Pd(OAc)2-Catalyzed Oxidative Coupling Reaction of Benzenes with Olefins in the Presence of Molybdovanadophosphoric Acid under Atmospheric Dioxygen and Air Masayuki Tani, Satoshi Sakaguchi, and Yasutaka Ishii* Department of Applied Chemistry, Faculty of Engineering, Kansai University, Suita, Osaka 564-8680, Japan
[email protected] Received October 23, 2003
The direct oxidative coupling reaction of benzenes with alkenes bearing an electron-withdrawing group was successfully achieved by the use of Pd(OAc)2/molybdovanadophosphoric acid (HPMoV) as the key catalyst under O2 or air atmosphere. Thus, the reaction of benzene with ethyl acrylate under air (1 atm) assisted by Pd(OAc)2/HPMoV afforded ethyl cinnamate as a major product in satisfactory yield (74%). This catalytic system could be extended to the coupling reactions between various substituted benzenes and alkenes through the direct aromatic C-H bond activation. In the reaction of benzene with ethyl acrylate under O2 (1 atm), the best turn-over number (TON) of Pd(OAc)2 reached was 121. This reaction provides a green route to cinnamate derivatives, which are important precursors of a variety of pharmaceuticals. Introduction The arylation of olefins (Heck-Mizoroki reaction) is frequently used as an important reaction in the synthesis of arene derivatives. Although the reaction is applicable to the coupling between various aryl halides and alkenes, there have been several drawbacks in this methodology. For example, the formation of undesired waste salts which come from aryl halides used as starting materials is unavoidable, and a stoichiometric amount or more of a base must be added to complete the reaction. As a result, the reaction is difficult to carry out in large scale in industry except for the synthesis of expensive pharmaceutical materials. Development of a novel catalytic method for the synthesis of arene derivatives through the direct aromatic C-H bond activation is a challenging subject in synthetic organic chemistry, since there has been a growing demand for the construction of a wastefree synthetic method.1 Although several transition metal compounds are known to activate stoichiometrically the aryl C-H bond,2 a limited number of methods have appeared for the (1) (a) Activation of unreactive bonds and organic synthesis; Murai, S., Ed.; Springer: New York, 1999. (b) Dyker, G. Angew. Chem. 1999, 111, 1808-1822; Angew. Chem., Int. Ed. 1999, 38, 1698-1712. (c) Jia, C.; Kitamura, T.; Fujiwara, Y. Acc. Chem. Res. 2001, 34, 633-639. (d) Jia, C.; Piao, D.; Oyamada, J.; Kitamura, T.; Fujiwara, Y. Science 2000, 287, 1992-1995. (e) Labinger, J. A.; Bercaw, J. E. Nature (London) 2002, 417, 507-514. (f) Ritleng, V.; Sinlin, C.; Pfeffer, M. Chem. Rev. 2002, 102, 1731-1770. (2) (a) Moritani, I.; Fujiwara, Y. Tetrahedron Lett. 1967, 1119-1122. (b) Jones, W. D.; Feher, F. J. Acc. Chem. Res. 1989, 22, 91-100. (c) Ryabov, A. D. Chem. Rev. 1990, 90, 403-424. (d) Chin, R. M.; Dong, L.; Duckett, S. B.; Partridge, M. G.; Jones, W. D.; Perutz, R, N. J. Am. Chem. Soc. 1993, 115, 7685-7695. (e) Gutierrtez, E.; Monge, A.; Nicasio, M. C.; Poveda, M. L.; Carmona, E. J. Am. Chem. Soc. 1994, 116, 791-792. (f) Shilov, A. E.; Shul’pin, G. B. Chem. Rev. 1997, 97, 2879-2932.
