Applied Catalysis A: General 366 (2009) 275–281

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

Ana C. Estrada a, Mario M.Q. Simoes b, Isabel C.M.S. Santos b, M. Graca P.M.S. Neves b,Artur M.S. Silva b, Jose A.S. Cavaleiro b, Ana M.V. Cavaleiro a,*a Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugalb Department of Chemistry, QOPNA, University of Aveiro, 3810-193 Aveiro, Portugal

A R T I C L E I N F O

Article history:

Received 24 April 2009

Received in revised form 3 July 2009

Accepted 9 July 2009

Available online 23 July 2009

Keywords:

Polyoxometalates

Polyoxotungstates

Oxidation

Hydrogen peroxide

Indane

Tetralin

A B S T R A C T

The homogeneous liquid phase oxidation of indane and tetralin with hydrogen peroxide catalysed by

tetrabutylammonium salts of iron(III)-substituted polyoxotungstates of general formula

[XW11Fe(H2O)O39]n�, X = P, Si or B is described. The system presented here gives rise to benzylic

monooxygenation and dioxygenation products. Indane oxidation reactions produce also dehydrogena-

tion and hydroperoxidation products. As a result, 1H-indene and indane hydroperoxide are formed.

Interestingly, tetralin gives rise to the cleavage of carbon–carbon bond, producing 4-(2-hydroxyphe-

nyl)butanal. In the present conditions, this aldehyde is probably arising from tetralin hydroperoxide.

Depending on the reaction conditions, moderate selectivities for the corresponding ketones are obtained,

affording conversions as high as 59% and 34% for indane and tetralin, respectively. In order to understand

the reactions pathway, the oxidation of 1-indanol, 1-indanone, 1H-indene, 1-tetralol and 1-tetralone is

also carried out with an iron(III)-substituted polyoxotungstate as catalyst and H2O2 as oxidant. The

results show that 1-indanol and 1-tetralol give an important contribution for the formation of the

corresponding ketones. As far as we know, the use of iron-substituted polyoxotungstates in the oxidation

of these arenes is presented for the first time.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

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 [1–4]. 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].

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

* Corresponding author. Tel.: +351 234370734; fax: +351 234370084.

E-mail address: anacavaleiro@ua.pt (Ana M.V. Cavaleiro).

0926-860X/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2009.07.022

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 [9,11–31]. 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 earlier work [32–39]. 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.

2. Experimental

2.1. Reagents and synthetic procedures

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

Scheme 1.

A.C. Estrada et al. / Applied Catalysis A: General 366 (2009) 275–281276

(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.

2.2. General oxidation procedure

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 reflux and aliquots werewithdrawn from the reaction mixture and injected into the GC–FIDor GC–MS (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 quantified by the titration of aliquots withceric sulphate 0.1 M using ferroin as indicator [43]. The amount ofH2O2 was determined considering the yields of the hydroperoxidesdetermined by GC–MS.

2.3. Instruments and methods

GC–FID and GC–MS analyses were performed using a Varian3900 apparatus and a Finnigan Trace GC/MS (Thermo Quest CEInstruments), respectively, equipped with fused silica capillary DB-5 type columns (30 m � 0.25 mm i.d.; 0.25 mm film 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 final 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.

1H and 13C NMR spectra were 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

Table 1Oxidation of indane with hydrogen peroxide catalysed by Fe(III)-substituted polyoxotu

Entry Catalyst Sub/Cat H2O2/Sub Consumed

H2O2 (mmol)

(

1 PW11Fe 667 2.0 0.75 3

2 4.0 1.25 3

3 333 2.0 1.69 1

4 4.0 3.00 1

5 SiW11Fe 667 2.0 0.81 3

6 4.0 1.66 3

7 333 2.0 0.99 1

8 4.0 1.18 1

9 BW11Fe 667 2.0 2.00 2

10 4.0 3.64 3

11 333 2.0 2.00 1

12 4.0 3.99 1

13 Without catalyst 4.0 n.d.d

a Reaction conditions: the substrate (1.0 mmol), the catalyst (1.5 or 3.0 mmol) and aqb Turnover number (TON) is defined as the amount of substrate converted per amouc Determined by GC.d Not determined.

tetramethylsilane (TMS) as internal reference. Preparative thinlayer chromatography was performed on silica gel (Merck silica gel60 GF254).

