Journal of Molecular Catalysis, 83 (1993) 107-116 Elsevier Science Publishers B.V., Amsterdam

MO163

Hydroxylation of aromatics with hydrogen peroxide catalyzed by vanadium (V ) peroxocomplexes

Mauro Bianchi, Marcella Bonchio, Valeria Conte*, Fausta Coppa, Fulvio Di Furia, Giorgio Modena, Stefano Moro and Stephen Standen’ Centro di Studio sui Meccanismi di Reazioni Organiche de1 CNR, Dipartimento di Chimica Organica, Universita’di Padova, Via Marzolo 1, I-35131 Padova (Italy); tel. (+39-49)831111, fax. (+39-49)831222

(Received November 19,1992; accepted March 18,1993)

Abstract

Benzene and substituted bensenes XCsHS (X = CHa, F, Cl, Br, NO,) are hydroxylated to the corresponding monophenols by H,OP in CHsCN in the presence of catalytic amounts of VO (0,) (PIC ) ( H20)2 where PIC is the anion of picolinic acid. Fair yields of hydroxylated ma- terials are obtained by adding the oxidant in portions, or continuously, and keeping the conversion of the substrate rather low ( < 3% ) . Although some degradation of the catalyst takes place during the reaction, such a process can be minimized by adding small amounts of acid. The catalytic reaction can be also carried out in a two-phase (H,O/dichloroethane) system.

Key words: aromatics; catalytic aromatic hydroxylation; hydroxylation; peroxocomplexes; two- phase oxidations; vanadium peroxocomplexes

Introduction

The hydroxylation of benzene to phenol by peroxovanadium complexes, e.g. 1 [ 11, is of obvious synthetic relevance, considering also [ 21 that the pro- cess may be rendered catalytic by adding of HzOz to 2, thus restoring 1.

It may be mentioned, in passing, that such a reaction is also of interest from a mechanistic standpoint. As an example, the role played by the picoli- nato ligand in determining such a reactivity is still poorly understood [ 31. While we are continuing our studies aimed at providing a rationale for the oxidative behavior of 1, we present here results that demonstrate the synthetic

*Corresponding author. ‘Erasmus student from the University of Newcastle upon Tyne.

0304-5102/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved.

108 M. Bianchi et al./J. Mol. Catal. 83 (1993) 107-116

scope of 1 as a catalyst of the hydroxylation of aromatic substrates both under homogeneous and two-phase conditions. In particular our data show that the catalytic reaction, where hydrogen peroxide is the oxidant, proceeds smoothly with benzene, giving fair yields of phenol, provided that the conversion of the substrate is kept rather low. Not only benzene but also substitute benzenes - e.g. toluene, halobenzenes and nitrobenzene - are oxidized by 1 to the corre- sponding monophenols, again in yields ranging from acceptable to fair. Some degradation of the catalyst, which is experienced under the conditions em- ployed, can be minimized by running the oxidation reactions in the presence of relatively small amounts of strong acid.

Experimental

Reagents and solvents Reagent grade V,O, and H,O, (70% w/v, Peroxid Chemie) were used

without further purification, The substrates and the solvents, all commercially available materials, were purified according to standard procedures and stored over 4A molecular sieves. Complex 1 was synthesized by following a reported procedure [ 11. HPCA, 4- (3-heptyl) -pyridine-2-carboxylic acid was prepared according to a literature method [4,5] and its identity confirmed by spectro- scopic (IR, ‘H- and 13C-NMR) and MS analyses.

Oxidation procedures and products analysis The homogeneous oxidation of aromatics was carried out in a jacketed

glass reactor with temperature control better than + 0.05 ’ C, by dissolving the desired quantity of complex 1 in 10 ml of CH3CN containing the substrate. Hydrogen peroxide was then added either stepwise, at controlled time inter- vals, or continuously via a syringe pump. The procedure for the two-phase oxidation carried out in a H,O/dichloroethane (DCE) system has been de- scribed previously [ 21. The oxidation products were determined by quantita- tive GC analysis: (a) glass column FFAP 3% on Chromosorb WAW DMCS 50 cm, for phenol (benzophenone internal standard); (b) glass column FFAP 3% on Chromosorb WAW DMCS 150 cm, for chloro- and fluorophenols (aceto- phenone and benzophenone as internal standards); (c) fused silica capillary column, 20 m, FFAP, i.d. 0.25 mm, for isomeric cresols and nitrophenols (ben- zophenone internal standard).

