 2.0 0.99</p><p>8 4.0 1.18</p><p>9 BW11Fe 667 2.0 2.00</p><p>10 4.0 3.64</p><p>11 333 2.0 2.00</p><p>12 4.0 3.99</p><p>13 Without catalyst 4.0 n.d.d</p><p>a Reaction conditions: the substrate (1.0 mmol), the catalyst (1.5 or 3.0mmol) andb Turnover number (TON) is dened as the amount of substrate converted per amc Determined by GC.d Not determined.the GCMS database and with the data from Ref. [29]. Compound(1e) was identied by its mass spectrum and by the triphenylpho-sphine test reported elsewhere [3]. The triphenylphosphine testwas also used to verify the possible formation of tetralinhydroperoxide. However, tetralin hydroperoxide was not detectedduring tetralin oxidation in these conditions.</p><p>For the chromatographic separation of the oxidation products,nal reaction mixtures were poured into water and extracted withdichloromethane. The organic phases were dried with anhydroussodium sulphate and concentrated using a rotary evaporator. Theresulting mixtures were then separated by silica gel thin layerchromatography, using dichloromethane as eluent. By thisprocedure the pure compound (2c) was obtained in a separatefraction and its identication was done by 1H [44] and 13C NMR[15,44].</p><p>4-(2-Hydroxyphenyl)butanal (2c) 1H NMR (CDCl3) d (ppm):1.891.94 (m, 2H, ArCH2CH2CH2CHO), 2.57 (dt, 2H, ArCH2CH2CH2CHO, J = 0.8 and 5.6 Hz), 2.62 (t, 2H, ArCH2CH2CH2CHO, J = 7.8 Hz), 5.77 (s, 1H, ArOH), 6.816.87 (m, 2H, H-3,5),7.077.13 (m, 2H, H-4,6), 9.82 (t, 1H, ArCH2CH2CH2CHO,J = 0.8 Hz). 13C NMR d (ppm): 22.1 (ArCH2CH2CH2CHO), 27.5(ArCH2CH2CH2CHO), 43.0 (ArCH2CH2CH2CHO), 115.7and 120.5 (C-3,5), 127.1 (C-2), 127.7 (C-4), 130.2 (C-6), 154.0</p><p>ngstates after 24 h of reactiona.</p><p>TON)b Conversionc (%) Selectivityc (%)</p><p>(1a) (1b) (1c) (1d) (1e)</p><p>33 50 23 11 40 16 10</p><p>47 52 10 7 30 15 38</p><p>93 58 18 12 44 14 12</p><p>96 59 11 8 35 12 34</p><p>40 51 16 14 60 8 2</p><p>54 53 19 10 39 13 19</p><p>27 38 19 11 36 14 20</p><p>86 56 21 5 32 11 31</p><p>73 41 4 11 47 12 26</p><p>60 54 23 3 65 6 3</p></li><li><p>.8) i</p><p>A.C. Estrada et al. / Applied Catalysis A: General 366 (2009) 275281 277Fig. 1. Conversion of indane with different H2O2/substrate molar ratios (2.0, 4.0 or 9Substrate: 1.0 mmol; CH3CN: 3.0 mL; reux.(C-1), 203.6 (ArCH2CH2CH2CHO). MS (EI) m/z (rel. int.%): 164(M+, 5), 146 (22), 131 (100), 129 (25), 115 (18), 91 (30), 77 (6).</p><p>3. Results and discussion</p><p>The oxidation of indane and tetralin was carried out inhomogeneous phase using hydrogen peroxide (H2O2/substratemolar ratio = 2.0, 4.0 and 9.8) in acetonitrile, at reux, in thepresence of catalytic amounts of the Keggin-type anions[XW11Fe</p><p>III(H2O)O39]n (XW11Fe), X = P, Si, B. The conversion of</p><p>Fig. 2. Time course for indane oxidation reactions catalysed by PW11Fe (&amp;), SiW11Fe(*) or BW11Fe (~) using 4.0 mmol of H2O2. Substrate: 1.0 mmol; catalyst:</p><p>1.5 mmol; acetonitrile: 3.0 mL; reux.</p><p>Fig. 3. The yield of products for indane oxidation reactions in the presence of PW11Fe an3.0 mL; reux.the substrates and the distribution of oxidation products dependon the catalyst and on the amount of hydrogen peroxide used. Forboth substrates, no oxidation products are detected in theexperiments performed in the presence of iodine, a well-knownradical scavenger [45]. This allows us to suggest that the oxidationof indane and tetralin is a radical process, as already registered forthe oxidation of other substrates in similar conditions[32,33,36,37].