At the end of the reactions with [PW11Fe(H2O)O39]4�, a drop ofthe reaction mixture was dried on a KBr pellet and the infraredspectrum measured in order to assess the stability of the catalysts.

2.4. Product chromatographic separation and characterization

Products (1a–c) and (2a and b) were identified by comparingtheir mass spectra with the information available from the GC–MSdatabase and also by GC co-injection of commercially availablestandards. The identity of compounds (1d) and (2d) was confirmedby comparing its mass spectra with the information available fromthe GC–MS database and with the data from Ref. [29]. Compound(1e) was identified 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.

For the chromatographic separation of the oxidation products,final 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 identification was done by 1H [44] and 13C NMR[15,44].

4-(2-Hydroxyphenyl)butanal (2c) – 1H NMR (CDCl3) d (ppm):1.89–1.94 (m, 2H, Ar–CH2–CH2–CH2–CHO), 2.57 (dt, 2H, Ar–CH2–CH2–CH2–CHO, J = 0.8 and 5.6 Hz), 2.62 (t, 2H, Ar–CH2–CH2–CH2–CHO, J = 7.8 Hz), 5.77 (s, 1H, Ar–OH), 6.81–6.87 (m, 2H, H-3,5),7.07–7.13 (m, 2H, H-4,6), 9.82 (t, 1H, Ar–CH2–CH2–CH2–CHO,J = 0.8 Hz). 13C NMR d (ppm): 22.1 (Ar–CH2–CH2–CH2–CHO), 27.5(Ar–CH2–CH2–CH2–CHO), 43.0 (Ar–CH2–CH2–CH2–CHO), 115.7and 120.5 (C-3,5), 127.1 (C-2), 127.7 (C-4), 130.2 (C-6), 154.0

ngstates after 24 h of reactiona.

TON)b Conversionc (%) Selectivityc (%)

(1a) (1b) (1c) (1d) (1e)

33 50 23 11 40 16 10

47 52 10 7 30 15 38

93 58 18 12 44 14 12

96 59 11 8 35 12 34

40 51 16 14 60 8 2

54 53 19 10 39 13 19

27 38 19 11 36 14 20

86 56 21 5 32 11 31

73 41 4 11 47 12 26

60 54 23 3 65 6 3

03 31 11 9 39 8 33

76 53 11 10 51 7 21

– 3 36 20 37 6 0

ueous 30% (w/w) H2O2 (2.0 or 4.0 mmol) are stirred in 3.0 mL of CH3CN at reflux.

nt of catalyst used.

Fig. 1. Conversion of indane with different H2O2/substrate molar ratios (2.0, 4.0 or 9.8) in the presence of PW11Fe (&), BW11Fe (~) and SiW11Fe (*) after 24 h of reaction.

Substrate: 1.0 mmol; CH3CN: 3.0 mL; reflux.

Fig. 2. Time course for indane oxidation reactions catalysed by PW11Fe (&), SiW11Fe

(*) or BW11Fe (~) using 4.0 mmol of H2O2. Substrate: 1.0 mmol; catalyst:

1.5 mmol; acetonitrile: 3.0 mL; reflux.

A.C. Estrada et al. / Applied Catalysis A: General 366 (2009) 275–281 277

(C-1), 203.6 (Ar–CH2–CH2–CH2–CHO). MS (EI) m/z (rel. int.%): 164(M�+, 5), 146 (22), 131 (100), 129 (25), 115 (18), 91 (30), 77 (6).

3. Results and discussion

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 reflux, in thepresence of catalytic amounts of the Keggin-type anions[XW11FeIII(H2O)O39]n� (XW11Fe), X = P, Si, B. The conversion of

Fig. 3. The yield of products for indane oxidation reactions in the presence of PW11Fe an

3.0 mL; reflux.

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].

3.1. Oxidation of indane (1)

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.

