Ozonolyses of Cyclopent-1-enylbenzenes in Micellar Aqueous Solutions

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Ozonolyses of Cyclopent-1-enylbenzenes in Micellar Aqueous Solutions Araki Masuyama,* Satoru Yamakawa, and Masatomo Nojima Department of Materials Chemistry, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan Received July 13, 2001. In Final Form: September 4, 2001 Ozonolyses of a series of water-insoluble cyclopent-1-enylbenzenes were carried out both in water and in micellar aqueous solutions composed of sodium 1-dodecyl sulfate (SDS) or polyoxyethylene (10) octylphenyl ether (TX-100). Because the conversion of every substrate in the micellar systems was higher than that in water, the solubilization of substrates by micelles was certainly effective in promoting the ozonolyses of insoluble substrates in an aqueous medium. Ozonolyses of these substrates yielded the corresponding water-participation products, oxoaldehydes, and the corresponding water-nonparticipation products, secozonides, almost exclusively. In water, sec-ozonides were formed predominantly in the ozonolyses of all substrates. The product distribution in micellar systems drastically varied with the combination of substrate and surfactant. The difference in the solubilizing mode of substrates by micelles seems to be the key in determining the product distribution. NMR studies on the location of substrates solubilized in SDS or TX-100 micelles supported this speculation very well.

Introduction Ozone has proved to be a powerful and clean oxidizing reagent in organic chemistry. It can be produced on the spot by passing oxygen (or air) through a generator, which is available from many types of equipment with various capacities. These characteristics make ozone an environmentally friendly oxidizing reagent in many chemical processes.1 Especially, ozone has been used in many filtration plants for supplied water and/or for high-level treatment of wastewater as we have already mentioned in a previous paper.2 By the way, the replacement of organic solvents by water in organic reactions is becoming a significant subject from the standpoint of environmentally benign processes.3 In connection with this, ozonations or ozonolyses of organic substrates in aqueous media are also oxidation methods of great promise. Yet, in contrast to the ozonolyses in organic solvents, there are only a small number of fundamental studies on the ozonolyses of alkenes in aqueous media. One of the serious problems of organic reactions in aqueous media is the poor solubility of most organic compounds in water. The solubilization of insoluble organic compounds into micelles or vesicular aggregations made from amphiphilic molecules in water is an effective * To whom correspondence should be addressed. Fax and telephone: +81-6-6879-7930. E-mail: [email protected]. (1) (a) Bailey, P. S. Ozonation in Organic Chemistry; Academic Press: London, 1978; Vol. 1 and 1982; Vol. 2. (b) McCullough, K. J.; Nojima, M. Peroxides from Ozonation. In Organic Peroxides; Ando, W., Ed.; John Wiley & Sons: Chichester, 1992; pp 661-728. (c) Ho, R. Y. N.; Liebman, J. F.; Valentine, J. S. Overview of the Energetics and Reactivity of Oxygen. In Active Oxygen in Chemistry; Foote, C. S., Valentine, J. S., Greenberg, A., Liebman, J. F., Eds.; Structure Energetics and Reactivity in Chemistry Series 2; Blackie Academic & Professional: London, 1995; pp 13-15. (d) Matsui, M. Ozonation. In Environmental Chemistry of Dyes and Pigments; Reife, A., Freeman, H. S., Eds.; John Wiley & Sons: New York, 1996; pp 43-60. (2) Masuyama, A.; Endo, C.; Takeda, S.; Nojima, M.; Ono, D.; Takeda, T. Langmuir 2000, 16, 368 and references therein. (3) (a) Li, C.-J.; Chan, T.-K. Organic Reactions in Aqueous Media; John Wiley & Sons: New York, 1997. (b) Organic Synthesis in Water; Grieco, P. A., Ed.; Blackie Academic & Professional: London, 1998.

