Strong Organic Acids as Efficient Catalysts for the Chloromethylation

Jan 20, 2009 - These acids stay in aqueous phase after the catalysis, and organic products are easily separated from the reaction mixture. The reactio...
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Ind. Eng. Chem. Res. 2009, 48, 1831–1839

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Strong Organic Acids as Efficient Catalysts for the Chloromethylation of m-Xylene: The Synthesis of 1,3-bis(Chloromethyl)-4,6-dimethylbenzene Tohru Kishida,†,‡ Noboru Ieda,†,‡ Takayoshi Yamauchi,‡ Kenichi Komura,† and Yoshihiro Sugi*,† Department of Materials Science and Technology, Faculty of Engineering, Gifu UniVersity, Gifu 501-1193, Japan, and Nissei Kagaku Kogyosho, 4-1-45 Miyahara, Yodogawa-ku, Osaka 532-0003, Japan

Strong organic acids, CCl3COOH, CHCl2COOH, CF3COOH, CH3SO3H, CF3SO3H, and p-CH3C6H4SO3H, effectively catalyzed the chloromethylation of m-xylene with hydrochloric acid and trioxane under organic and aqueous biphasic conditions. They were active enough at 2-10 wt % against m-xylene to yield chloromethyl2,4-dimethylbenzene (I) and 1,3-bis(chloromethyl)-4,6-dimethylbenzene (II), although they accompanied trischloromethylation products, trischloromethyl-1,3-dimethylbenzenes (III). The highest yield of II, as high as around 70%, was obtained by using CF3SO3H under appropriate conditions because it has the strongest acid strength among the acids. These acids stay in aqueous phase after the catalysis, and organic products are easily separated from the reaction mixture. The reaction may start the formation of chloromethanol from formaldehyde, and strong organic acids can assist the stabilization of chloromethyl carbocation-acid complex formed by the dehydration of chloromethanol. The resulting complex gives I by nucleophilic attack at o- and p-positions of methyl groups, and II is formed through consecutive attack of the complex against I. The chloromethylation of biphenyl (BP) in the presence of strong acids also gave the chloromethylated products; however, large amounts of acid are necessary for the efficient formation of the products due to the low reactivity of BP. Introduction

Scheme 1. Chloromethylation of m-Xylene

Recently, pyromellitic dianhydride and trimellitic anhydride have been used for a wide range of advanced materials for electronic and mechanical purposes.1,2 The aerobic or nitric acid oxidation of durene (1,2,4,5-tetramethylbenzene),3,4 or the hydroformylation of pseudocumene (1,2,4-trimethylbenzene), and subsequent oxidation of the aldehyde5 have been used as current practical procedures. However, there are drawbacks of environmental burden, such as NOx formation, corrosiveness, use of toxic boron trifluoride, etc. The oxidation of 1,3bis(chloromethyl)-4,6-dimethylbenzene is an alternative way to solve the drawbacks6-9 if an environmentally conscious catalyst for the chloromethylation is developed. The chloromethylation of aromatic hydrocarbons has been well-documented in previous papers.6-32 The reaction of aromatic hydrocarbons with hydrochloric acid and trioxane or paraformaldehyde as formaldehyde precursor gave the chloromethylated products without a catalyst, although the rate is slow and not enough for practical chemical processes.13-20 Lewis acids such as zinc chloride, stannic chloride, aluminum chloride, and born trifluoride are well-known catalysts for the reaction.20-26 Among these acids, zinc chloride is an effective catalyst in hydrochloric acid solution;10,11,16,21,22 however, a large amount of catalyst against to substrate is required to obtain a reasonable product yield. These catalysts, in general, suffer from the inherent problems of tedious workup procedures, corrosiveness, high susceptibility to water, difficulty of catalyst recovery, environmental hazards, waste control, etc. It is important to replace these catalysts with environmentally conscious catalysts that are active under mild conditions and can be easily recovered after the reactions and reused for new reactions.32 To attain these purposes in the chloromethylation of aromatic hydrocarbons, we proposed rare earth metal triflates, particularly * Corresponding author. Phone: +81-58-293-2597. Fax: +81-58293-2653. E-mail: [email protected]. † Gifu University. ‡ Nissei Kagaku Kogyosho.