Results and discussion

As stated above, the aim of our study was the development of a catalytic procedure for the direct hydroxylation of benzene by using dilute hydrogen peroxide in the presence of 1.

M. Bianchi et al./J. Mol. Catal. 83 (1993) 107-116 109

Previous investigations have shown that acetonitrile is a suitable solvent for the stoichiometric oxidation of benzene by 1 [ 1,2]. In addition, dilute hy- drogen peroxide is soluble in such a solvent. Therefore, our experiments under homogeneous conditions, aimed mainly at examining some of the general fea- tures of the reaction, were run in CH,CN. The results are collected in Table 1.

There are several items of information that can be obtained from the data of Table 1. An important one is the observation that, in the experiments where excesses of the oxidant over the catalyst are added from the beginning, the amount of phenol produced is almost the same as that found in the stoichio- metric oxidation of benzene by 1 (entries l-3 ) . Having established, by direct experiments, that dioxygen is produced and that other products arising from overoxidation of benzene or phenol are not formed, it may be suggested that an extensive decomposition of hydrogen peroxide takes place under such con- ditions. By contrast, when the same excess of hydrogen peroxide is added step- wise or in various portions (entries 4-8)) the yields of phenol, based on hydro- gen peroxide, rises to 79% (entry 8).

While this behavior confirms that 1 may be restored from the dimer 2, (see Eqn. 1) by addition of hydrogen peroxide, it also indicates that the reac- tion of 1 with an excess H,O, leading to dioxygen is faster than the reaction of 1 with benzene to yield phenol. Therefore, a necessary prerequisite for the use of hydrogen peroxide in this system is its slow addition during the reaction. These results have a mechanistic significance that will be discussed elsewhere

[61. It should be mentioned that the conversion of benzene must be kept rather

low. Indeed, when smaller excesses of the substrate over the oxidant are used the yields drop rapidly [ 21. Although this can be considered a severe limitation of the system, it is not unexpected. In fact, in similar reactions, including the

TABLE 1

Hydroxylation of benzene” with HzOz in CH,CN (10 ml) catalyzed by 1

No. 1 I-W, Charging T t PhOH Yieldb (molX 105) (molX105) (“C) (h) (molX 10’) (%)

1 4.8 20 2 3.4 (70) 2 4.8 24.0 1x24” 20 4 3.9 14 3 4.8 48.0 lX4F 20 7 4.9 9.5 4 4.8 48.0 4x12” 20 8 12.5 24 6 4.8 48.0 5x6’ 20 9 13.7 39 5 4.8 48.0 0.03 eq/mind 40 6 35.0 66 7 4.8 48.0 0.06 eq/mind 50 3 35.5 67 8 2.0 48.0 0.06 eq/mind 50 5 38.0 79

*Benzene 2000 mol X 10e5 bMeasured as (mol phenol/m01 oxidant) X 100; oxidant = 1 +H202. “Defined as number of portions x mol x lop5 of Hz02. dH,Oz added via a syringe pump.

110 M. Bianchi et al./J. Mol. Catal. 83 (1993) 107-116

radical attack on aromatic rings by species such as HO’, phenol is found to be considerably more reactive than benzene [ 71. From a general point of view, it may be recalled that the need to keep the conversion of the substrate at a low level is typical of autoxidative processes such as the autoxidation of saturated hydrocarbons to produce hydroperoxides [ 8 1.

Information provided by our parallel mechanistic studies has revealed that the course of the hydroxylation of benzene by 1 is affected by the presence of strong acids. In particular, it has been observed that added acid enhances the reaction rates measured both as disappearance of the oxidant or as formation of phenol. Such an effect, which might be related to the anionic nature of the active species in solution, tentatively described as a radical anion [ 2,3], has been examined here in greater detail. The relevant results are collected in Table 2.