catalytic C-H bond activation of arenes. Since the success of the activation of aryl C-H bonds through ortho-chelation by Ru complexes by Murai et al.,3 similar catalytic reactions that can activate the ortho C-H bond of substituted arenes have been reported.4 However, it is still very difficult to carry out the direct activation of the C-H bond of nonsubstituted aromatic compounds such as benzene through a catalytic process.5 Fujiwara and co-worker have reported the stoichiometric and catalytic oxidative coupling of arenes with alkenes through the cleavage of the aromatic C-H bond assisted by Pd compounds.6 In 1978, Matveev et al. reported the Pd(II)catalyzed arylation of ethylene in the presence of molybdovanadophosphoric acid.7 The Pd-catalyzed oxidative (3) (a) Murai, S.; Kakiuchi, F.; Sekine, Tanaka, Y.; Kamatani, A.; Sonoda, M.; Chatani, N. Nature 1993, 366, 529-531. (b) Kakiuchi, F.; Sekine, S.; Tanaka, Y.; Kamatani, A.; Sonoda, M.; Chatani, N.; Murai, S. Bull. Chem. Soc. Jpn. 1995, 68, 62-83. (c) Kakiuchi, F.; Yamakazu, M.; Chatani, N.; Murai, S. Chem. Lett. 1996, 111-112. (d) Kakiuchi, F.; Sato, T.; Yamakazu, M.; Chatani, N.; Murai, S. Chem. Lett. 1999, 19-20. (e) Kakiuchi, F.; Murai, S. Acc. Chem. Res. 2002, 35, 826834. (4) (a) Miura, M.; Tsuda, T.; Satoh, T.; Nomura, M. Chem. Lett. 1997, 1103-1104. (b) Miura, M.; Tsuda, T.; Satoh, T.; Pivsa-Art, S. J. Org. Chem. 1998, 63, 5211-5215. (c) Lenges, P. C.; Brookhart, M. J. Am. Chem. Soc. 1999, 121, 6616-6623. (d) Ritleng, V.; Sutter, J.; Pfeffer, M.; Sirlin, C. Chem. Comm. 2000, 129-130. (e) Trost, B. M.; Toste, F. D.; Greenman, K. J. Am. Chem. Soc. 2003, 125, 4518-4526. (5) (a) Chen, H. Y.; Schlecht, S.; Semple, T. C.; Hartwig, J. F. Science 2000, 287, 1995-1997. (b) Matsumoto, T.; Taube, D. J.; Periana, R. A.; Taube, H.; Yoshida, H. J. Am. Chem. Soc. 2000, 122, 7414-7415. (c) Ishiyama, T.; Takagi, J.; Ishida, K.; Miyaura, N.; Anastasi, N. R.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 390-391. (d) Cho, J. Y.; Tse, M. K.; Holmes, D.; Maleczka, R. E.; Smith, M. R. Science 2002, 295, 305-308. (e) Tsukada, N.; Mitsuboshi, T.; Setoguchi, H.; Inoue, Y. J. Am. Chem. Soc. 2003, 125, 12102-12103. (6) (a) Fujiwara, Y.; Takaki, K.; Taniguchi, Y. Synlett 1996, 591599. (b) Jia, C.; Lu, W.; Kitamura, T.; Fujiwara, Y. Org. Lett. 1999, 1, 2097-2100 and references therein. (7) Taraban’ko, V. E.; Kozhevnikov, I. V.; Matveev, K. I. Kinet. Katal. 1978, 19, 1160-1166.
10.1021/jo035568f CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/21/2004
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coupling of arenes with alkenes using peroxides and benzoquinones as oxidants has been studied by several authors.8 The oxidative coupling reaction with molecular oxygen as a terminal oxidant is very useful from environmental and economical points of view, but attempts with dioxygen or air as a terminal oxidant are not fully successful and the turnover numbers of catalysts are still less than 10.9 Recently, a few oxidative arylations of alkenes with dioxygen as a terminal oxidant have been reported. For instance, Matsumoto et al. have obtained styrene from benzene and ethylene catalyzed by a Rh complex in acetic acid with high dioxygen pressure (5.4 atm).10 The Ru-catalyzed reaction of benzene with acrylates under the influence of O2 (2 atm) and CO (6.1 atm) at 180 °C is reported by Milstein et al.11 Quite recently, Jacobs et al. reported the Pd(OAc)2-catalyzed coupling reaction of benzene with olefins under 0.4-1.6 MPa (416 atm) of O2 without solvent at 90 °C.12 However, these reactions must be carried out at high reaction temperature or high pressure of dioxygen. Therefore, it has been desired for a long time to develop an efficient catalytic method for the coupling through the direct aromatic C-H bond activation under mild conditions. In the course of our study on the activation of benzenes with the Pd(II)/molybdovanadophosphoric acid (HPMoV) catalytic system,13 we have recently disclosed a highly efficient catalytic system for oxidative coupling of benzene with acrylates under atmosphere dioxygen in a communication.14 In this paper, we wish to report in detail our works on the coupling reaction of arenes with alkenes (eq 1).
