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OXIDATION OF METHYLBENZENES BY HETEROPOLY COMPOUNDS IN SOLUTION UDC 542.943.7:547.534.2:541.49

T. A. Gorodetskaya, I. V. Kozhevnikov, K. I. Matveev, V. N. Sidel'nikov, and R. I. Maksimovskaya

Methylbenzenes (ArH) are oxidized in solutions of strong oxidizing agents such as Co(III), Mn(III), Fe(III), and Ce(IV) to give mixtures of aromatic alcohols (or esters), aldehydes, acids, and C--C coupling products, namely, diarylmethanes and higher oligomers. The product ratio depends on the reaction conditions [i, 2]. The oxidation of Ar by heteropolyanions CoW1204o s- proceeds analogously [3, 4]. These reactions are noncatalytic since the oxidation of the reduced forms of these metal compounds by oxygen under the reaction is impossible. In the present work, we studied the oxidation of toluene, p-xu mesitylene, durene, pentamethylbenzene, hexamethylbenzene and their mixtures by PMoY~-nV~04o ~3+nJheteropoly compounds (HPC-n, n = 0-4) which proceeds by the equation:

ArH ~ HPC ---~HiHPC @ products

(1)

The reduced form of the HPC, namely, H.HPC (heteropolyblue) is oxidized by oxygen under the conditions of the major reaction. Duelto this property of HPC-n, the oxidation of methylbenzenes may be carried out catalytically similarly to other oxidation reactions in the presence of HPC-n [5]. EXPERIMENTAL We used chemically pure HPC-0. Samples of HPC-n (n = 1-4) were obtained according to Tsigdinos [6] and our earlier work [7] and a sample of K4HCoW~=O4o was obtained according to Baker and McCutcheon [8]. Samples of HCP with n = 0-3 ~ere introduced as acids, while HPC-4 was added as the sodium salt. Chemically pure samples of the methylbenzenes were used without further purification and were oxidized in sealed glass ampuls in aqueous acetic acid acidified with 1 M H2S04 in a homogeneous or two-phase system depending on the amount of acetic acid in the solvent. The experiments in the two-phase system were carried out with rapid mixing. ArH was taken in large excess relative to HPC. The products were separated by ether extraction from the reaction mixture diluted with water. The yields were found by gas--liquid chromatographyon a l-m column packed with 15% Carbowax on Chromatone N-AW-DMCS. The products were identified by IR spectroscopy on a Specord IR-75 spectrometer and by chromato-mass spectrometry using an LKB-2091 spectrometer. The kinetic studies were carried out in the homogeneous system at 25~ The reaction was followed relative to change in the concentration of vanadium(IV) in the heteropolyanion spectrophotometrically using an SF-26 spectrometer at 640 nm and the accumulation of the products by gas--liquid chromatography. Reproducible results were obtained only with the use of HPC solutions maintained for not less than two days to establish equilibrium for the dissociative decomposition of the polyanions. The radicalcations were recorded by ESR spectroscopy on a Bruker ER-200 spectrometer using a rapid-mixing flow apparatus. Solutions of HPC and ArH were mixed upon passing through a T-tube mixer and frozen by liquid nitrogen. The IH, Sip, and 5~V NMR spectra were taken on a Bruker CXP300 spectrometer at about 20~ RESULTS AND DISCUSSION Oxidation Products of Methylbenzenes~ Products (I)-(XIII) were identified (Table i). These products are usually formed in the oxidation of these ArH by strong oxidizing agents. The oxidation of benzene and toluene proceeds very slowly, p-Xylene is oxidized to a mixture of p-methylbenzaldehyde, p-methylbenzyl alcohol, and 2,4',5-trimethyldiphenylmethane. Their analogs were found in the oxidation of mesitylene, durene, and pentamethylbenzene. In the case of hexamethylbenzene, C-C coupling occurs through replacement of a methyl group and forInstitute of Catalysis, Siberian Branch, Academy of Sciences of the USSR, Novosibirsk~ Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheska, No. 5, pp, i010~i017, May, 1984. Original article submitted February 17, 1983.

