and iridium-catalyzed oxidative coupling of benzoic acids ... - CiteSeerX

Pure Appl. Chem., Vol. 80, No. 5, pp. 1127–1134, 2008. doi:10.1351/pac200880051127 © 2008 IUPAC

Rhodium- and iridium-catalyzed oxidative coupling of benzoic acids with alkynes and alkenes* Tetsuya Satoh‡, Kenji Ueura, and Masahiro Miura‡ Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan Abstract: The oxidative coupling of benzoic acids with alkynes and alkenes effectively proceeds in the presence of a Rh catalyst and an appropriate oxidant to produce the corresponding isocoumarin and phthalide derivatives, respectively. Interestingly, by using an Ir catalyst in place of Rh, the acids and alkynes undergo 1:2 coupling accompanied by decarboxylation to afford naphthalene derivatives exclusively. Keywords: rhodium; iridium; oxidative coupling; C–H bond cleavage; C–C coupling; isocoumarin; phthalide; naphthalene. INTRODUCTION The intermolecular coupling of aromatic substrates with alkenes or alkynes by transition-metal catalysis is now recognized to be a powerful tool to construct various π-conjugated vinylarene frameworks. The regioselective C–C coupling is usually carried out using halogenated or metallated aromatic reagents [1]. From the atom-economic point of view, a promising alternative is the oxidative coupling of unactivated aromatic substrates with the unsaturated compounds via C–H bond cleavage [2]. This allows the direct introduction of vinyl groups on aromatic substrates without pre-halogenation and -metallation. However, there is a substantial problem of forming a mixture of regioisomers of substituted aromatic products. One of the most promising methods to overcome the problem is to utilize the proximate effect by coordination of a functional group in a given substrate to the metal center of a catalyst, leading to regioselective C–H bond activation and functionalization. Since Murai et al. reported the ruthenium-catalyzed ortho-alkylation of aromatic ketones as pioneering work in this area [3], various catalytic alkylation and arylation of aromatic compounds bearing oxygen- or nitrogen-containing groups through C–H activation have been successfully developed [4]. In contrast, the chelation-assisted version of oxidative coupling to produce regioselectively vinylated arenes has been less explored. As the rare examples, we demonstrated that 2-phenylphenols, N-(arylsulfonyl)-2-phenylanilines, and benzoic acids undergo chelation-controlled direct oxidative vinylation in the presence of a Pd/Cu catalyst [5]. After our reports, the direct vinylation of amides [6], N-(pyridylmethyl)indole [7], and N,N-dimethylbenzylamines [8] has also been developed. The reaction of benzoic acids, among these substrates, seems to be of particular interest because of their wide availability as aryl sources [9]. The reactions of benzoic acid with styrene and an acrylate afford isocoumarin and phthalide derivatives, respectively, via ortho-vinylation and subsequent oxidative or non-oxidative cyclization [5a]. *Paper based on a presentation at the 14th International Symposium on Organometallic Chemistry Directed Towards Organic Synthesis (OMCOS-14), 2–6 August 2007, Nara, Japan. Other presentations are published in this issue, pp. 807–1194. ‡Corresponding authors

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Isocoumarin and phthalide nuclei are found in various natural products that exhibit a broad range of interesting biological properties [10]. Although these reactions have high potential to provide atomeconomical routes to such heterocycles [11,12], their efficiency is moderate to low: decomposition of the homogeneous palladium-based catalyst into inactive bulk metal seems to be involved [13]. Moreover, the coupling partners are so far limited to some alkenes, and the reactions with other unsaturated substrates including alkynes are unexplored. In the context of our study of catalytic coupling of benzoic acid derivatives [14], we have succeeded in finding that the direct oxidative coupling of benzoic acids with internal alkynes can be realized by using Rh [15,16] in place of Pd as the principal catalyst component to afford isocoumarin derivatives in good to excellent yields. Furthermore, by using an Ir catalyst, the corresponding naphthalene derivatives can be produced selectively from the same combination of substrates accompanied by decarboxylation. This represents a new example of aromatic homologation by the coupling of ArX and two alkyne molecules [17]. In the Rh catalysis, acrylates can be used in place of alkynes to afford phthalides with good yields. In this account, we briefly summarize the results obtained for these reactions. ISOCOUMARIN SYNTHESIS We initially examined the oxidative coupling of benzoic acid (1a) with diphenylacetylene (2a, 1.2 equiv). When these substrates were treated in the presence of [Cp*RhCl2]2 (0.5 mol %) and Cu(OAc)2H2O (4 equiv) in o-xylene at 120 °C for 6 h under N2, 3,4-diphenylisocoumarin (3a) was formed in 95 % yield, along with a small amount of 1,2,3,4-tetraphenylnaphthalene (4a, 5 %) (eq. 1, Cp* = η5-pentamethylcyclopentadienyl) [18]. None or trace amounts of 3a were obtained in the case using RhCl3H2O, Rh(acac)3, [RhCl(cod)]2, or [RhCl(C2H4)2]2 in place of [Cp*RhCl2]2 (acac = acetylacetonate, cod = cyclooctadiene). Under the conditions using [Cp*RhCl2]2 as the catalyst, p-, m-, and o-substituted benzoic acids (1b–f) and 1-naphthoic acid (1g) (eq. 2) also underwent the coupling with 2a efficiently to produce the corresponding isocoumarins 3b–g. The reaction of 1g needed refluxing the solvent for the smooth coupling. From the reactions of dialkylacetylenes such as 4-octyne (2b) and 8-hexadecyne (2c) with 1a, the corresponding 3,4-dialkylisocoumarins 3h,i were obtained in good yields.