Inspection of the data of Table 2 confirms that added CH3S03H reduces the reaction times while it does not significantly affect the yields of phenol, provided that the acid is not in large excess over the catalyst. As an example, for a l&fold excess of CH,SO,H over 1 the yield of phenol drops to 45% (entry 5). As observed, in the absence of acid (entry 8 of Table 1 ), and also in its presence, the yield of phenol is increased by decreasing the concentration of 1. At variance with the results obtained in the absence of acids, under acidic conditions the initial concentration of benzene can be lowered up to 0.5 mol

TABLE 2

Hydroxylation of benzene with HZ02 in CH&N (10 ml) catalyzed by 1 in the presence of CH,SOIH

No. PhH 1 H+ Hz& Charging” T t PhOH Yieldb

(molX105) (molX10’) (molX106) (molX105) (“C) (min) (molXl0”) (%)

1 2000 4.8 20 120 3.4 70

2 2000 5.0 1.0 25 <5 3.4 69

3 2000 5.0 5.0 25 <5 3.4 70

4 2000 5.0 10.0 25 <5 3.0 60

5 2000 5.0 77.0 25 <5 2.3 45

6 2000 10.0 10.0 25 <5 3.4 69

7 2000 1.0 1.0 25 <5 0.8 82

8 1000 5.0 5.0 25 15 3.4 69

9 500 5.0 5.0 25 <5 3.4 70

10 100 5.0 5.0 25 <5 3.0 60

11 2000 4.9 5.0 24.5 5x4.9 40 18 20.4 69

12 2000 1.2 1.2 6.0 5x1.2 40 18 5.1 71

13 1000 1.2 1.2 6.0 5x1.2 40 24 5.2 72

14 1000 1.2 1.2 10.8 9x1.2 40 60 7.3 73

15 500 1.1 1.1 10.2 9x1.1 40 60 7.3 75

“Defined as in note c, Table 1

“Defined as in note b. Table 1.

M. Bianchi et al. jJ. Mol. Catal. 83 (1993) 107-l 16 111

1-l without sensibly affecting the yield of phenol (entries 12-15, Table 2)) thus indicating that higher conversions of benzene may be attained.

A further important aspect that should always be taken into consideration in processes like the one examined here is, of course, the stability of the cata- lyst. In particular, one must ascertain the extent to which 1 undergoes some degradation, reducing its catalytic ability. To this end we have measured the yield of phenol obtained after every addition of hydrogen peroxide, both in the absence and in the presence of acid. A decrease of such yields would be an indication of a reduced effectiveness of the catalyst. The results obtained are reported graphically in Fig. 1.

It may be observed that, in the absence of added acid (curve A), a contin- uous decrease of phenol production is observed, so that the yield drops from a value of 77.% obtained for the first addition of hydrogen peroxide to a value of 59% when the fifth portion of the oxidant has been added. The nature of the reaction leading to degradation of the catalyst has not been investigated in detail. At any rate, the possibility that such a reaction is related to the increas- ing amount of water in solution, resulting from the reduction of hydrogen per- oxide, has been directly tested. In fact, such an increase could accelerate the hydrolysis of the picolinato ligand, thus modifying the nature of the vanadium species in solution. However, independent experiments carried out under stoi- chiometric conditions by using 1 (4.8~ 10T3 mol l-l) as an oxidant for ben-

0 I 1 I I 0 2 4 6 8

I rddltlon of HOOH

3

Fig. 1. Decrease of the phenol yield as a function of H,Op addition. (A) 1 (1 x lop3 mol I-‘), Hz02 (1X10-3mo11-‘eachaddition)PhH (lmoll-‘),inCH,CNat60”C. (B) 1 (1x10-3mo11-‘), CH,S03H (1X10-3moll-1)H,0z (1X10-3mo11-‘eachaddition)PhH (lmoll-‘),inCH3CN at 40°C. (C ) Two-phase conditions (see entry 7, Table 3 ) .

112 M. Bianchi et al.fJ. Mol. Catal. 83 (1993) 107-116

zene (1 mol 1-l) in CHJZN in the presence of H,O (0.11 mol l-l) show that the yield of phenol is only slightly lower than that obtained in the absence of water under otherwise identical experimental conditions. In particular, in the absence of water the yield is 62% whereas in its presence it is 59%. It should be mentioned however, that in the presence of much larger amounts of water, i.e. 5 vol.-% in CH,C!N, 1 is practically unreactive.