Results and Discussion To confirm the optimum reaction conditions, the reaction of benzene (1a) with ethyl acrylate (2a) was chosen as a model reaction and carried out under various reaction conditions. Table 1 shows the representative results for the reaction of 1a with 2a catalyzed by Pd(OAc)2 combined with heteropoly acids. The coupling reaction catalyzed by Pd(OAc)2 combined with (8) (a) Tsuji, J.; Nagashima, H. Tetrahedron 1984, 40, 2699-2702. (b) Mikami, K.; Hatano, M.; Terada, M. Chem. Lett. 1999, 55-56. (c) Boele, M. D. K.; van Strijdonck, G. P. F.; de Vries, A. H. M.; Kamer, P. C. J.; de Vries, J. G.; van Leeuwen, P. W. N. M. J. Am. Chem. Soc. 2002, 124, 1586-1587. (9) (a) Fujiwara, Y.; Moritani, I.; Danno, S.; Teranishi, S. J. Am. Chem. Soc. 1969, 91, 7166-7169. (b) Fujiwara, Y.; Asano, R.; Moritani, I.; Teranishi, S. J. Org. Chem. 1976, 41, 1681-1683. (10) (a) Matsumoto, T.; Yoshida, H. Chem. Lett. 2000, 1064-1065. (b) Matsumoto, T.; Periana, R. A.; Taube, D. J.; Yoshida, H. J. Catal. 2002, 206, 272-280. (11) Weissman, H.; Song, X.; Milstein, D. J. Am. Chem. Soc. 2001, 123, 337-338 and references therein. (12) Dams, M.; De Vos, D. E.; Celen, S.; Jacobs, P. A. Angew. Chem., Int. Ed. 2003, 42, 3512-3515. (13) Yokota, T.; Sakaguchi, S.; Ishii, Y. Adv. Synth. Catal. 2002, 344, 849-854. (14) Yokota, T.; Tani, M.; Sakaguchi, S.; Ishii, Y. J. Am. Chem. Soc. 2003, 125, 1476-1477.
1222 J. Org. Chem., Vol. 69, No. 4, 2004
TABLE 1. Oxidative Coupling of Benzene (1a) with Ethyl Acrylate (2a) under Several Conditionsa yield (%)b entry
catalyst
base
1 2c 3 4 5d 6d 7e 8f 9 10 11 12 13g 14h
Pd(OAc)2 Pd(OAc)2
NaOAc NaOAc NaOAc
Pd(OAc)2 Pd(OAc)2 Pd(acac)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2
NaOAc NaOAc NaOAc NaOAc LiOAc CsOAc KOAc NH4OAc NaOAc NaOAc
conv
(%)b
100 100 3 21 63 100 93 26 83 21 19 18 90 16
3aa
4aa
5
74 6 0 6 53 75 51 20 66 13 11 12 74 10
13 81 0 0 2 16 5 0 5 0 0 0 8 0
4 0 0 4 8 1 16 0 7 1 0 0 6 3
a 1a (30 mmol) was allowed to react with 2a (1.5 mmol) in the presence of Pd-species (0.1 mmol), HPMo11V1 (47 mg, ca. 0.02 mmol), base (0.08 mmol), and acetylacetone (0.1 mmol) in EtCOOH (5 mL) at 90 °C for 2.5 h. b Conversion and yield were based on 2a. c The reaction was carried out for 5 h. d In the absence of acetylacetone. e 1a (6 mmol) was used. f AcOH was used instead of EtCOOH. g Under air (1 atm) for 5 h. h Under Ar (1 atm)