0568-5230/84/3305-0929508.50

9 1984 Plenum Publishing Corporation

929

TABLE i. Oxidation of Methylbenzenes (ArH) by Heteropoly Compound (HPC) in 74:26 (v/v) Acetic Acid-Water, [HPC] = 0.i0 mole/liter, [H2SO~] = 1.0 mole/liter, 1 h, 90~ i0 ml HPC Solution Volume HPC

ArH, mmotes

Yield, mole % relative to H PC

Products

I

HPC-4 HPC-4 a HPC-4

Toluene, 30. p-Xylene, 30

Benzaldehyde p-Methylbe nzaldehyde (I)

p-Xylene, 30

(I)

HPC-4 b

P-Xylene, 80

HPC - 4 c HPC-4 d

p-Xylene, 30 p-Xylene, 80

HPC~4

Mesitylene, 30

HPC-4 e

Duren% 3

4

p-Methylbenzyl alcohol (II) 4,2',5'-Trimethyldiphenylmethane (III)

(I) (H) (IIi) 1,4-Dimethyl- 2-bromobenzene (hi) (I) . (ii). (m) 3,5-Dimethylbenzaldehyde (IV) 3,5-Dimethylbenzyl alcohol (V) 3,5,2',4',6 - Pe nta methyldiphe nylmethane (VI) 3,4,6-Trime,thytbenzaldehyde (VII) 2,4,5,2',3 ,5 ,6'-Hepta'methyldiphenylmethane (VIII) \

/era -.//--% ...~/

~--\\=/ /

\

/

\

4 16 i7 34 22 65 71 30 84 24 10i

200 ll 13 42 18 25

/

\OH2--~----)(IX)§ /~\

20

+ 2 isomers Tetramer

HPC-0 c f C oW1%o HPC-4 e HPC-4 e

Durene, 3 Durene, 3 Pentamethylbenzene, 5

(VIII)

(viii)

2,3,5,6-Tetramethylbenzaldehyde (X) Nonamethytdiphe@lmethane (XI) (2 isomers) Hexamethylbenzene, Pentamethylbenzyl acetate (XII) 5 Decamethyldiphenylmethane (XIII)

12 50 Traces 120 i6 i2

aIn water. bin the presence of 0.05 mole/liter HBr. CIn 92:8 (v/v) HOAc--H20. dThree cycles of ArH oxidation and HPC reoxidation by oxygen: [HPC-4] = 0.05, [H2S04] = 0.50 mole/liter, P02 = 0.9 MPa. e3 h. f[K4HCoWIa04o] = 0.05 mole/liter. mation of diphenylmethane (XIII). This was noted previously for the reaction of hexamethylbenzene with other oxidizing agents [9]. The oxidation of mixtures of different ArH gave mixed coupling products (XIV)-(XXI) (Table 2). Mass spectral data for the products of oxidative coupling are given in Table 3. Molecular ions were detected for all the products. The presence of methylene bridges in the coupling products was confirmed by the detection of CH2 groups by PMR spectroscopy. The oxidation of methylbenzenes with four or more methyl groups gives, in addition to diarylmethanes, oligomers of a higher order (see Tables 1 and 2). These products are insoluble in aqueous acetic acid and are readily separated from the reaction mixture. Thus, the oxidation of durene by HPC-4 gives a white product with mp 232-236~ and molecular weight 437 (found ebullioscopically in benzene) in 20% yield relative to HPC. Chromato-mass spectrometric analysis indicated that this product consisted of a trimer and a tetramer. The trimer consisted of three isomers of undetermined composition (see Table 3). The probable structure of one of these isomers is (IX). We should note that the tetramer was previously found in the oxidation of durene by the action of the HPC + Pd(II) redox system [i0]. It was mistakenly considered to be the product of the coupling of the ArH systems catalyzed by Pd(II). In fact, it is a polyarylmethane and Pd(II), as we have established, has virtually no effect on this reaction.