Sc(OTf)3, as the effective catalyst to synthesize chloromethylated hydrocarbons.26-28 Wang et al. recently reported the use of 1-ethyl-3-methylimidazolium tetrafluoroborate, [emim]BF4, in the chloromethylation of aromatic hydrocarbons.29 However, rare earth metal triflates, particularly Sc(OTf)3, and [emim]BF4 are very expensive for the use of practical chemical processes. It is important to develop environmentally conscious catalysts with high activity and reasonable prices. The chloromethylation of aromatic hydrocarbons using strong acids has been proposed by some workers;7,30,31 however, the systematic works on their catalytic properties, such as activity and have not been clarified yet. In this paper, we describe the catalytic properties of some acids in the chloromethylation of m-xylene (see Scheme 1). We found that some of the strong organic acids are active for the chloromethylation of m-xylene under organic-aqueous biphasic conditions and that they are environmentally conscious catalysts. We also applied the strong acids to the chloromethylation of biphenyl (BP) (see Scheme 2). Experimental Section Catalysts. All chemicals were obtained commercially and used without further purification. Reaction Procedures. Typical procedures for the chloromethylation of m-xylene are as follows: a mixture of m-xylene (3.0 g, 28.3 mmol), trioxane (1.3 g, 14.2 mmol), 35% aqueous hydrochloric acid 14.74 g (141.5 mmol), and trichloroacetic acid (93 mg, 0.57 mmol) were stirred in a 30 mL round flask for 5 h

10.1021/ie8014022 CCC: $40.75  2009 American Chemical Society Published on Web 01/20/2009

1832 Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 Scheme 2. Chloromethylation of Biphenyl

Table 1. Chloromethylation of m-Xylene in the Presence of Organic Acidsa yield (%) acid

conversion (%)

I

II

pKab

none CH3SO3H CF3SO3H p-CH3C6H4SO3H CH3COOH CF3COOH CCl3COOH CHCl2COOH CH2ClCOOH C2H5COOH HCOOH H2SO4 H3PO4 ZnCl2c

42 63 99 78 70 88 93 82 62 54 68 64 58 100

39 40 56 53 55 64 57 68 48 45 54 52 46 70

2 22 41 24 14 23 33 12 14 9 13 11 12 25

-2.0 -13.0 -2.6 4.8 0.0 0.77 1.3 2.9 4.7 3.8 2.0 2.2

a Reaction conditions: m-xylene, 3.0 g (28.3 mmol); trioxane, 1.28 g (14.2 mmol); conc. HCl, 14.7 g (141.5 mmol); acid, 2 mmol; temperature, 70 °C; period, 5 h. b See refs 32 and 33. c 28.3 mmol of ZnCl2 was used.

at 80 °C. After cooling, the organic products were extracted with cyclohexane and dried over anhydrous Na2SO4. The solvent was evaporated in vacuo, and organic residue was resolved in cyclohexane again and analyzed by Shimadzu high-performance liquid chromatograph (HPLC) SPD-10Avp using the column of Finepack SIL C-18-5 φ4.6 × 250 mm (JASCO, Tokyo, Japan). Each product was separated by silica gel column chromatography and identified by 1H and 13C NMR, IR spectroscopies, and elemental analysis. 1,3-bis(Chloromethyl)-4,6-dimethylbenzene. Results: m.p. ) 100.7-101.8 °C. IR (KBr): 656 (C-Cl) cm-1. NMR: 1H NMR(CDCl3); 2.386 (s, 6H, CH3), 4.574 (s, 4H, CH2), 7.045 (s, 1H, ArH), 7.254 (s, 1H, ArH) (ppm). 13C NMR(CDCl3); 18.38, 44.34, 131.21, 133.25, 133.51, 137.95 (ppm). Elemental analysis: C, 59.14, H, 5.96% calculated from C10H12Cl2. Observed: C, 59.01, H, 5.97%. Results and Discussion Type of Acid on the Chloromethylation. The chloromethylation of m-xylene was examined with hydrochloric acid and trioxane as formaldehyde precursor by using strong acids as catalysts. Typical results are shown in Table 1 under the following conditions: acid/m-xylene ) 0.02 (mol/mol); trioxane/ m-xylene ) 0.5 (mol/mol); HCl/m-xylene ) 5.0 (mol/mol); reaction temperature ) 70 °C; and reaction period ) 5 h. m-Xylene gave chloromethyl-2,4-dimethylbenzene (I) and 1,3bis(chloromethyl)-4,6-dimethylbenzene (II) without a catalyst; however, the yield of II was CCl3COOH > CH3SO3H ≈ CF3COOH ≈ p-CH3C6H4SO3H ≈ CHCl2COOH. Among the acids, the yields of II by sulfonic acids are due to stronger acidity than those by acetic acids. It is useful to compare the catalysts performances on the basis of the rate of formation of II. Table 2 shows the conversion of