H4PMo11V1O40‚nH2O (HPMo11V1, n ) 30) proceeded smoothly in the presence of small amounts of acetylacetone and NaOAc under atmospheric O2 in propionic acid at 90 °C for 2.5 h to give coupling products, ethyl cinnamate (3aa) (74%) and ethyl β-phenylcinnamate (4aa) (13%), together with ethyl 3-propionyloxyacrylate (5) (4%) derived from 2a and propionic acid (entry 1). When the reaction was prolonged to 5 h under these conditions, double coupling product 4aa was formed in good yield (81%) (entry 2). Needless to say, the reaction did not take place at all without Pd(OAc)2 (entry 3). It was found that a small amount of a base like NaOAc is necessary to carry out the coupling in higher conversion (entry 4). Removing acetylacetone from the present catalytic system resulted in the decrease of the coupling products, 3aa and 4aa (entry 5). It is interesting to note that the acetylacetone ligand is not necessary when Pd(acac)2 was employed in place of Pd(OAc)2 (entry 6). This fact indicates the importance of acetylacetone ligand in the present reaction. It has been reported that the oxidative arylation of benzene with ethylene to styrene by Rh and Pd complexes is accelerated by adding a small amount of acetylacetone and the formation of a byproduct like vinyl acetate is depressed.10 When the amount of 1a was reduced from 30 mmol to 6 mmol, the amount of 5 that is formed by the reaction of 2a with propionic acid increased from 4% to 16% and the yield of 3aa decreased to 51% (entry 7). The use of acetic acid as the solvent resulted in considerable decrease of 3aa and 4aa (entry 8). Although the effect of several alkali metal acetates in the present coupling was examined, the addition of lithium acetate furnished 3aa in 66% yield, and potassium, cesium, and ammonium acetates led to 3aa in very low yields (entries 9-12). There results show that the NaOAc is the best base in the present catalytic system. From the practical synthetic viewpoint, it is important to carry out the reaction under air in place of pure O2. Interestingly, it was found that the reaction smoothly took place even under air (1 atm), and 3aa was obtained
Oxidative Coupling Reaction of Benzenes with Olefins TABLE 2. Effect of HPA on Oxidative Coupling of
TABLE 3. Oxidative Coupling of Benzene (1a) with
Benzene (1a) with Ethyl Acrylate (2a)a
Various Alkenes by the Pd(OAc)2/HPMo11V1 Catalytic System under O2 (1 atm)a
yield (%)b entry
HPA
conv (%)b
3aa
4aa
5
1 2 3 4 5 6 7 8 9
H4PMo11V1O40‚nH2O (HPMo11V1) none H5PMo10V2O40‚nH2O (HPMo10V2) H6PMo9V3O40‚nH2O (HPMo9V3) H7PMo8V4O40‚nH2O (HPMo8V4) H3PMo12O40‚nH2O (HPMo12) HPMo11W1O40‚nH2O (HPMo11W1) HPW10V2O40‚nH2O (HPW10V2) HPW12O40‚nH2O (HPW12)
100 14 100 96 76 52 44 21 14
74 9 73 70 62 43 37 14 9
13 0 14 13 4 1 1 0 0
5 1 4 5 5 4 4 0 1
a A mixture of 1a (30 mmol), 2a (1.5 mmol), Pd(OAc) (0.1 2 mmol), HPA (ca. 0.02 mmol, calculated as n ) 30), NaOAc (0.08 mmol), and acetylacetone (0.1 mmol) in EtCOOH (5 mL) was placed in a round-bottom flask (30 mL) equipped with a balloon filled with O2 and allowed to react under stirring at 90 °C for 2.5 h. b Conversion and yield were based on 2a.
FIGURE 1. Time dependence curves for the oxidative coupling of 1a with 2a under air (1 atm).