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 profile (the valuesof conversion increase along the first 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.

Regarding product yields, it was found that (1e) and (1c)account together for more than half (50–72%) of the products after

d BW11Fe with 4.0 mmol of H2O2. Substrate: 1.0 mmol; catalyst: 1.5 mmol; CH3CN:

Fig. 4. Fraction of the initial H2O2 present in solution during the indane oxidation

catalysed by PW11Fe (&), SiW11Fe (*) or BW11Fe (~) using 4.0 mmol of H2O2.

Substrate: 1.0 mmol; catalyst: 1.5 mmol; acetonitrile: 3.0 mL; reflux.

A.C. Estrada et al. / Applied Catalysis A: General 366 (2009) 275–281278

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).

Table 2Oxidation of (1a), (1b) and (1c) with H2O2 catalysed by PW11Fea.

Entry Substrate Conversion (%)b [Time]

1 41c

[7 h] [10]

2 40

[7 h] 100

3 88

[24 h] [85]

4 0 No products

[24 h]

a Reaction conditions: the substrate (1.0 mmol), the catalyst (3.0 mmol) and 4.0 mmb Determined by GC.c When the same reaction is repeated in the absence of PW11Fe, no reaction produc

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).

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) asthe major product (58%), followed by (1i) (18%), (1g) (14%) and (1f)(10%) (Table 2, entry 1). It is worth to refer that products resultingfrom 1H-indene oxidation are not detected in the indane oxidation.These results can be justified 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].

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 minor by-product(Table 2, entry 3). Ketone (1c) does not react when submitted to thesame reaction conditions of indane (Table 2, entry 4). Thus 1-indanone is obtained from 1-indanol through the same mechanismpreviously studied for benzylic alcohols [46]. As 1H-indene (1a) isnot observed as a product of 1-indanol oxidation reactions, this

Products [Selectivity (%)]b

[14] [58] [18]

[15]

ol of aqueous 30% (w/w) H2O2 are stirred in CH3CN at reflux.

ts are observed.

Scheme 2.

A.C. Estrada et al. / Applied Catalysis A: General 366 (2009) 275–281 279

result allows us to exclude the possibility of (1a) arising from thedehydration of (1b). Therefore, 1H-indene is the result of oxidativedehydrogenation of indane in the conditions presented here.

The stability of the catalyst was assessed by infrared spectro-scopy for reactions performed with PW11Fe and substrate/catalystmolar ratio equal to 333 (H2O2/substrate = 4.0 and 9.8). Theinfrared spectra of the residues obtained from the solution at theend of the reaction (24 h) presented the characteristic bands of[PW11Fe(H2O)O39]4� and/or [PW11Fe(OH)O39]5� [47,48] at 1063,954, 882 and 809 cm�1, suggesting that no decompositionoccurred in the reaction conditions.

3.2. Oxidation of tetralin (2)

Oxidation of tetralin with hydrogen peroxide in the presence ofthe iron(III)-substituted polyoxotungstates results in the forma-tion of products (2a–d) (Scheme 2). The distribution pattern of the

Table 3Oxidation of tetralin with hydrogen peroxide catalysed by Fe(III)-substituted polyoxot

Entry Catalyst Sub/Cat H2O2/Sub Consumed H2O2 (mm

1 PW11Fe 667 2.0 1.4

2 4.0 3.1

3 333 2.0 1.3

4 4.0 3.6

5 SiW11Fe 667 2.0 0.7

6 4.0 2.3

7 333 2.0 0.7

8 4.0 1.2

9 BW11Fe 667 2.0 2.0

10 4.0 3.8

11 333 2.0 1.9

12 4.0 4.0

13 Without catalyst 4.0 n.d.d

a Reaction conditions: the substrate (1.0 mmol), the catalyst (1.5 or 3.0 mmol) and aqb Determined by GC.c Turnover number (TON) is defined as the amount of substrate converted per amoud Not determined.

Fig. 5. Time course (conversion and fraction of the initial H2O2 present in solution) for tetr

of H2O2. Substrate: 1.0 mmol; catalyst: 1.5 mmol; acetonitrile: 3 mL; reflux.

products is dependent on the catalyst used and on the reactiontime (Table 3).