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Figure 1. Influence of reaction period on the chloromethylation of m-xylene. Reaction conditions: m-xylene, 3.0 g (28.3 mmol); organic acid, 0.57 mmol; trioxane, 1.28 g (14.2 mmol); conc. HCl, 14.7 g (141.5 mmol); temperature, 70 °C. Legend: (9), conversion; (O), I; (b), II; (2), III. Table 2. Catalytic Activity of Organic Acids in the Chloromethylation of m-Xylenea yield (%) acid

conversion (%)

I

II

CCl3COOH CHCl2COOH CF3COOH CH3SO3H CF3SO3H p-CH3C6H4SO3H

78 65 70 47 81 69

56 59 54 38 41 52

22 6 16 9 38 16

a Reaction conditions: m-xylene, 3.00 g (28.3 mmol); conc. HCl, 14.7 g (141.5 mmol); trioxane, 1.28 g (14.2 mmol), acid, 0.57 mmol; temperature, 70 °C; reaction period, 2 h.

m-xylene and the yields of II and III after 2 h of reaction. The yield of II (i.e., the rate of formation of II) is the highest for CF3SO3H and is around 10-20% for CCl3COOH, CHCl2COOH, CF3COOH, and p-CH3C6H4SO3H; however, CHCl2COOH gave only 6% yield. These results suggest that the catalytic activity is related to acid strength, as shown in Table 1. It is worthwhile to notice the highest yield of CF3SO3H: this is due to the strongest acid strength among the acids, although the further formation of III was enhanced. Influence of Reaction Temperature. Figure 2 shows the influence of reaction temperature on the chloromethylation in the presence of organic acids: CCl3COOH, CHCl2COOH, and CF3COOH for acetic acids, and CH3SO3H, CF3SO3H, and p-CH3C6H4SO3H for sulfonic acids, under the following conditions: acid/m-xylene ) 0.02 (mol/mol); trioxane/m-xylene )

0.5 (mol/mol); HCl/m-xylene ) 5.0 (mol/mol); reaction period ) 5 h. The catalytic activities of these acids increased with the increase in reaction temperature. I was predominantly formed at lower temperatures; however, the yields of I were saturated for all the acids, and the yields of II increased with the temperatures. Particularly, the yield of II increased with the decrease in the yield of I. The highest yields of II were 50%, 40%, and 30% for CF3COOH, CCl3COOH, and CHCl2COOH, respectively, at 90 °C. The relatively low yields of II may be due to the low amount of trioxane in the substrate. On the other hand, the chloromethylation by sulfonic acids gave similar results by acetic acids at the lower temperatures. The formation of II was enhanced with the increase in reaction temperatures: 55% at 90 °C for CF3SO3H. However, CH3SO3H and p-CH3C6H4SO3H had a lower yield of II (45-50%) among the sulfonic acids: this is due to the lower acidities of CH3SO3H and p-CH3C6H4SO3H compared with the other sulfonic acids. The formation of III was not significant under the conditions. Influence of Acid Amount. Figure 3 shows the influence of acid amount on the chloromethylation of m-xylene at 70 °C by using organic acids: CCl3COOH, CHCl2COOH, and CF3COOH for acetic acids, and CH3SO3H, CF3SO3H, and p-CH3C6H4SO3H for sulfonic acids, that were examined under the following conditions: trioxane/m-xylene ) 0.5(mol/mol); HCl/m-xylene ) 5.0 (mol/mol); reaction period ) 5 h. These acids enhanced catalytic activities even in the presence of small amounts, although I was the predominant product. Further addition of

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Figure 2. Influence of reaction temperature on the chloromethylation of m-xylene. Reaction conditions: m-xylene, 3.0 g (28.3 mmol); organic acid, 0.57 mmol; trioxane, 1.28 g (14.2 mmol); conc. HCl, 14.7 g (141.5 mmol); period, 5 h. Legend: see Figure 1.