in 74% yield after 5 h (entry 13). However, the reaction under Ar (1 atm) was difficult (entry 14).15a The yield of coupling products, 3aa and 4aa, was markedly influenced by the combination of Pd(OAc)2 with heteropoly acids (HPA) which serve as reoxidation catalysts of the reduced Pd(0) to Pd(II) during the reaction course (Table 2). When H4PMo11V1O40‚nH2O (HPMo11V1) was employed as the reoxidation catalyst, 3aa, 4aa, and 5 were obtained in 74%, 13%, and 5% yields, respectively (entry 1). The reaction was difficult to in the absence of HPA (entry 2). The catalytic potential of various HPMoV compounds was affected by the vanadium content in the HPMoV catalysts (entries 3 to 5). The reaction by the use of 12-molybdophosphoric acid (HPMo12) not involving V ion brought about 3aa in somewhat lower conversion (52%) (entry 6). Molybdotungstphosphoric acid (HPMo11W1) and tungustvanadophosphoric acid (HPW10V2) were found to be less efficient than HPMoV (entries 7 and 8). Figure 1 shows the time-dependence curves for the oxidative coupling of 1a with 2a, using the Pd(OAc)2/ HPMo11V1 system under air atmosphere. The yield of 3aa increased with time to reach 74% after 5 h, and then the 3aa gradually decreased in contrast to an increase of 4aa. After 15 h, the yields of 3aa and 4aa were 1% and 81%, respectively. The time dependence of the reaction clearly shows that 4aa is formed by the further oxidative coupling of the resulting 3aa with 1a. (15) (a) Kozhevnikov, I. V. Chem. Rev. 1998, 98, 171-198. (b) Nomiya, K.; Yagishita, K.; Nemoto, Y.; Kamataki, T. J. Mol. Catal. A: Chemical 1997, 126, 43-53.
a Benzene (30 mmol) was allowed to react with alkene (1.5 mmol) in the presence of Pd(OAc)2 (0.1 mmol), HPMo11V1 (47 mg, ca. 0.02 mmol), acetylacetone (0.1 mmol), and NaOAc (0.08 mmol) in propionic acid (5 mL) at 90 °C for 2.5 h. b The number in parentheses shows the yield of β-phenylcinnamates. c The reaction was carried out for 12 h.
To extend the present reaction, 1a was allowed to react with various alkenes under the same conditions as entry 1 in Table 1. These results are shown in Table 3. 1a reacted smoothly with several acrylates to give the corresponding cinnamates (68-88%) along with β-phenylcinnamate (11-16%) (entries 1 to 5). The arylation of 4-phenyl-2-butenone (2e) under these reaction conditions gave 1,1-diphenyl-3-butenone (3ae) in 93% yield (entry 6). Styrene (2f) and acrylonitrile (2g) reacted with difficulty to generate the corresponding product, 3af and 3ag, in 31% and 9% yields, respectively (entry 7 and 9). 3af, however, reacted smoothly with 1a to give the triphenylethylene (4af) in 76% yield (entry 8). Table 4 summarizes the oxidative coupling of various arenes with 2a under several conditions. The reaction of toluene (1b) with 2a afforded an isomeric mixture of coupling products 3ba (70%) consisting of o-3ba:m-3ba: p-3ba ) 15:41:44 (entry 1). Anisole (1c) coupled with 2a even at 60 °C to give a mixture of coupling products 3ca (o-3ca:m-3ca:p-3ca ) 17:7:76) in 73% yield (entry 2). It is interesting that the reaction of chlorobenzene (1d) and bromobenzene (1e) with 2a gave the corresponding coupling products, 3da and 3ea, in 74% and 49% yields, J. Org. Chem, Vol. 69, No. 4, 2004 1223
Tani et al. TABLE 4. Oxidative Coupling of Various Arenes with Ethyl Acrylate (2a) by the Pd(OAc)2/HPMo11V1 Catalytic System under O2 (1 atm)a
FIGURE 2. TON of Pd(OAc)2 combined with various HPMoV compounds for the reaction of 1a with 2a. Reaction conditions: 1a (45 mmol) was allowed to react with 2a (3 mmol) in the presence of Pd(OAc)2 (0.03 mmol), HPMoV (0.02 mmol), NaOAc (0.08 mmol), and acetylacetone (0.03 mmol) in EtCOOH (5 mL) under O2 (1 atm) at 90 °C for 12 h.
a Arene (30 mmol) was allowed to react with ethyl acrylate (1.5 mmol) in the presence of Pd(OAc)2 (0.1 mmol), HPMo11V1 (47 mg, ca. 0.02 mmol), acetylacetone (0.1 mmol), and NaOAc (0.08 mmol) in propionic acid (5 mL). b Ratio of ortho:meta:para. c Ethyl cinnamate (2a) was obtained in 5%. d Arene (5 mmol) was used. e 2,3Dimethoxy cinnamate was formed in 3% yield. f Ratio of 2,3methylenedioxy cinnamate (3ga): 3,4-methylenedioxy cinnamate (3′ga). g Arene (3 mmol) was used. h 5 wt % of Pd(OAc)2/C was used instead of Pd(OAc)2.