procedure to solve this intrinsic problem.4 In the case of reactions of organic substrates with ozone, some research groups have reported ozonolyses of unsaturated lipids or related compounds in aqueous micellar or vesicular systems mainly to investigate the damage to the living tissues caused by inhaled ozone.5 In this work, we explored the ozonolyses of simple alkenes in water and in micellar aqueous solutions composed of conventional sodium 1-dodecyl sulfate (SDS) or polyoxyethylene (10) octylphenyl ether (TX-100). We selected a series of substituted or unsubstituted cyclopent1-enylbenzenes as substrates because we had already confirmed that the ozonolysis of cyclopent-1-enylbenzene (1a) proceeded cleanly and afforded the corresponding secozonide, 1-phenyl-6,7,8-trioxabicylo[3.2.1]octane, almost quantitatively in nonparticipating solvents such as ether.6 An outline of the reaction and a list of the substrates used in this work are summarized in Scheme 1. The conversion of substrates and the molar ratio of reaction products, oxoaldehyde 3/ozonide 2, were evaluated in water and in micellar aqueous systems. As shown below, the 3/2 ratios were significantly influenced by the substituent on substrates 1 and the nature of the surfactants. To understand this, we tried to get information about the location of substrates solubilized in surfactant micelles by the NMR technique. Results and Discussion Preliminary Experiments. Before starting this study, we had expected that the ozonolyses of cyclopent-1enylbenzenes yielded different products depending on the (4) (a) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: London, 1975. (b) Masuyama, A.; Fukuoka, K.; Wakita, M.; Nojima, M. Chem. Commun. 2000, 1727 and references therein. (5) (a) Aravena, D.; Lissi, E. A. Free Radical Biol. Med. 1985, 1, 327. (b) Giamalva, D. H.; Church, D. F.; Pryor, W. A. J. Am. Chem. Soc. 1986, 108, 6646. (c) Pryor, W. A.; Das, B.; Church, D. F. Chem. Res. Toxicol. 1991, 4, 341. (d) Pryor, W. A.; Wu, M. Chem. Res. Toxicol. 1992, 5, 501. (6) Kawamura, S.; Takeuchi, R.; Masuyama, A.; Nojima, M.; McCullough, K. J. J. Org. Chem. 1998, 63, 5617.

10.1021/la011081v CCC: $20.00 © 2001 American Chemical Society Published on Web 11/01/2001

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Masuyama et al. Scheme 1

Scheme 2

reaction medium as follows: Ozonolyses of substrates dispersed in water would afford the corresponding waterparticipation products (oxoaldehydes), while ozonolyses of substrates solubilized in micelles would give the corresponding nonparticipation products (ozonides) as illustrated in Scheme 2. The results of preliminary experiments, however, were quite different from this expectation. Ozonolysis of cyclopent-1-enylbenzene (1a) in water gave the corresponding ozonide 2a as the major product (the average molar ratio of 3a/2a was 0.7), while the reaction in a SDS micellar solution resulted in the predominant formation of the corresponding oxoaldehyde 3a (the average molar ratio of 3a/2a was 3.3). These results prompted us to explore the ozonolyses of cyclopent-1-enylbenzenes bearing various

substituents systematically both in water and in micellar solutions. Ozonolyses of Cyclopent-1-enylbenzenes (1a-e) in Water, in SDS Micellar Solutions, and in TX-100 Micellar Solutions. All substrates 1a-e used in this work (10 mg) were not soluble in water (20 mL); compounds 1a, c, d were oily droplets, and compounds 1b, e were powdery solids dispersed in water under our ozonolytic conditions. In this work, a 30 mM solution of TX-100 was used as a nonionic surfactant micellar medium because this TX-100 concentration was enough to solubilize 10 mg of every substrate completely. The resultant system after sonication toward 20 mL of an aqueous dispersion composed of SDS (160 mM) and substrates 1b-e (10 mg) was slightly turbid. This is consistent with the fact that