When the conversion values obtained are compared, BW11Fe isthe more efficient catalyst in the presence of a substrate/catalystmolar ratio of 667, independently of the H2O2/substrate molarratio used. Tetralin conversions above 30% are observed for theseconditions. For the other catalysts and for the same reactionconditions, the values of tetralin conversion observed staybetween 14% and 21% (Table 3 and Fig. 5).

Aiming to optimize the values of tetralin conversion, thesubstrate/catalyst molar ratio was increased to 333, keeping theamount of oxidant as 2.0 or 4.0 mmol. For these conditions,conversion of tetralin increased slightly for PW11Fe and SiW11Fe(Table 3). BW11Fe continues to be the more active catalyst, but forthis amount of catalyst the values of tetralin conversion slightlydecrease. This decrease of conversion may be related to the fasterconsumption/decomposition of H2O2 in the presence of BW11Fe(Fig. 5). Again, the best TON values are observed when thesubstrate/catalyst molar ratio of 667 is used. PW11Fe and SiW11Fecontinue to show a similar kinetic profile while BW11Fe has adifferent behaviour. Low values of conversion are observed whenthe reaction is carried without catalyst (entry 13).

Ketone (2b) is always the major product and the amount ofoxidant does not seem to influence the selectivity of the reactions.Apparently, 1-tetralol is an important primary intermediate in the

ungstates after 24 h of reactiona.

ol) (TON)c Conversionb (%) Selectivityb (%)

(2a) (2b) (2c) (2d)

93 14 14 75 0 12

120 18 5 75 0 12

60 18 4 57 5 34

77 23 4 57 2 38

113 17 5 45 45 5

140 21 3 56 0 41

73 22 7 43 28 22

80 24 3 41 34 22

220 33 8 60 6 26

227 34 7 64 2 27

93 28 6 65 17 12

83 25 6 64 8 22

– 7 24 67 5 4

ueous 30% (w/w) H2O2 (2.0 or 4.0 mmol) are stirred in 3.0 mL of CH3CN at reflux.

nt of catalyst used.

alin oxidation catalysed by PW11Fe (&), SiW11Fe (*) or BW11Fe (~) using 4.0 mmol

Table 4Oxidation reactions of (2a) and (2b) with H2O2 catalysed by PW11Fea.

Entry Substrate Conversion (%)b [time] Products [selectivity (%)]b

1

30

[7 h] [100]

2 65

[24 h] [77] [23]

3 0 No products

[24 h]

a Reaction conditions: the substrate (1.0 mmol), the catalyst (3.0 mmol) and 4.0 mmol of aqueous 30% (w/w) H2O2 are stirred in CH3CN at reflux.b Determined by GC.

A.C. Estrada et al. / Applied Catalysis A: General 366 (2009) 275–281280

oxidation of tetralin, and further oxidation of 1-tetralol occurs with30% of conversion after 7 h of reaction affording 100% selectivityfor 1-tetralone (Table 4, entry 1). Increasing the reaction time for24 h, the oxidation of 1-tetralol occurs with 65% of conversion andaffords also 1-tetralone as the major product, along with 1,4-dihydroxytetralin (2d) as a minor product (Table 4, entry 2). On theother hand, the oxidation of (2b) does not occur in the reactionconditions presented here (Table 4, entry 3). Furthermore, product(2c) that is observed in tetralin oxidation reactions does not occurin (2a) and (2b) oxidation reactions in similar conditions.

It is known from the literature that the primary product oftetralin autooxidation is usually tetralin hydroperoxide [25,26]. Inour studies it was not possible to detect tetralin hydroperoxide. Inthe present conditions tetralin hydroperoxide is probably con-verted into 1-tetralol and 1-tetralone and, most likely, product (2c)is also arising from tetralin hydroperoxide (Scheme 3). This C–Cbond cleavage is similar to the well-known industrial process for

Scheme

simultaneous production of phenol and acetone [49]. The reactionmechanism, as shown in Scheme 3, is mainly an acid catalysedreaction. Protons may be added to the solvent by the(TBA)4Hx[XW11Fe(H2O)O39] (where x = 0, 1 or 2 and X = P, Si orB) salts used or by the H2O2 [50,51]. Thus the protonated tetralinhydroperoxide can loose a water molecule to form the inter-mediates A1 (via 1,2 shift-migration of the phenyl group from theadjacent carbon to oxygen atom) or B1 (via C–C bond cleavage)followed by water addition to afford (2c) (Scheme 3).