930

TABLE 2. Competitive Oxidation of Methylbenzenes (ArH) by Heteropoly Compounds (HPC) in 74:26 (v/v) Acetic Acid-adater, [HPC] = 0.i, [H2SO~] = 1.0 mole/liter, 3 h, 90~ I0 ml HPC Solution Volume HPC HPC-2* HPC-4

.~rH, mmoles

Yield, mole % relative to HPC

Products

Dnrene, 2 + benzene 2O Durene, 2 + toluene, 20

VII) t3,4, 6-T rimethyldiphenytmethane (XIV) (vii) (viii)

6 5 13 24

81

HPC-4

3,4,6,2",4',6'-Hexamethyldiphenyimethane (XVI

Durene, 2 + mesitylene, 20

/

-.//--5-cm-/~/

_

/%/HPC-4

H PC-2* HPC-2*

Durene, t + naphthalene, 2

-\

i5

_/--t-isomer (xvii)

/

(VII)

Pentamethylbenzene, 0.4 + benzene, 20 Hexamethylbenzene, 0.4 + benzene, 20

7

(vnI) 2,4,5-Trimethylbenzylnaphthatene + isomer (XVIII) 2,3,5,6-Tetramethyldiphenylmethane and 3,4,5,6-tetramethyldiphenylmethane (XtX) 2:3,4,5, 6- Pe nt amethyldipt~enylmetha ne (XX) \

/

-- /\ /~-/--G -

/

120

\

\

/

~..x a s//7----K ~..n ~ -,7--~ u --\=/--u -\=/--. \

/

a-isomer (xxI)

6

20 4 5 8

*In 92:8 (v/v) HOAc--H20. In the case of the oxidation of p-xylene, the yield of the coupling products increases the amount of water in the solvent; the yields of alcohol and benzaldehyde drop concurrently (see Table I). The reaction is markedly accelerated in the presence of bromide ions. However, in this case, bromination of the ArH system occurs along with the oxidation [ii]. These results are considered below. The composition of the products of reaction (i) is virtually independent of the type of HPC (HPC-n with n from 0 to 4, CoW~2). In the reaction with HPC-n with n > 0, V(V) is the major oxidizing agent. This is supported by the finding that the weaker oxidizing agent HPC-O is significantly less active (see Table i). The product yields in the noncatalytic oxidation of methylbenzenes is not greater than the amount corresponding to the amount of V(V) in the HPC. The strong one-electron oxidizing agent CoIIIw12 (Eo = 1 V [5]) quantitatively oxidizes durene to (VIII). Reaction (i) with HPC-n may be carried out catalytically by introducing reoxidation of the blue by oxygen. This is more readily accomplished for n ~ 2 [5]. In the catalytic variant, the products of the oxidation of p-xylene by HPC-4 and unreacted ArH were extracted wit[ heptane and the solution of the blue was oxidized by oxygen in an autoclave with 0.9 MPa oxygen pressure at 90~ for 1 h. Repeated oxidation of p-xylene was then carried out. The activity of the starting and regenerated catalyst was the same. Three oxidation cycles gave products (I)-(III) in 325 mole % total yield relative to HPC-4 (see Table i). Kinetics and Mechanism. The generally accepted mechanism for the oxidation of methylbenzenes by metal compounds includes the formation of an aromatic+radical-cation as an intermediate which is converted to carbocation by the elimination of H and further oxidation [i,

2] ,+ - - H

ARCH3 Ye~ ARCH3 The attack

of

Ar'H

by ArCH~ leads

to

the

+

. --e i

+

, > ARCH2 - - - ~ A r C H ~

formation

of

the

coupling

ARCH2 + q- A r ' H - ~ A r C H 2 A r ' q- H +

(2) product

(3)

931

TABLE 3. Mass Spectra of the Coupling Products of Methylbenzene (molecular ions are underlined) ~m/z, %

Compound (III)

195, [ 21O, 100 1 73

i18, ] i80, 70 24

165, 22

t05, 20

(vI)

223, t00

238, 75

132, 68

1t9, 224, 23 I 22

193, 20

(VIII)

t46, 100

132, 38

2 6 6 , 25t, 36 27

133, 17

.(IX) (3 isomers)

i32, I00 399,

t96, if

77, 16

17~

133.