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Figure 1. Conversion of substrates 1 and molar ratio of oxoaldehydes 2/ozonides 3 in ozonolyses of cyclopent-1-enylbenzenes (1a-e) (a) in water, (b) in SDS (160 mM) micellar solutions, and (c) in TX-100 (30 mM) micellar solutions. Each plot is the mean value which is obtained by adding all the individual measurement values and dividing the sum by the number of measurements (at least five times). An error bar means the range of obtained values.

nonionic surfactants are better solubilizing agents for nonpolar organic compounds than are anionic surfactants bearing the same lipophilic chain length.7 In contrast, substrate 1a was solubilized completely in a 20 mM solution of SDS, and the resultant solution became clear. It was confirmed that the result of ozonolysis of substrate 1a in a 20 mM solution of SDS was identical with the result for the same substrate carried out in a 160 mM solution of SDS, and, therefore, a 160 mM solution of SDS was applied to the ozonolyses for all kinds of substrates in the later study. Figure 1 shows the results of ozonolyses of a series of substrates 1a-e (a) in water, (b) in SDS (160 mM) micellar solutions, and (c) in TX-100 (30 mM) micellar solutions. In each graph, the numbers on the left y-axis indicate the molar ratio of oxoaldehydes 3 to ozonides 2, and the figures on the right y-axis mean the conversion/% of substrates 1 after the reaction with 10 equiv of ozone. The conversion of every substrate both in the anionic and in the nonionic micellar systems was greater than the conversion in water. This clearly demonstrates that the solubilization of substrates by micelles is certainly effective in promoting the ozonolyses in aqueous medium. The efficiency of ozonolyses in nonionic micellar systems is higher than that in anionic micellar systems. The difference may be ascribed in part to the higher solubilizing ability of TX-100 micelles. Since aqueous dispersions of SDS and substrates 1b-e are slightly turbid as mentioned above, two states consisting of both the dispersion of substrates and the solubilization of substrates into SDS micelles are suggested. This effect may induce lower conversions for 1c-e using the SDS system. The conversions of the substrates are relatively large for 1a, b as compared with those for 1c-e in water. Yet this reason is not clear. The results of the product distribution in water were particularly surprising; water-nonparticipation products, ozonides 2, were formed predominantly in ozonolyses of all substrates. In contrast to this, every substrate was reacted with ozone to afford the corresponding waterparticipation products, oxoaldehydes 3, as the major product in the SDS micellar systems. These results were quite different from our original expectation as depicted in Scheme 2. The product distribution in the micellar systems varied widely depending on the structure of (7) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley & Sons: New York, 1989; Chapter 4.