the acids increased the formation of II with a decrease of the yields of I. The highest yields of II were 60%, 65%, and 60% for CF3COOH, CCl3COOH, and CHCl2COOH, respectively, and 50-56% for CH3SO3H, CF3SO3H, and p-CH3C6H4SO3H by the addition of 15-20% against m-xylene. In these cases, the increase in the yield of II occurred by relatively smaller amounts of sulfonic acids, CH3SO3H, CF3SO3H, and p-CH3C6H4SO3H. These results show that the amount of acid is an important key for the synthesis of II. The formation of III was increased to 5-15% by the addition of large amounts of acids for all the acids. These acetic and sulfonic acids gave a reasonable level of yields of the chloromethylated products in spite of the low ratio of trioxane to m-xylene. These results mean that formaldehyde produced by the hydrolysis of trioxane is stable from the condensation or another side reaction and works effectively as the substrate of the chloromethylation. Influence of Trioxane. Figure 4 shows the influence of the amount of trioxane on the chloromethylation of m-xylene at 70 °C in the presence of the organic acids: CCl3COOH, CHCl2COOH, and CF3COOH for acetic acids, and CH3SO3H, CF3SO3H, and p-CH3C6H4SO3H for sulfonic acids, under the following conditions: acid/m-xylene ) 0.02 (mol/mol); HCl/ m-xylene ) 5.0 (mol/mol); and reaction period ) 5 h. The conversion of m-xylene increased with increasing the amount of trioxane. The principal product is I at the low amounts for all acids, and the formation of I was saturated in the range of 0.5-1.0 of trioxane/m-xylene ratio. The formation of II was increased with the increase in the amounts of trioxane ac-

companying the decrease in the yield of I. The increase in the yield rapidly occurred for CF3COOH and CCl3COOH: the yields of II reached 55% and 61%, respectively, at trioxane/m-xylene ) 2.5. The chloromethylation with CHCl2COOH produced principally I at low ratios and gradually increased to 50% at trioxane/m-xylene ) 2.5. Similarly, the formation of II was enhanced with the increase in the amount of trioxane by using sulfonic acids, CH3SO3H, CF3SO3H, and p-CH3C6H4SO3H. Particularly, the yield of II reached 60% by using CF3SO3H at trioxane/m-xylene ) 2.5. The catalytic activity of CF3SO3H was higher than those of CH3SO3H and p-CH3C6H4SO3H. These results show that the amount of trioxane was the most important key for the high-yield synthesis of II. However, the increase in the amounts of trioxane accompanied the formation of III, particularly for CCl3COOH and CH3SO3H. The formation of III was accompanied by the decrease in the yield of II in the presence of large amounts of trioxane. However, CF3COOH and CF3SO3H did not form III, even at large amounts of trioxane. They are the best catalysts for the selective synthesis of II among the acids in this study. Influence of the Concentration of Hydrochloric Acid. The influence of the concentration of HCl at 70 °C was examined by using organic acids: CCl3COOH, CHCl2COOH, and CF3COOH for acetic acids, and CH3SO3H, CF3SO3H, and p-CH3C6H4SO3H for sulfonic acids, under the following conditions: acid/m-xylene ) 0.02 (mol/mol); trioxane/m-xylene ) 0.5; and reaction period ) 5 h). The influence of the concentration of HCl was examined by the addition of diluted hydro-

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Figure 3. Influence of acid amount on the chloromethylation of m-xylene. Reaction conditions: m-xylene, 3.0 g (28.3 mmol); trioxane, 1.28 g (14.2 mmol); conc. HCl, 14.7 g (141.5 mmol); organic acid, 0-5.66 mmol; temperature, 70 °C; period, 5 h. Legend: see Figure 1.

