Letter Cite This: Org. Lett. 2018, 20, 1328−1332
pubs.acs.org/OrgLett
Rapid Access to Ortho-Alkylated Vinylarenes from Aromatic Acids by Dearomatization and Tandem Decarboxylative C−H Olefination/ Rearomatization Hung-Chang Tsai, Yen-Hsiang Huang, and Chih-Ming Chou* Department of Applied Chemistry, National University of Kaohsiung, 700 Kaohsiung University Road, Nanzih District, Kaohsiung 81148, Taiwan S Supporting Information *
ABSTRACT: A two-step straightforward method for the preparation of ortho-alkylated vinylarenes from readily available benzoic acids is described. The synthetic route involves the dearomatization of benzoic acids by Birch reduction providing alkylated cyclohexa-2,5-dienyl-1-carboxylic acids. The diene subsequently undergoes a decarboxylative C−H olefination followed by rearomatization to deliver ortho-alkylated vinylarene. Mechanistic studies suggest that a Pd/Ag bimetallic catalytic system is important in the tandem decarboxylative C−H olefination/rearomatization step.
A
available from diverse natural and synthetic sources. Additionally, carboxylic groups can serve as traceless directing groups7 or can subsequently be further functionalized by decarboxylative coupling. Decarboxylative coupling has the added benefit that only CO2 is generated as a byproduct, instead of toxic metal halides resulting from other commonly used methods, therefore providing a green and site-selective aryl C−H functionalization protocol.8 Recently, Linker and co-workers described a two-step, regioselective preparation of alkylated arenes by the ipsosubstitution of benzoic acid derivatives using Birch reductive alkylation followed by decarbonylative rearomatization (Scheme 1a).9 More recently, Studer’s group extended this approach for the preparation of highly substituted arenes via sequential Birch reductive alkylation, allylic alkylation, decarboxylative γ-arylation, and rearomatization.10 We envisioned that aformentioned methods reported by Linker and Studer could be modified and combined with direct C−H functionalization to provide an efficient synthetic route for preparing ortho-alkylated vinylarenes from readily available benzoic acid derivatives. Herein, we describe such an approach where alkyl groups are introduced to the ipso-position of benzoic acids by Birch reductive alkylation to give alkylated cyclohexa-2,5dienyl-1-carboxylic acids. The diene subsequently undergoes a tandem decarboxylative C−H olefination/rearomatization (DCHORE) to complete the regioselective installation of vinyl motifs (Scheme 1b). Using this synthetic route, ortho-
lkylated vinylarenes are ubiquitous structures in numerous biologically active compounds, pharmaceuticals, and natural products and are important synthetic intermediates for fine chemicals and functional organic π-materials.1 Therefore, the development of simple and efficient methods for the construction of alkylated vinylarenes has attracted significant attention. Conventional synthetic approaches for the introduction of alkyl substituents on aromatic rings include Friedel−Crafts alkylation2 and metal-catalyzed Csp2−Csp3 cross-coupling reactions.3 However, limitations to the Friedel−Crafts reaction include carbocation rearrangement, moderate regioselectivity, and polyalkylation. The metal-catalyzed coupling reaction is the most straightforward approach for Csp2−Csp3 bond formation. However, these reactions require prefunctionalized substrates and often suffer from low yield due to a competing β-hydride elimination reaction. With respect to the installation of vinyl motifs on aromatic rings, classical Wittig-type olefination4 or Mizoroki−Heck-type reactions5 are commonly used. Unfortunately, these multistep reactions often generate phosphorusand halogen-containing toxic byproducts. Thus, the development of practical, straightforward, and environmentally benign synthetic methods, in particular for ortho-alkylated vinylarenes, is profoundly important. Transition-metal-catalyzed direct C−H functionalization of arenes for the preparation of vinylarenes has been heavily investigated. Using directing group strategies, these reactions can deliver regioselective C−H olefination products in a predictable and economical fashion.6 Among a variety of directing groups, carboxylate moieties serve as promising candidates with several advantages. Carboxylic acids are nontoxic, highly stable, easy to handle, and commercially © 2018 American Chemical Society