coupling of 1a with 2a. It was found that TON of Pd(OAc)2 was markedly affected by the vanadium content in the HPMoV catalysts. Increasing V content in HPMoV resulted in the decrease of TON of the Pd(OAc)2. The highest total TON (76) was obtained when HPMo11V1 was employed. In the oxidative coupling reaction of benzene to biphenyl under O2, we reported that a 1:1 mixture of HPMo11V1 and HPMo12 as the reoxidation catalyst was the most efficient system for the Pd-catalyzed oxidative coupling of benzene to biphenyl under O2 (1 atm).13 In the present reaction, the similar synergy effect of HPMo11V1 and HPMo12 was observed. Thus, the reaction by Pd(OAc)2 combined with a 1:1 mixture of HPMo11V1 and HPMo12 gave the best TON (121) of Pd (eq 2). Recently Jocob
respectively, in preference to the Mizoroki-Heck reaction (entries 3 and 4). In the reaction of 1d with 2a, no ethylcinnamate 3aa based on the Mizoroki-Heck reaction was observed, but a small amount (5%) of 3aa was formed when 1e was employed. The reaction of 1,2dimetoxybenzene (1f) with 2a afforded ethyl 3,4-dimetoxy cinnamate (3fa) (67%) and a small amount of its isomer, 2,3-dimetoxy cinnamate (3%) (entry 5). The reaction of 1,2-methylenedioxybenzene (1g) with 2a led to a 40:60 mixture of 2,3- and 3,4-methylenedioxy cinnamates (3ga and 3′ga) in 65% yield (entry 6). The reaction of 2-methylthiopene (1h) with 2a under these conditions did not take place at all. It is interesting to note that Pd(OAc)2 supported on active carbon, Pd(OAc)2/C, was efficient for the coupling of 1h with 2a to give the corresponding coupling product, 3ha, in satisfactory yield (86%) (entry 7). Owing to the strong coordination of the sulfur atom of the 1h to the Pd(OAc)2, the coordination of olefin 2a to the Pd species may be difficult. However, the coordination of 2a to the Pd on carbon needed for the coupling with 1h may be possible, since 1h is strongly adsorbed on activated carbon. Furan (1i) gave the corresponding coupling product 3ia (47%) along with double coupling product 4ia (11%). Figure 2 shows the turn-over number (TON) of Pd(OAc)2 combined with various HPMoV for the oxidative
reported that the TON of Pd(OAc)2 reached 762 in the reaction of anisole with ethyl trans-cinnamate under 8 atm of O2 at 90 °C.12 To our knowledge, our result is the best TON for the oxidative coupling of 1a with 2a under atmospheric dioxygen (1 atm). To obtain further information on HPMoV used, 31P NMR spectra of HPMo11V1 (a), HPMo12 (b), and a 1:1 mixture of HPMo12 and HPMo11V1 (c) were measured (Figure 3). A single peak was observed at δ -2.26 in HPMo12, while HPMo11V1 indicated principally two main peaks, δ -2.26 and -1.61, and several small peaks. One of two main peaks obtained from HPMo11V1 was the same as that of HPMo12. Another peak (δ -1.61) is attributed
1224 J. Org. Chem., Vol. 69, No. 4, 2004
Oxidative Coupling Reaction of Benzenes with Olefins
31 P NMR spectra of (a)HPMo12, (b)HPMo11V1, and (c) a 1:1 mixture of HPMo12 and HPMo11V1 in acetic acid.
FIGURE 3.