chloric acid in water (see Figure S1 in Supporting Information). In the absence of hydrochloric acid, the reaction of m-xylene with formaldehyde is expected to be hydroxymethylated by the attack of protonated formaldehyde species (+CH2OH). However, no such reaction occurred in the absence of hydrochloric acid for all acids. These results mean the chloromethylation is much faster than the hydroxymethylation, even if formaldehyde is activated by strong acid, and that hydrochloric acid is essential for the reaction of m-xylene with formaldehyde. The activities of the chloromethylation increased with the concentration of hydrochloric acid. The principal product was I for the acetic acids, and the formation of II was less than 30% by using concentrated hydrochloric acid. The yield of II was the highest for CCl3COOH among the acetic acids and decreased in the following order: CF3SO3H >> CH3SO3H > p-CH3C6H4SO3H. The higher yield of II was obtained for sulfonic acids than acetic acids at a lower concentration of hydrochloric acid. The yield of II reached 40% for CF3SO3H; however, CH3SO3H and p-CH3C6H4SO3H showed lower activities (22% for CH3SO3H and 28% for p-CH3C6H4SO3H). The formation of III was negligible even in concentrated hydrochloric acid for all acids under these conditions. Among the acids, CF3SO3H had high catalyst performances on the chloromethylation in the high concentration of hydrochloric acid. These results mean that the amount of HCl is a less important factor compared to the other factors, such as reaction period, reaction temperature, acid amount, and trioxane/m-xylene ratio.

Tuning of the Chloromethylation of m-Xylene. From the results discussed in previous sections, the reactions were carried out under the optimized conditions: acid/m-xylene ) 10 (mol/ mol); trioxane/m-xylene ) 2.5 (mol/mol); and HCl/m-xylene ) 5 (mol/mol) at 80 °C for 6 h to know the potentialities of our system (Figure 5). The yield of II was the highest for CCl3COOH and CF3SO3H (70%), and the yields were decreased in the following order: CF3SO3H ≈ CCl3COOH > CHCl2COOH ≈ CH3SO3H ≈ p-CH3C6H4SO3H. The increases in the yield of II accompanied the decrease in the yield of I and the increase in the yield of III. The formation of III for CCl3COOH was particularly high compared to that for CF3SO3H. These results indicate that CF3SO3H has the best performance among the acids, although further tuning of the reaction conditions is necessary for the practical manufacturing of II. Chloromethylation of Biphenyl. Table 3 summarizes typical results of the chloromethylation of biphenyl (BP) in the presence of acids under the following conditions: acid/BP ) 5.0 (mol/ mol); trioxane/BP ) 7.5 (mol/mol); HCl/BP ) 16.2 (mol/mol); reaction temperature ) 80 °C; and reaction period ) 5 h. The reactivity of BP was much lower than that of m-xylene discussed in previous sections. The severe reaction conditions, particularly the large acid amounts, were necessary for high conversion of BP. We examined the reaction by the use of large amount of the acids compared to the case of m-xylene (∼70 times). The principal products under these conditions were 4-(chloromethyl)biphenyl (IVa) and 2-chloromethylbiphenyl (IVb) in

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Figure 4. Influence of trioxane on the chloromethylation of m-xylene. Reaction conditions: m-xylene, 3.0 g (28.3 mmol); organic acid, 0.57 mmol; conc. HCl, 14.7 g (141.5 mmol); trioxane, 0-6.3 g (0-70.1 mmol); temperature, 70 °C; period, 5 h. Legend: see Figure 1. Table 3. Chloromethylation of Biphenyl in the Presence of Acidsa yield (%) acid

conversion (%)

IVa

IVb

Va

Vb

none CF3SO3H p-CH3C6H4SO3H CCl3COOH CHCl2COOH CH2ClCOOH CH3COOH H2SO4 H3PO4

46 92 97 90 99 78 66 74 69

29 50 44 51 30 48 42 47 43

14 23 20 22 26 21 19 20 20

2 13 24 12 31 7 4 6 5

1 6 8 5 13 3 2 2 2

a Reaction conditions: biphenyl, 1.5 g (9.7 mmol); trioxane, 6.55 g (72.7 mmol); conc. HCl, 16.4g (157 mmol); acid, 50 mmol (5 equiv against BP); temperature, 80 °C; period, 5 h.

Figure 5. Tuning of reaction conditions for the chloromethylation of m-xylene. Reaction conditions: m-xylene, 3.0 g (28.3 mmol); organic acid, 2.8 mmol; conc. HCl, 14.7 g (141.5 mmol); trioxane, 6.3 g (70.1 mmol); temperature, 80 °C; period, 5 h.