Received: January 7, 2018 Published: February 13, 2018 1328
DOI: 10.1021/acs.orglett.8b00064 Org. Lett. 2018, 20, 1328−1332
Letter
Organic Letters Scheme 1. Preparation of Ortho-Alkylated Vinylarenes from Readily Available Benzoic Acids Using Dearomatization and Tandem Decarboxylative C−H Olefination/Rearomatization Strategy
Upon replacement of Pd(TFA)2 with Pd(OAc)2 or PdCl2, only isopropylbenzene was observed instead of the desired product (entries 4 and 5). In the absence of Pd catalyst or replacing Ag2CO3 with AgNO3, isopropylbenzene was the major product (entries 6 and 7). Increasing the catalyst loading to 10 mol % and the amount of 2a to 1.5 equiv did not improve the reaction yield (entry 8). The yield was slightly enhanced to 67% by using 2 equiv of 1a (entry 9). Gratifyingly, the yield increased significantly to 90% by switching the solvent system to 5% DMSO/dioxane (entry 10). With the optimized reaction conditions in hand, we investigated the substrate scope of a palladium-catalyzed DCHORE reaction. Initially, various 1-alkylated cyclohexa2,5-dienyl-1-carboxylic acids 1b−l were prepared from a variety of commercially available aromatic carboxylic acids derivatives using the well-established Birch reductive alkylation.11 The R2 substituent can be selected on the basis of the respective alkyl halides. Compound 1 was reacted with methyl or isobutyl acrylates under optimized conditions to give ortho-alkylated vinylarenes 3aa−jb (Scheme 2). Isopropyl, methyl, n-hexyl, and
alkylated vinylarenes with a broad substrate scope were readily prepared as described below. 1-Isopropylcyclohexa-2,5-diene-1-carboxylic acid 1a was prepared from benzoic acid by Birch reductive alkylation in quantitative yield. Next, a tandem decarboxylative C−H olefination/rearomatization was tested using methyl acrylate. In an initial attempt, compound 1a (1.1 equiv) was treated with methyl acrylate 2a (1.0 equiv) under conditions of 5 mol % of Pd(TFA)2 and Ag2CO3 (2.0 equiv) in 5% of DMSO/DMF at 120 °C for 2 h. The DCHORE reaction produced the desired ortho-alkylated vinylarene 3aa in 44% isolated yield (Table 1,
Scheme 2. Palladium-Catalyzed Tandem Decarboxylative C−H Olefination/Rearomatization (DCHORE) of 1 with Acrylates 2a
Table 1. Optimization of Tandem Decarboxylative C−H Olefination/Rearomatization (DCHORE) of 1a with Methyl Acrylatea,b
entry
1a/2a
Pd (mol %)
solvent (1:19)
3aa (%)
1 2 3 4 5 6 7 8 9 10
1.1:1 1.1:1 1.1:1 1.1:1 1.1:1 1.1:1 1.1:1 1:1.5 2:1 2:1
Pd(TFA)2 (5.0) Pd(TFA)2 (5.0) Pd(TFA)2 (5.0) Pd(OAc)2 (5.0) PdCl2 (5.0)
DMSO/DMF DMSO/DMF DMSO/DMF DMSO/DMF DMSO/DMF DMSO/DMF DMSO/DMF DMSO/DMF DMSO/DMF DMSO/dioxane
44c 66 0d 24e 0e 0e 0e,f 42 67 90
Pd(TFA)2 Pd(TFA)2 Pd(TFA)2 Pd(TFA)2
(5.0) (10.0) (10.0) (10.0)
a
Reaction carried out with 1 (0.5 mmol) and 2 (0.25 mmol); the yield was calculated on the basis of 2. bReaction conducted on a 1 mmol scale of 2a. cBenzoquinone as oxidant. d1 equiv of 1g and 2 equiv of 2 were reacted for 18 h; the yield was calculated on the basis of 1g. e Combined yields and ratio of two regioisomers.
a
Reaction conducted at 0.25 mmol scale under air environment. Isolated yield. c2 equiv of Ag2CO3. dReaction was performed at 60 °C, and starting material was recovered. eIsopropylbenzene was the major byproduct. f3 equiv of AgNO3.