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© 2008 IUPAC, Pure and Applied Chemistry 80, 1127–1134

Rh- and Ir-catalyzed oxidative doupling

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It was found that the amount of the Cu salt could be reduced to 5 mol %, when the reaction of 1a with 2a was conducted under air. Thus, the aerobic oxidative coupling using a catalyst system of [Cp*RhCl2]2/Cu(OAc)2H2O proceeded efficiently in DMF at 120 °C to afford 3a in 96 % yield (eq. 3). Under similar conditions, a series of isocoumarins 3b–i were also obtained from 1a–f and 2a–c in good to excellent yields.

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A plausible mechanism for the reaction of benzoic acid (1a) with alkyne 2 is illustrated in Scheme 1, in which neutral ligands are omitted. Coordination of the carboxylate oxygen to Cp*RhX2(III) gives a Rh(III) benzoate A. Subsequent ortho-rhodation to form a rhodacycle intermediate B [19], alkyne insertion, and reductive elimination occur to produce isocoumarin 3. The resulting Cp*Rh(I) species may be oxidized in the presence of a Cu(II) salt to regenerate Cp*RhX2(III). Under air, Cu(I) formed in the last step may also be reoxidized to Cu(II). It should be noted that no deposition of deactivated bulk metal is observed during the reaction, while this often occurs in Pd/Cu-catalyzed reactions [13].

Scheme 1

PHTHALIDE SYNTHESIS Under the aerobic conditions, benzoic acid (1a) also reacted with n-butyl acrylate (5a) smoothly (Scheme 2) [18]. Interestingly, disubstitution at both the ortho-positions occurred to afford 7-vinylphthalide 6a as a 1:2 coupling product in 66 % isolated yield. The reaction of 1a with ethyl acrylate (5b) also proceeded efficiently in o-xylene to give the corresponding phthalide 6b in 76 % yield.


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Scheme 2

In this reaction, a rhodacycle intermediate, generated in a similar manner to that in the reaction with alkynes (B in Scheme 1), may undergo alkene insertion and successive β-hydride elimination to form the ortho-monovinylated benzoic acid. Prior to the nucleophilic cyclization, the second vinylation takes place to lead to the divinylated product 6. In contrast to the acrylates, N,N-dimethylacrylamide (5c) and acrylonitrile (5d) reacted with 1a in a 1:1 ratio under similar conditions to afford 7c and 7d (eq. 4). In these cases, the cyclization exclusively occurs after the first vinylation (see Scheme 2), as occurs in the Pd-catalyzed reaction [5a].

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NAPHTHALENE SYNTHESIS Treatment of 1a with 2a (2 equiv) in the presence of [Cp*RhCl2]2 (1 mol %) and Cu(OAc)2H2O (4 equiv) in mesitylene at 180 °C for 2 h gave a mixture of 3a and 4a in 81 and 19 % yields, respectively (eq. 5). The naphthalene 4a became a predominant product, when the reaction was conducted using Ag2CO3 (2 equiv) in place of the Cu salt as an oxidant.