The data referring to the same experiments but carried out in the presence of 1 X 10T3 mol 1-l CH,SO,H are shown in Fig. 1, curve B. The reader may notice that the addition of the acid has a beneficial effect on the stability of the catalyst. In fact, although the yield of phenol measured after the first ad- dition of hydrogen peroxide is almost the same as that obtained in the absence of the acid (see also the data of Table 2), its value then remains practically unchanged when subsequent portions of hydrogen peroxide are added (from

1 79 to 73% over ten additions of H202). To summarize the information obtained by the experiments in CH3CN

under homogeneous conditions it may be stated that the best conditions for catalytic benzene hydroxylation are those where one uses a large excess of the substrate over the oxidant, hydrogen peroxide. In turn, the use of an excess of the oxidant over the catalyst must be avoided by proceeding to a stepwise ad- dition of hydrogen peroxide. Finally, acidic conditions are recommended to prevent - or at least greatly reduce -the degradation of the catalyst.

Taking advantage of these conclusions, we turned to a two-phase ( HzO/ DCE) system, where the peroxovanadium complex is formed in an acidic aqueous phase simply by addition of hydrogen peroxide to V,O,. Then the ligand HPCA [ 4- (3-heptyl) -pyridine-2-carboxylic acid], which should have coordinating properties almost identical to those of picolinic acid, being, of course, much more lipophilic, extracts the peroxocomplex into the organic phase

- + WO), n vqo2)~- PhOH M-I

Org. Phase = 1,2dichloroethane Ho

=H3

Fig. 2.

TA

BLE

3

Hyd

roxy

lati

on o

f be

nze

ne

wit

h H

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lyze

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V(V

)*

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he

two-

phas

e sy

stem

Hz0

: D

CE

(5

: 5 m

l);

at 6

0°C

No.

P

hH

V

(V)

(mol

x105

) (m

olX

10

5)

Aci

d (t

ype;

mol

x 10

’) H

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(mol

X

105)

H

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Ab

Ch

argi

ng

t P

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(mol

X 1

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(h)

(mol

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)

1

1100

2.0

H

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,; 120

420

2 11

00

2.0

H

,SO

,; 120

200

3 55

0 2.0

H

2S0.

,; 120

200

4 28

0 2.0

H

,SO

,; 120

200

5 1100

2.0

H

,SO

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56

6 1000

1.2

H

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,; 120

28

7 1100

1.2

C

H,S

O,H

; 12

0 28

8 550

1.4

C

H,S

O,H

; 12

0 28

9 550

1.4

C

HB

SO

sH; 6

0 28

10

550

1.1

C

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; 15

29

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4

4

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4

4

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4

5

5

5

5

8x52

7.5

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12.0

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5.5

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15.0

25x8

3.5

22

11.0

25x8

4.3

15

7.5

25x2.5

2.5

13

24.0

25x1.2

2.0

10

35.0

25x1.2

1.2

11

41.2

25x 1

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1.2

8

28.7

25x1.2

1.2

10

34.0

25X

1.2

1.2

12

41.7

25x1.2

1.2

12

40.0

“V(V

) =

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th

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ol X

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“At

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C.

114

TABLE 4

M. Bianchi et al./J. Mol. Catal. 83 (1993) 107-116

Hydroxylation of substituted benxenes ArX with 1 in CHaCN (10 ml) at 40°C

No. X Substrate 1 CH,SOaH H,Os XArOH Yield” (molX105) (molX106) (molX105) (molX10”) (molX105) (W)

1 CH, 2000 4.8 2.7 66’ 2 CH, 2000 4.8 4.8 2.7 68” 3 CH, 2000 4.8 4gb 35.9 68 4 F 2000 4.8 3.1 65 5 F 2000 4.8 4.8 3.2 67 6 Cl 2000 4.8 2.8 58 7 Cl 2000 4.8 4.8 2.8 59 8 Cl 2000 4.8 34b 24.8 63 9 Br 2000 4.8 2.3 49

10 Br 2000 4.8 4.8 2.4 51 11 NO2 2000 4.8 1.5 31 12 NOs 2000 4.8 4.8 2.9 62 13 NO, 1000 1.0 1 1Od 4.1 37

“Defined as note b in Table 1. bH,O, added via a syringe pump. “2% Benzylic alcohol and 8% benxaldehyde also obtained. dHzO, added in 10 subsequent portions.

where the hydroxylation of benzene takes place. The process is depicted in Fig. 2.