</p><p>3.1. Oxidation of indane (1)</p><p>The results obtained for indane oxidation (Scheme 1) usingdifferent H2O2/substrate and substrate/catalyst molar ratios aresummarized in Table 1. Very low values of conversion are observedin the absence of catalyst (entry 13). Comparing the values ofconversion obtained after 24 h of reaction in the presence of thethree polyoxotungstates studied, the higher conversion is alwaysobserved for H2O2/substrate molar ratio equal to 4.0. In fact,reducing or increasing the H2O2/substrate molar ratio causes, insome cases, a considerable decrease in the conversion (Fig. 1).Although in most cases slightly better values of conversion areobtained for substrate/catalyst molar ratio of 333, the bestturnover numbers (TON) are observed for a substrate/catalystmolar ratio of 667.</p><p>The time course of indane oxidation is also dependent on thecatalyst added (Fig. 2). In the conditions presented here, reactionswith PW11Fe and SiW11Fe show a similar kinetic prole (the valuesof conversion increase along the rst 24 h of reaction), while in thepresence of BW11Fe the value of conversion after 3 h is almost thesame to the one observed after 24 h of reaction.</p><p>n the presence of PW11Fe (&amp;), BW11Fe (~) and SiW11Fe (*) after 24 h of reaction.Regarding product yields, it was found that (1e) and (1c)account together for more than half (5072%) of the products after</p><p>d BW11Fe with 4.0 mmol of H2O2. Substrate: 1.0 mmol; catalyst: 1.5mmol; CH3CN:</p></li><li><p>24 h, varying only the relative proportions. For a H2O2/substratemolar ratio of 4.0 and a substrate/catalyst molar ratio of 667,hydroperoxide (1e) is the main product along 24 h of reaction forPW11Fe, whereas in the presence of BW11Fe, ketone (1c) is alwaysthe major product (Fig. 3). When a H2O2/substrate molar ratio of2.0 is used instead, 1-indanone is always the major product alongthe reaction, independently of the catalyst used (Table 1).</p><p>Consumption of H2O2 is always higher and faster in thepresence of BW11Fe, when compared with SiW11Fe and PW11Fe inthe same reaction conditions. In the presence of BW11Fe, forsubstrate/catalyst molar ratio of 667 and 4.0 mmol of H2O2 permmol of substrate, the consumption of H2O2 is comparatively veryrapid and almost complete after 9 h of reaction (only 9% of H2O2 ispresent). In the case of SiW11Fe and PW11Fe, the consumption ismoderate, and 58% and 69 % of H2O2 are still present after 24 h ofreaction, respectively (Fig. 4).</p><p>In order to understand how the reaction products are formed,the oxidation of possible intermediates was analysed. The catalyticoxidation of (1a) occurs with 41% of conversion and affords (1h) asthemajor product (58%), followed by (1i) (18%), (1g) (14%) and (1f)(10%) (Table 2, entry 1). It is worth to refer that products resultingfrom1H-indene oxidation are not detected in the indane oxidation.These results can be justied by the fact that indane oxidationoccurs via a radical process (proved by the iodine test). In contrast,the catalytic oxidation of 1H-indene in the presence of polyox-otungstates occurs by a non-radical process [39].</p><p>When submitted to the same reaction conditions as indane, 1-indanol (1b) gives, as expected, the corresponding carbonylproduct (1-indanone) after 7 h of reaction (Table 2, entry 2). After24 h of reaction, (1b) affords also (1d) as a m...</p></li></ul>

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