substrates and the type of surfactants. In the SDS micellar systems, ozonolysis of substrate 1d gave the corresponding oxoaldehyde 3d as the major product, but the average molar ratio of 3d/2d was only 1.2. Substrate 1e bearing an n-octyl chain on the benzene ring was predominantly converted to the corresponding oxoaldehyde 3e by ozonolysis (3e/2e ) 13.6). The profile of the product distribution in the TX-100 micellar systems was different from that in the SDS micellar systems. As shown in Figure 1c, oxoaldehydes were predominantly formed in the case of substrates 1a and 1b, and ozonides became major products in the ozonolyses of substrates 1c-e. How can we explain these unexpected and eccentric results? The following speculations were proposed. First, water-insoluble substrates were reacted with ozone in an oily droplet or a solid state, so water-nonparticipation products would be predominantly formed in water. Only reaction with ozone on the surface of droplets or solids of substrates afforded the corresponding water-participation products. Second, a wide variety of product distributions would be attributed to the difference in the solubilizing manner by micelles toward substrates. Taking the second point into consideration, NMR studies on the location of substrates solubilized in micelles were carried out. NMR Studies on the Location of the Benzene Moiety of Substrates Solubilized in SDS or TX-100 Micelles. Plenty of methods have been proposed to estimate the location of organic materials solubilized in micelles.7 One of the convenient methods is to detect alterations in the NMR chemical shifts of various protons of surfactant molecules constituting micelles induced by the ring current of aromatic moiety of solubilizates.8 In general, upfield shifts are observed in the protons of surfactants located in the neighborhood of the aromatic ring(s) of solubilizates embedded in the micelles. So, the upfield shift (∆δ) of every position of protons of SDS or TX-100 (50 mM in D2O) by the addition of cyclopent-1enylbenzenes (6 mM) was monitored at 25 °C in this work.9 The changes in the chemical shift of these surfactants by the addition of benzophenone were also measured under the same conditions as a reference. Figure 2 shows the (8) (a) Fendler, E. J.; Day, C. L.; Fendler, J. H. J. Phys. Chem. 1972, 76, 1460. (b) Ganesh, K. N.; Mitra, P.; Balasubramanlan, D. J. Phys. Chem. 1982, 86, 4291. (c) Fornasiero, D.; Grieser, F.; Sawyer, W. H. J. Phys. Chem. 1988, 92, 2301. (d) Wasylishen, R. E.; Kwak, J. C. T.; Gao, Z.; Verpoorte, E.; MacDonald, B.; Dickson, R. M. Can. J. Chem. 1991, 69, 822.

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Figure 2. Aromatic substrate (6 mM)-induced changes in the 1H NMR chemical shifts, ∆δ, of the various protons in the surfactant molecules (50 mM in D2O, at 25 °C): (a) SDS and (b) TX-100. Positive values mean upfield shift of protons of surfactants by the addition of substrates.

relation between the ∆δ value and the position of the protons of (a) SDS and (b) TX-100. In the case of SDS micelles, the benzene ring of substrates 1c, d bearing a tert-butyl substituent seems to be located in the hydrophobic core part of the micelle on the average. The benzene ring of three other substrates 1a, b, e, however, seems not to be located in the hydrophobic core part of the micelle because the upfield shift is very small at any proton of surfactants. The latter results were in great contrast to the results in the ∆δ value by the addition of benzophenone. In the case of TX-100 micelles, it is noticed that the ∆δ values at the D, E, F, and G protons of TX-100 are relatively large for substrates 1c-e bearing a tert-butyl or an n-octyl substituent. So it is estimated that the benzene ring of these substrates will be located in the hydrophobic core part of the micelle on the average. Estimation of Solubilizing Modes of Cyclopent1-enylbenzenes in Micelles. Solubilizing manners of substrates in SDS or TX-100 micelles can be imagined on the basis of the results of both the product distribution and the chemical shift changes of surfactants by the addition of substrates. Of course, the following models may be simplified too much, but these profiles illustrate the effect of the microscopic environment of the reaction medium on the product distribution in ozonolyses of a series of cyclopent-1-enylbenzenes very well. A stylized illustration of the typical solubilizing manner of substrates 1a-e in the SDS and TX-100 micelles is shown in Figure 3. (9) The concentration of substrates used for NMR measurements is high as compared with those for the ozonolysis. Yet it has been already reported that the solubilizing mode of probes into micelles will not change drastically until the probe/surfactant molar ratio goes up to at least 0.3 in the study on the chemical shift changes of surfactants by the addition of substrates. In our NMR study, the ratio was 0.12 ()6 mM/50 mM).