the ratio (2.0-2.5)/1.0 accompanying the formation of 4,4′bis(chloromethyl)biphenyl (Va) and 2,4′-bis(chloromethyl)biphenyl (Vb) in the ratio 70/30. The highest yield of Va was 31% by using CHCl2COOH. The use of large amounts of acid to 10-70 times against BP enhanced the formation of Va and Vb. However, the use of a large amount of CF3SO3H resulted in the formation of gel products due to further cross-reaction

of the chloromethylated products. Similar differences in reactivity of m-xylene and BP were also observed in the presence of rare earth metal triflates as the catalyst,26-28 although the differences in the reactivity for organic acids in this work were much larger than those for Sc(OTf)3. These results mean that the organic acids in this study are less active than Sc(OTf)3 for the chloromethylation, particularly, of BP. The formation of IVb in addition to Vb with 2,4-substitution was due to the substitution of CH2Cl moiety at o- and p-directions of phenyl group. The isomer distributions in chloromethyl- and dichloromethylbiphenyl using strong organic acids are quite similar to those using the rare earth metal triflates.28 These results suggest that the catalyzed reactions by strong organic acids and rare earth metal triflates occur through

Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 1837 Scheme 3. Plausible Mechanism of the Chloromethylation of m-Xylene by Strong and Soft Organic Acids

the mechanism with similar natures. These differences in m-xylene and BP are due to the solubility of both substrates: BP is much less accessible against the species for the chloromethylation because of lower solubility of BP in water and/or of acids compared to that of m-xylene.34 Catalytic Aspects of Acids in the Chloromethylation. The catalysis under our conditions occurs in a heterogeneous mixture composed of m-xylene, trioxane, and organic acids in aqueous hydrochloric acid. Hydrochloric acid itself cannot accelerate effectively the chloromethylation in spite of its strong acidity. The addition of strong and soft organic acids, such as acetic acids: CCl3COOH, CHCl2COOH, and CF3COOH, and sulfonic acids: CH3SO3H, CF3SO3H, and p-CH3C6H4SO3H, enhanced the chloromethylation of m-xylene. These results mean that these acids act as keys for the catalysis: a strong acid assists the formation of the key intermediate, which cannot be formed from hydrochloric acid. There are tendencies that the catalytic activity depends on the acid strength and softness. These results suggest that the stabilization of the intermediate is important for the reaction. The reaction profiles shown in Figures 1-4 and Figure S1 in the Supporting Information suggest that the chloromethylation occurred consecutively, I to II and II to III. The formation of II and III increased with temperature with all acids. Particularly, the formation of III was enhanced by the strong and soft organic acids such as CF3SO3H and CCl3COOH with a large amount of trioxane. From the results shown in previous sections, we propose Scheme 3 as a plausible mechanism for the catalysis. Formaldehyde was formed from trioxane before the reaction with m-xylene, and that formaldehyde is an actual reactant for the chloromethylation. It is considered that the activation of formaldehyde is a first step for the enhancement of the chloromethylation. First, trioxane yields formaldehyde by acid catalysis of hydrochloric acid (HCl) and the organic strong and soft acid (H+A-). Then, the reaction of formaldehyde and hydrochloric acid gives chloromethanol (VII). This step is accelerated by HCl and/or H+A- through the six-membered transition states VI, and then, VII reacts with H+A- to yield chloromethyl acid complex (IX) with carbocation natures through the transition state VIII. Because strong acids such as CF3COOH, CCl3COOH, CHCl2COOH, CH3SO3H, CF3SO3H, and p-CH3C6H4SO3H have softer natures than HCl and other

inorganic acids, they can easily form the complex IX by the removal of water through the transition states VII. Particularly, CF3SO3H accelerates the formation of IX from VIII because of its highest acid strength. The complex IX reacts with aromatic hydrocarbon to yield I by electrophilic substitution with formation of the acid. The attack of IX against m-xylene occurs in electron-rich o- and p-directions of methyl groups, yielding I because of the inductive acceleration by two methyl groups. The attack against m-position from methyl groups does not occur even though it has less steric hindrance. The chloromethylation of I gives II successively by a similar mechanism because methyl groups of m-xylene enhance the second chloromethylation. Some of III is also formed from II. This mechanism is close to the classical zinc chloride catalysis10,11 and to the catalysis by rare earth metal triflates.26,27 HCl plays important roles for the chloromethylation, although it is not a highly potential catalyst for the total reaction: it accelerates key steps of the formation of chloromethanol (ClCH2OH), resulting in the formation of chloromethyl complex with strong organic acids. The reaction of VII with HCl should be supposed to give CH2Cl2 with two C-Cl covalent bonds; however, no such formation of CH2Cl2 was observed under our conditions. These results suggest that hydrochloric acid cannot act well in the step VII to IX because it is harder than organic acids. The chloromethylation of m-xylene proceeds effectively even at prolonged reaction periods and/or at high temperatures. These results suggest that the formaldehyde species are stable under our conditions, although they are easily oligomerized by aldoltype condensation in the presence of strong acids. The chloromethylation using strong organic acids as the catalysts in this study occurs under organic and aqueous biphasic reaction system. The characteristic feature of the system is easy separation of organic products and aqueous catalyst solution. Separated catalyst solution can be reused after the adjustment of the amounts of trioxane, hydrochloric acid, and acid. Another feature of catalysis by strong acid is the low possibility of the formation of carcinogenic chloromethyl methyl ether and/or bis(chloromethyl)ether, which has been suspicious during the chloromethylation with the catalysis by ZnCl2 and/or in acid solution with limited amounts of water, because the hydrolysis of these chloromethyl ethers is predominant over their formation in the aqueous solvent as reaction medium.