FIGURE 5. A possible reaction path for the coupling of arene with acrylate.
complex [A] reacts with an alkene like acrylate 2 to give a σ-alkyl-palladium(II) complex [B]. β -Hydride elimination of B liberates Pd-H to give the coupling product 3. The Pd-H is readily reduced to Pd(0), and the resulting Pd(0) is reoxidized by [HPMoV]ox to generate Pd(II). The [HPMoV]red is oxidized with O2 to [HPMoV]ox. Conclusion FIGURE 4. The time dependence of 3aa and 3a-da in two separate reactions of benzene 1a or benzene-d6 (1a-d) with 2a under the same conditions as entry 1 in Table 1.
to HPMo11V1.13,15 Several small peaks around δ -1 may be based on HPMo12-XVX (X g 2). These results indicate that HPMo11V1 available from commercial source is a mixture of HPMo12, HPMo11V1, and HPMo12-XVX (X g 2). The 31P NMR spectrum from a 1:1 mixture of HPMo12 and HPMo11V1, which led to the best TON, showed two main peaks based on HPMo12 and HPMo11V1. Although the synergistic effect by mixing of two different types of HPMoV is not clear at this stage, a new signal was not observed by mixing HPMo12 and HPMo11V1. To obtain mechanistic information on the present reaction, the initial rate of the reaction of benzene 1a with 2a was compared with that of benzene-d6 (1a-d) with 2a under the same conditions (Figure 4). It was found that the reaction rate of 1a with 2a was about four times faster than that of benzene-d6 (1a-d) with 2a, i.e. kH/kD Z 4. The same results (kH/kD Z 4) were obtained by the reaction of a 1:1 mixture of 1a and 1a-d with 2a under these conditions. A kinetic isotope effect for the coupling of 1a with methyl acrylate is reported to be kH/kD ) 2.10 No scrambling of deuterium was observed. These facts indicate that the cleavage of the C-H bond of 1a is the slowest step in a sequence of reactions. The coupling reaction is considered to proceed via a reaction path similar to that proposed by Fujiwara et al. A plausible reaction path is shown in Figure 5. The reaction may be initiated by the electrophilic attack of a Pd(II) to arene 1 leading to a σ-aryl-palladium(II) complex [A]. From the labeled experiment, this step is considered to be the slowest. The σ-aryl-palladium(II)
An efficient catalytic system for the oxidative coupling of arenes with alkenes through the aromatic C-H bond activation was developed. Thus, oxidative coupling of benzene with acrylates using dioxygen as terminal oxidant could be achieved by Pd(OAc)2 combined with HPMoV as reoxidation catalyst under O2. The reaction was smoothly promoted under air (1 atm) to give the corresponding coupling products in satisfactory yields. Furthermore, this catalytic system could be extended to the reaction of various aromatic compounds with alkenes. By the use of a 1:1 mixture of HPMo11V1 and HPMo12, the best TON (121) of Pd was obtained in the coupling reaction of benzene 1a with ethyl acrylate 2a.
Experimental Section General. All solvents and reagents ware purchased from commercial sources and used without further purification. All heteropoly acids were obtained from Nippon Inorganic Colour & Chemical Co., Ltd., except for HPMo12 (from Wako). Analytical TLC was performed on Merck TLC Plastic sheets F254 silica gel 60, using UV light and I2. GLC analysis was performed on Shimadzu GC-17A equipped with a flame ionization detector, using a 0.22 mm × 25 m capillary column. Mass spectra were determined at an ionization energy of 70 eV. Visible spectra were recorded on a Shimadzu UV-2500PC spectrometer. NMR spectra were recorded on a JEOL JEMEX-270. 