In the case of SDS micelles, it is surmised that a large portion of substrates 1a and 1b will be solubilized in the neighborhood of the hydrophilic shell part of the micelles on the average. The benzene ring of 1a and 1b will stick out into the bulk phase because the ∆δ values of the protons of SDS at all positions are small and the waterparticipating products, oxoaldehydes 3a and 3b, are predominantly formed. It has been also reported that aromatic solubilizates, such as 1-methylindole or acridine, are located near the headgroup region of the SDS micelles.8b In the case of substrate 1c with a tert-butyl group, the benzene ring will be embedded in the hydrophobic part of the micelles. Moreover, in the case of substrate 1d, not only the benzene ring but also the cyclopentene moiety will be embedded in the hydrophobic part on the average because the additional n-butyl chain in 1d will contribute to the lipophilicity of the substrate. The drastic depression of formation of oxoaldehyde 3d supports this solubilizing model. In contrast to this, the maximum formation of oxoaldehyde was found for the ozonolysis of 1e bearing an n-octyl chain. The n-octyl chain will be effectively solubilized in the core part, and the cyclopentene moiety will stick out into the bulk phase. In the TX-100 micelles, the remarkably high oxoaldehydes 3/ozonides 2 ratio in ozonolyses and the upfield shift in 1H NMR of substrates 1a and 1b lead to a speculation that the cyclopentene ring in these substrates will stick out into the bulk phase or locate in the hydrophilic shell part of the micelles. NMR studies demonstrate that the solubilizing manner of substrates 1c-e bearing lipophilic substituents on their benzene ring in the TX100 micelles is quite different from that in the SDS micelles. Consistent with this, ozonolyses of these substrates in the TX-100 micelles afforded the corresponding ozonides as the major products. The benzene moiety of

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Figure 3. A stylized illustration of the typical solubilizing manner of substrates 1a-e in the SDS (the upper line) and TX-100 (the bottom line) micelles. A gray-shaded circle in every micelle is imaged as a hydrophobic core part.

these substrates will be located in the core part of TX-100 aggregates because the upfield shifts of protons in the lipophilic chain of TX-100 by the addition of these substrates are much higher than those by the addition of substrates 1a and 1b. It should be additionally mentioned that the conversion process from substrates to the corresponding oxoaldehydes by ozonolysis might be influenced by the surface properties of micelles; the charged surface and the noncharged surface on micelles will probably make a different contribution to the solubilization of substrates into micelles. Unfortunately, an ambiguous explanation on this point is brought out in this study. So the corresponding charged and water-insoluble substrates will be designed to explore the effect of the surface properties of micelles on ozonolysis in a micellar system. In summary, efficiency of ozonolyses of cyclopent-1enylbenzenes in aqueous media is improved by application of micellar systems to these reactions. In addition, the distribution of ozonolytic products drastically varies with both the structure of substrates and the kind of surfactants. For the latter issue, it is surmised that the difference in the solubilizing mode by micelles toward substrates will be the key to the explanation of the product distribution. Information about ozonolyses of alkenes in water or in micellar solutions obtained in this work will contribute to the promotion of application of this methodology to laboratory or industrial processes. Experimental Section Materials. SDS (specially prepared reagent grade) was purchased from Nacalai Tesque, Inc. (Kyoto, Japan). Its purity was confirmed by its 1H NMR in D2O and by measuring the relation between the surface tension and concentration in water. Its critical micelle concentration (cmc) was found to be 8.0 mM at 25 °C, and no hysteresis was observed in the plots of surface tension versus concentration for this reagent. TX-100 (practical grade) was purchased from Wako Pure Chemical Ind., Ltd. (Osaka, Japan) and was used without any treatment. Its cloud