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Interestingly, it was found that 4a can be obtained exclusively under Ir catalysis [18a]. Thus, the reaction in the presence of [Cp*IrCl2]2 (2 mol %) and Ag2CO3 (2 equiv) in o-xylene at °C for 6 h afforded 4a in 88 % yield and did not give 3a (eq. 6). Bis(4-methoxyphenyl)- (2d) and bis(4-chlorophenyl)acetylene (2e) also reacted with 1a efficiently to give the corresponding © 2008 IUPAC, Pure and Applied Chemistry 80, 1127–1134


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1,2,3,4-tetraarylnaphthalenes 4b and 4c in 64 and 91 % yields. In contrast to the Rh-catalyzed reactions (eqs. 1 and 3), the Ir-catalyzed reactions with dialkylacetylenes were sluggish. Thus, treatment of 1a with 2b and 2c gave the corresponding tetraalkylnaphthalenes in low yields (10–20 %, by GC-MS). The reactions of 1b and 3,5-dimethylbenzoic acid (1h) with 2a proceeded smoothly to give naphthalenes 4d and 4e in 95 and 93 % yields, respectively.

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The reaction of 2-chlorobenzoic acid (1i) with 2a gave not only an expected naphthalene, 5-chloro-1,2,3,4-tetraphenylnaphthalene (4f), but also its isomer, 6-chloro-1,2,3,4-tetraphenylnaphthalene (4'g) (eq. 7). The possible pathway toward the latter unexpected naphthalene via isomerization will be discussed below (Scheme 3). Furthermore, the reactions of other 2-substituted benzoic acids 1f,j,k with 2a proceeded involving the isomerization to exclusively form 6-substituted 1,2,3,4-tetraphenylnaphthalenes 4'h (= 4d), 4'i, and 4'j in 60–88 % yields.

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A plausible mechanism for the reaction of benzoic acids 1 with diarylacetylenes 2 by the Ir catalysis is illustrated in Scheme 3, in which neutral ligands are omitted. A seven-membered iridacycle intermediate D appears to be generated in a manner similar to that to a rhodacycle C in Scheme 1. Then, D undergoes decarboxylation to form a key, five-membered iridacycle intermediate E, rather than C–O reductive elimination, which is a preferable pathway in the Rh catalysis. Subsequently, the second alkyne insertion and C–C reductive elimination occur to produce naphthalene 4. The resulting Ir(I)X species may be oxidized in the presence of the silver salt to regenerate Ir(III)X3. In the cases with 2-substituted benzoic acids, the key intermediate E may undergo rearrangement driven by steric reasons through protonation and cycloiridation to form an isomeric iridacycle F, which reacts with alkyne to afford 6-substituted naphthalene 4'.


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Scheme 3

SUMMARY We have demonstrated that the oxidative coupling reaction of benzoic acids with internal alkynes can be performed in the presence of a Rh catalyst and a Cu oxidant to selectively give the corresponding 1:1 coupling products, isocoumarin derivatives. The reaction proceeds catalytically with respect to both the metals under air. Under the aerobic conditions, the coupling of benzoic acids with acrylates takes place efficiently to give phthalides. On the other hand, the acids and alkynes have been found to react in a ratio of 1:2 under Ir catalysis to produce naphthalene derivatives exclusively accompanied by decarboxylation. Rh and Ir catalyst systems for oxidative C–C coupling reactions have been less explored than those with Pd. The present catalyst systems and related ones are expected to be applicable to other coupling reactions. Work is underway toward further development of the catalysis. ACKNOWLEDGMENT This work was partly supported by a Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan. REFERENCES 1. For example, see: (a) J. Tsuji. Palladium Reagents and Catalysts, 2nd ed., John Wiley, Chichester (2004); (b) A. de Meijere, F. Diederich. Metal-Catalyzed Cross-Coupling Reactions, 2nd ed., Wiley-VCH, Weinheim (2004); (c) K. Fagnou, M. Lautens. Chem. Rev. 103, 169 (2003); (d) I. P. Beletskaya, A. V. Cheprakov. Chem. Rev. 100, 3009 (2000). 2. (a) C. Jia, T. Kitamura, Y. Fujiwara. Acc. Chem. Res. 34, 633 (2001); (b) Y. Fujiwara, I. Moritani, S. Danno, S. Teranishi. J. Am. Chem. Soc. 91, 7166 (1969). 3. S. Murai, F. Kakiuchi, S. Sekine, Y. Tanaka, A. Kamatani, M. Sonoda, N. Chatani. Nature 366, 529 (1993).



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