There are, in principle, some advantages connected with the procedure summarized in Fig. 2 as compared with the homogeneous case. Among these one may mention the possibility of forming the catalyst in situ instead of iso- lating the peroxocomplex. Moreover, the organic phase may be either an im- miscible solvent containing the substrate, or it may be benzene itself. In both cases, the isolation of the product is easier than from CH,CN.

In fact, even though the yields of phenol are generally lower than those obtained under homogeneous conditions, the data reported in Table 3 appear to confirm that the two-phase system is an attractive procedure. Of course, some of the drawbacks already observed in the homogeneous oxidation are still present here. Thus, the conversion of the substrate must also in this case be kept rather low to maximize the yield of phenol. Here, too, the decomposition of the oxidant takes place to an appreciable extent. However, the yields may be increased, as in the homogeneous system, by keeping the oxidant-vanadium ratio as low as possible, i.e. by proceeding to continuous additions of hydrogen peroxide. The stability of the catalyst (Fig. 1, curve C) is comparable to that observed in CH$N in the absence of acid, probably because only relatively small amounts of acid are transferred into the organic phase from the aqueous one. This appears to be confirmed by the finding that the yields are somewhat larger when the more lipophilic CH3S03H is used instead of H&SO,. In conclu-

M. Bianchi et al./J. Mol. Catal. 83 (1993) 107-116 115

sion, the two-phase system allows the catalytic hydroxylation of benzene to phenol and, on the basis of practical considerations, it may be considered at least as attractive as the homogeneous one.

The last aspect investigated in this work has been the possibility of hy- droxylating substituted benzenes. We considered substrates containing either an electron-withdrawing or an electron-donating substituent, and the corre- sponding results obtained under homogeneous conditions are reported in Table 4.

While the data of Table 4 confirm the general features already observed in benzene hydroxylation, we were pleased to find that all the substrates can be hydroxylated and that the yields are, generally speaking, satisfactory. It should be recalled that the hydroxylation of halobenzenes and nitrobenzene by radicals such as HO* is rather difficult and that the yields of such reactions are rather low [ 91. On the other hand, the hydroxylation under the conditions studied here is rather unselective as far as the isomeric distribution of substi- tuted phenols is concerned. As an example, for toluene and nitrobenzene, we find o : m :p ratios of 58 : 19 : 23 and 26 : 56 : 18, respectively. These results, while providing further evidence for the radical nature of the reaction, may be com- pared with those obtained when HO- is the reacting species. In this latter case the isomeric distribution obtained in the hydroxylation of toluene and nitro- benzene with H,O,/Fe (II) is o : m :p = 55 : 15 : 29, and 24 : 30 : 46, respectively [9]. These findings also have a mechanistic significance which will be dis- cussed elsewhere [ 61.

The possibility of extending the two-phase procedure to substituted ben- zenes has been tested only in the case of nitrobenzene, which has been hydrox- ylated by using reaction conditions identical to those of entry 9 of Table 3. As a result, a 13.5% total yield of isomeric nitrophenols has been obtained.

In conclusion, the data reported above may be taken as evidence that the catalytic hydroxylation of aromatics by HzOz in the presence of picolinato complexes of vanadium is a viable synthetic procedure providing results which are at least comparable with those obtained with other systems involving direct hydroxylation. It is hoped that a better understanding of the reaction may reduce some of the drawbacks revealed by our investigation.

Acknowledgments

This research was carried out within the framework of ‘Progetto Finaliz- zato Chimica Fine II’ of CNR. Financial support by MURST is also gratefully acknowledged.

116 M. Bianchi et al./J. Mol. Catal. 83 (1993) 107-116

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