point (Tcp, 1 wt %) and cmc (at 25 °C) were 69 °C and 0.25 mM, respectively. A series of substrates, cyclopent-1-enylbenzene (1a), 1-(cyclopent-1-enyl)-4-methoxybenzene (1b), 1-tert-butyl-4-(cyclopent1-enyl)benzene (1c), 1-tert-butyl-4-(3 or 4-butylcyclopent-1enyl)benzene (1d), and 1-(cyclopent-1-enyl)-4-n-octylbenzene (1e), were prepared from the corresponding bromobenzene derivatives, and cyclopentanone was prepared by the established Grignard reaction followed by a dehydration reaction. These compounds were finally isolated by Kugelrohr distillation under reduced pressure or by silica gel column chromatography. Authentic ozonides 2a-e were prepared by the ozonolysis of the corresponding cyclopent-1-enylbenzenes (1a-e) in ether at -78 °C.6 Authentic oxoaldehydes 3a-e were obtained by the reduction of the corresponding ozonides 2a-e with triphenylphosphine in benzene at room temperature. Purification methods and the characteristic data of newly prepared compounds are given in the Supporting Information. Methods. 1H and 13C NMR spectra were recorded with a JEOL JNM-EX270 spectrometer. Normal mass and high-resolution mass spectra (HRMS) were measured on a JEOL JMS-DX303 mass spectrometer. The surface tension of surfactants was recorded at 20 °C with a Wilhelmy tensiometer (Kyowa CBVPA3; platinum plate). Ozonolysis was carried out with a Nippon Ozone model ON1-2 ozonator; dry oxygen containing about 2% ozone was introduced at a speed of 50 L h-1 (1 mmol of ozone per 3 min) to an aqueous dispersion or to a micellar aqueous solution of surfactant. Typical procedures of ozonolysis are as follows: A substrate was dissolved in a micellar aqueous solution of surfactant (50 mL) at room temperature. The concentration of substrate was set at 2.5 mM. The concentration of surfactants in each reaction system has already been mentioned in the Results and Discussion section. Twenty milliliters of the solution was poured into a two-neck pear-shaped flask equipped with an ozoneinlet glass tube and a squibb type of glass adapter (inside volume: ca. 50 mL) for exhausting gas and quenching foam. Ten equivalents of ozone toward substrate were bubbled into the dispersion or micellar solution containing a substrate. When heavy foam was developed, a few drops of n-propanol were added to the solution as a foam-quenching agent. It was confirmed that this quantity of n-propanol did not participate in the ozonolysis of a series of substrates at all. It was confirmed by the NMR

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measurement that these ozonides did not decompose in a mixture of water and MeCN at least for 12 h at room temperature. The aqueous solution was extracted with ether (2 × 100 mL), and the combined ether extract was dried with MgSO4. After careful evaporation of the solvent, 10 mL of eluent (MeCN:H2O) containing ethylbenzene as an internal standard for HPLC analyses was added to the residue. The resultant solution was directly injected into a HPLC apparatus [column, TOSOH ODS80Ts (4.6 mm i.d. × 150 mm length); column temperature, 40 °C; flow rate, 0.80 mL min-1; detector, TOSOH RI-8020 (refractive index detector)]. Three kinds of eluents with different compositions were used according to the reaction systems. The volume ratios of MeCN to H2O were 6/4 for the ozonolytic products of substrates 1a and 1b, 8/2 for substrate 1c, and 9/1 for substrates 1d and 1e. The contents of unreacted substrates 1, ozonides 2, and oxoaldehydes 3 in the ozonolytic product were determined by the calibration curves which were plotted independently for each compound using ethylbenzene as an internal standard. Ozonolyses of each substrate 1 in a reaction system were carried out at least five times to confirm their reproducibility and to clarify the range of errors of the contents of each product.

Masuyama et al. For the measurement of difference in the 1H chemical shift of surfactants between the absence and the presence of substrates, D2O and TMS were used as a solvent and as an external standard, respectively. Caution. Since organic peroxides are potentially hazardous compounds, they must be handled with due care. Avoid exposure to strong heat or light, mechanical shock, oxidizable organic materials, or transition metal ions. No particular difficulties were experienced in handling any of the organic peroxides with the reaction scales and aqueous media used in these experiments.

Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research Area (12650835) from the Ministry of Education, Culture, Sports, Science, and Technology. Supporting Information Available: Purification methods and the characteristic data of newly prepared substrates 1, authentic ozonides 2, and authentic oxoaldehydes 3. This material is available free of charge via the Internet at http://pubs.acs.org. LA011081V