1H and 13C NMR were measured at 270 and 67.5 MHz, respectively, in CDCl3 with Me4Si as the internal standard. 31P NMR was measured at 109.25 MHz in AcOH with 85% H3PO4 in a sealed capillary as the external standard. The chemical shifts were reported on the δ scale with resonances upfield of H3PO4 (δ 0) as negative. General Procedure for Oxidative Coupling of Benzene (1a) with Ethyl Acrylate (2a). A solution of Pd(OAc)2 (0.1 mmol), H4PMo11V1O40‚nH2O (HPMo11V1, n ) 30) (46.7 mg, ca. 0.02 mmol), NaOAc (0.08 mmol), acetylacetone (0.1 mmol),
J. Org. Chem, Vol. 69, No. 4, 2004 1225
Tani et al. benzene (1a) (30 mmol), and ethyl acrylate (2a) (1.5 mmol) in propionic acid (5 mL) was placed in a round-bottom flask (30 mL) equipped with a balloon filled with O2, and the mixture was allowed to react under stirring at 90 °C for 2.5 h. The reaction gave ethyl cinnamate (3aa), β-phenylcinnamate (4aa), and 3-propionylacrylate (5) in 74%, 14%, and 5% yields, respectively. All yields were detected by GLC analysis with nonane as internal standard. The products were characterized by 1H and 13C NMR and GC-MS, respectively. 3ac, 3ad, 3ba, 3ca, 3da, 3ea, 3fa, and 3ga were obtained by the independent esterification of the corresponding cinnamic acids available from commercial source and identified through the comparison of the isolated products with authentic samples. 3aa, 3ab 3af, 4af, and 3ag were commercially available and 3ae11 3ha,16 3ia,11 4aa,11 and 4i′a11 were reported previously. 3ae: 1H NMR (270 MHz, CDCl3) δ 1.88 (s, 3H), 0.98 (s, 1H), 7.19-7.42 (m, 10H); 13C NMR (67.5 MHz, CDCl3) δ 30.3, 127.3, 127.8, 128.3, 128.6, 129.3, 138.8, 140.6, 153.8, 200.0. 3ha: 1H NMR (270 MHz, CDCl3) δ 1.31 (t, J ) 7.0 Hz, 3H), 2.47 (s, 3H), 4.20 (q, J ) 7.0 Hz, 2H), 6.08 (d, J ) 15.7 Hz, 1H), 6.68 (d, J ) 3.4 Hz, 1H), 7.02 (d, J ) 3.4 Hz, 1H), 7.67 (d, J ) 15.7 Hz, 1H); 13C NMR (67.5 MHz, CDCl3) δ 14.4, 15.8, 60.3, 115.4, 126.3, 131.4, 137.2, 137.4, 143.7, 166.8. 3ia: 1H NMR (270 (16) Schmodt, R. R.; Hirsenkorn, R. Tetrahedron 1983, 39, 20432054.
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MHz, CDCl3) δ 1.32 (t, J ) 7.2 Hz, 3H), 4.24 (q, J ) 7.2 Hz, 2H), 6.31 (d, J ) 15.7 Hz, 1H), 7.42 (d, J ) 15.7 Hz, 1H), 6.457.47 (m, 3H); 13C NMR (67.5 MHz, CDCl3) δ 14.4, 60.4, 112.1, 114.5, 115.9, 130.8, 144.5, 150.8, 166.8. 3i′a: 1H NMR (270 MHz, CDCl3) δ 1.33 (t, J ) 7.2 Hz, 6H), 4.26 (q, J ) 7.2 Hz, 4H), 6.42 (d, J ) 15.7 Hz, 2H), 6.64 (s, 2H), 7.38 (d, J ) 15.7 Hz, 2H); 13C NMR (67.5 MHz, CDCl3) δ 14.4, 60.6, 116.5, 118.0, 129.9, 152.3, 166.4. Data for new compounds: ethyl cinnamate-d5 (3a-da): 1 H NMR (270 MHz, CDCl3) δ 1.27 (t, J ) 7.1 Hz, 3H), 4.19 (q, J ) 7.1 Hz, 2H), 6.36 (d, J ) 16.0 Hz, 1H), 7.62 (d, J ) 16.0 Hz, 1H); HRMS (EI) calcd for C11H7D5O2 [M]+ 181.1151, found 181.1142. 3-Propionyloxyacrylate (5): 1H NMR (270 MHz, CDCl3) δ 1.21 (t, J ) 7.8 Hz, 3H), 1.29 (t, J ) 7.0 Hz, 3H), 2.50 (q, J ) 7.8 Hz, 2H), 4.20 (q, J ) 7.0 Hz, 2H), 5.70 (d, J ) 12.6 Hz, 1H), 8.30 (d, J ) 12.6 Hz, 1H); 13C NMR (67.5 MHz, CDCl3) δ 8.60, 14.3, 27.2, 60.4, 105.6, 149.3, 166.0, 170.2; HRMS (EI) calcd for C8H12O4 [M]+ 172.0736, found 172.0747.
Acknowledgment. This work was partially supported by a Grant-Aid for Scientific Research (KAKENHI) (S) (No. 15036265) from the Japan Society for the Promotion of Science (JSPS). All heteropoly acids except HPMo12 were contributed by Nippon Inorganic Colour & Chemical Co., Ltd. JO035568F