The oxidation of indane and tetralin with the catalytic systemhere described was found to occur through radical formation, asobserved previously for cycloalkanes in similar conditions [36]. Webelieve that the radical mechanism proposed for the oxidation ofcyclooctane with excess of H2O2 in the presence of the Fe-monosubstituted Keggin anions [36] applies also in the case of thesubstrates studied here, conducting to the formation of initialhydroxylated or hydroperoxylated products. The results observed

3.

A.C. Estrada et al. / Applied Catalysis A: General 366 (2009) 275–281 281

were dependent on several factors, like the H2O2/substrate molarratio, time of reaction or catalyst used. Several possible concurrentreactions may occur, namely the hydroperoxidation or hydro-xylation of the substrates, the decomposition of formed hydro-peroxide, the transformation of the products by the action of theacidity of the reaction media, or the dismutation of H2O2. Therelative importance of the several reactions is not easy to assess.Also, in view of previous results, it is highly probable that theactivation of H2O2 may occur simultaneously at W and Fe. In theseconditions it is also difficult to evaluate the reasons that lead to thedifferent catalyst performances.

Taking the series of Keggin anions [XW12O40]n�, it is known thatthey vary in their hydrolytic stability in the presence of H2O2, protonaffinities and redox behaviour [52,53]. The same is expected tohappen with the Fe-monosubstituted anions. In our studies we werenot able to find so far any direct correlation between the knowncatalysts properties, like charge, basicity or redox potentials [42],and the outcome of the reactions. Kholdeeva et al. have reported theinfluence of the number of protons (in a series of compounds withKeggin anions monosubstituted by titanium) on the homolytic orheterolytic H2O2 activation: fewer protons conduct to homolyticcleavage of peroxide [54]. In our case, the number of protons on theanions (0–2) seems to be associated with the homolytic cleavagethat originates radical formation.

4. Conclusions

The oxidation of indane, tetralin, 1-indanol and 1-indanone withhydrogen peroxide, in the presence of catalytic amounts oftetrabutylammonium salts of iron(III)-substituted Keggin-typepolyoxotungstates was studied for the first time. The conversionand selectivity for tetralin and indane oxidation reactions are foundto be dependent on the polyoxotungstate used, on the reaction timeand on the amount of oxidant added. This study demonstrates thatiron(III)-substituted Keggin-type polyoxotungstates are efficientcatalysts, giving 59% of conversion for indane and 34% for tetralin,using the environmentally safe hydrogen peroxide as oxidant.

In the presence of BW11Fe, the conversions obtained after 3 h ofreaction for both substrates are significantly higher than thoseregistered using the other catalysts. Additionally, the boroncatalyst affords almost the same substrate conversion after 3 or24 h of reaction.

Based on the recent literature [8,11–13,16,27,29], the oxidationreactions of these substrates give always the respective ketones asmajor products. In general, the system presented here also yieldsketones (1c) and (2b) as major products after 24 h of reaction,along with hydroperoxide (1e) and diol (2d). Besides theseproducts, in the case of tetralin, the carbon–carbon bond oxidativecleavage of tetralin hydroperoxide to afford a carbonyl compoundis also observed, a very important reaction in synthetic organicchemistry. Here we report the oxidative cleavage of the hydro-peroxide to compound (2c) under mild and environmentallyfriendly conditions, using hydrogen peroxide as oxidant.

Acknowledgements

Thanks are due to the University of Aveiro and FCT (Fundacaopara a Ciencia e a Tecnologia) for funding. Ana C. Estrada alsothanks FCT for a PhD grant (SFRH/BD/21883/2005).

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