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The compounds with 1,2,4- or 1,2,4,5-substitution of benzene with methyl and chloromethyl groups are important intermediates for advanced materials. m-Xylene gave I and II by the chloromethylation by organic strong acid discussed above. p-Xylene also gives 1,2,4- and 1,2,4,5-substituted benzenes, chloromethyl-2,5-dimethylbenzene and 1,4-bis(chloromethyl)2,5-dimethylbenzene, although p-xylene was less active than m-xylene. These 1,2,4- and 1,2,4,5-substituted benzenes, thus obtained by the proposed chloromethylation, were oxidized to trimellitic and pyromellitic acids by conventional liquid-phase oxidation in acetic acid using cobalt based catalyst. Particularly, II is more easily oxidized than parent durene and 2,4,5-trimethylbenzaldehyde.8,9 These results suggest that the combination of chloromethylation of m-xylene by strong and soft organic acids and subsequent oxidation is a promising and environmentally conscious manufacturing method for trimellitic and pyromellitic acids. These acids also accelerate the chloromethylation of polyaromatic hydrocarbons, such as BP, although large amounts of acids are necessary for the effective formation to chloromethylated products.28 The chloromethylation also occurs through a similar mechanism, although the attack of IX to BP is very slow due to low solubility of BP compared to m-xylene34 under organic-aqueous biphasic condition. Similar differences of reactivities between m-xylene and BP were also observed in the chloromethylation by Sc(OTf)3.26,27 Conclusions Strong organic acids, CF3COOH, CCl3COOH, CHCl2COOH, CF3SO3H, CH3SO3H, and p-CH3C6H4SO3H, are highly active for the chloromethylation of m-xylene. These acids work well in catalytic amounts. Principal products in the chloromethylation of m-xylene were chloromethyl-2,4-dimethylbenezene (I) and 1,3-bis(chloromethyl)-4,6-dimethylbenzene (II) with 1,2,4- and 1,2,4,5-substitutions. The formation of II accompanied the formation of III. CF3SO3H has the best catalytic performances for the formation of II among the acids. The chloromethyl organic acid complex is the key intermediate: hydrochloric acid plays an important role as a source of chloromethyl group in the products, although it cannot work as an effective catalyst for the chloromethylation. The catalysis by strong organic acids easily occurs in heterogeneous mixture of m-xylene, trioxane, and organic acid in aqueous hydrochloric acid. Since the acid stays in the aqueous phase after the reaction, the catalyst was easily separated from the products and recycled for further reaction. Among the acids, CF3SO3H is the most active among the acids for the chloromethylation of m-xylene to I and II. The proposed chloromethylation devotes to environmentally conscious manufacturing for trimellitic and pyromellitic acids by the oxidation of I and II by conventional cobalt-based catalysts. These acids or their anhydrides are key materials for advanced materials. However, the acids proposed in this study are lowly active for the chloromethylation of biphenyl under our conditions because the low solubility of biphenyl in water prevents the catalysis. Further aspects of the catalysis, the application to organic syntheses, and practical chemical processes are under investigation. Acknowledgment A part of this work was financially supported by a Grant-in Aid for Scientific Research (B) 16310056 and 19310060, the Japan Society for the Promotion of Science (JSPS).

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ReceiVed for reView May 16, 2008 ReVised manuscript receiVed November 18, 2008 Accepted November 20, 2008 IE8014022