231 E

91,

20~

147, 16

1t7

9'. H

131, i0

267

146, 100

399, t33, 265, 278, 58 58 ~ 40 33 2 6 5 , 278, :400, 266, 69 47 37 36 266, 25t, 3 9 9 , 265, 56 41 23 t4

t46, 27 25t, 32 147, 14

40( 24 146 29 267 13

26{ 2( 26~ 12 278 i3

t58, i0 279, t2 57, 10

279 tC 383 i0 400 9

D urene itetramer

132, t00

398, 83

399, 26

266 t2

263 i3

78, II

249 9

(XI) (2 isomers)

t60, 100 t46, t00

t46, 2 9 4 , 279, 98 - - ~ 50 160, 294, L 279, 95 40 29

147, 26 i47~ 26

161, 19 t61, 17

145 17 t31 16

9i 15 91 13

280, 14 145, t2,

131 12 i36 12

(XlII)

160, 100

308,, 136, 24 22

16t, 18

293, t8

145, 8

309 7

91 7

105, 7

159. 7

(XIV)

195, 100

210,. t80, 65 21

165, 20

91, 20

t79, 17

196, 16

89 i4

i19, 14

2it i3

(xvi)

132,252,237,

t33,117,

100 132, 100 132, t00

25 22 t33,237, 43 31 t33, 37t, 50 29

317 ,71

tt 250, -'28 '! 16 237, 250, 28 18

91, 8

.)38, 6

207, 6

)-22, 5

i05, 5

207, 12 91, 9

91, 12 tt7, 9

105, tl 372, 9

!51, 9 ;56, 7

t17. 8 207, 7

245, 100 245, 100

260, 99 260, 64

132, 93 132, 55

230, 69 26I, 34

tt5, 55 tt7, 28

229, 51 115, 17

261, 47 246, t6

[4t, 44 91, 15

~15, 43 29, 14

246, 43 t41, i3

209, 224___2 _, 133, t00 82 24 209, 224, I 179, 100 69 21

91, 22 i33, t9

194, 2t t94, t8

t79, 20 91, t7

210, 19 210, 17

A7, 19

77, I

46, t7 93,

11

10

225, 16 225, t0

238, 100

223, 74

147, 42

239, 18

224, 17

208, 16

9t, 15

60, 15

6t, 13

193, 12

t46, 100 132, I00

397, 43 133, 67

t33, 39 397, 46

265, 24 265, 38

t47, 24 278, 24

132, 2O 398, i6

249, 19 266, t0

,98, 16 58,

63, 12 35, 8

145, 10 91, 8

100

(xwt) (2 isomers)

(XVlII) (2 isomers)

(xlx) (2 isomers)

-(xx) ~'(xxI) (2 isomers)

~

I

t33, 68

2 6 5 , 278, 49 37

8

i[

531, 9

The reaction of ARCH2 + with the solvent gives an alcohol or ester, which may be oxidized further to the aldehyde or acid ARCH2+ + H 2 0 - + A r C H 2 0 H

+ H+

ArCH~ + + H O A c - + A r C H 2 O A c + H

(4) +

(5)

ArCH~ + radical-cations were detected by ESR spectroscopy in the oxidation of methylbenzenes by metal salts [12, 13]. The above results for the products of reaction (i) are in accord with mechanism (2)-(5). The solvent effect on the ratio of the oxidation products may be explained by competition of steps (3)-(5). 932

[Y(/tg]/0 motes/liter