b
methylene methyl ester substituted cyclohexa-2,5-dienyl-1carboxylic acids are compatible with DCHORE and lead to the corresponding ortho-alkylated vinylarenes 3aa−db in good yields. It is worth mentioning that we are able to introduce long alkyl chains or a methyl group at the ortho-position of vinylarenes. Indeed, these compounds are not easily accessible using classic Friedel−Crafts alkylation. In addition, o-methylene ester vinylarenes 3da and 3db can be utilized in the tandem conjugate addition−Dieckmann condensation12 for the formation of fused bicyclic compounds that are common scaffolds used in the preparation of active pharmaceuticals and natural products.
entry 1). In the crude NMR spectra, a significant amount of isopropylbenzene was present, suggesting that the decarboxylation/rearomatization of 1a occurred as a side reaction using these conditions. Increasing the amount of Ag2CO3 to 3 equiv significantly improved the yield to 66% (entry 2). No reaction occurred when the reaction temperature was lowered to 60 °C (entry 3). 1329
DOI: 10.1021/acs.orglett.8b00064 Org. Lett. 2018, 20, 1328−1332
Letter
Organic Letters
A steric effect was observed for reaction of 5j−l and slightly influenced the reaction yields. 3,4-Dimethoxystyrene, when used as a coupling partner, produced 5t in good yield. Similarly, isopropyl-, methyl-, n-hexyl-, and tert-butyl-substituted cyclohexa-2,5-dienyl-1-carboxylic acids are also compatible with this procedure (5p−s). Again, the regioisomers were observed when the 3-methyl- or 3-fluoro-substituted substrates were reacted with styrene (5n and 5o). Additionally, substituted 1,4dihydro-1-naphthoic acids also reacted smoothly with styrene to give the corresponding styrylnaphthalenes having extended π-conjugation (5u and 5v). To obtain insight into a possible reaction mechanism, several experiments were carried out as shown in Scheme 4. First,
In the case of 1-benzylcyclohexa-2,5-dienyl-1-carboxylic acid 1e, however, fast decarboxylation followed by rearomatization occurred, resulting in the production of diphenylmethane as the sole isolated product. By replacing the silver oxidant with benzoquinone, the desired coupling product 3ea was obtained in 32% yield (Scheme 2). As expected, olefinated products 3fa−hb were obtained upon treatment of 2-methyl- or 4-tertbutyl-substituted cyclohexa-2,5-dienyl-1-carboxylic acids with acrylates under standard conditions. To our delight, the methyl group at the 3-position of the cyclohexa-2,5-dienyl-1-carboxylic acid was well tolerated, and reactions proceeded with excellent regioselectivity to deliver 3ia and 3ib. On the other hand, when the 3-fluoro substrate was reacted with methyl or isobutyl acrylates, 3ja and 3jb were isolated with its corresponding regioisomers as mixtures in combined yields of 67% and 81%, respectively (see the Supporting Information). This low regioselectivity may be due to the small, nonsterically demanding size of the fluorine atom. In addition, the fluorine atom can also act as a directing group that enhances the acidity of the ortho-hydrogen. This enhancement of acidity may direct the decarboxylative C−H olefination to occur at a neighboring site.13 Stilbene structures are commonly found in natural products14 and organic materials.1a,15 To further expand the scope of DCHORE, we examined the reaction with respect to styrene coupling partners. Gratifyingly, a wide range of substituents and substitution patterns are tolerated under similar reaction conditions affording the corresponding ortho-alkylated stilbenes in moderate to good yields (Scheme 3). Para-substituted
Scheme 4. Control Experimentsa
a
Scheme 3. Palladium-Catalyzed Tandem Decarboxylative C−H Olefination/Rearomatization (DCHORE) of 1 with Various Styrenes 4a
Yields were determined by 1H NMR of crude reaction mixtures.
olefinated cyclohexa-2,5-dienyl-1-carboxylic acid 6 was successfully isolated in 62% yield by stopping the reaction before decarboxylation followed by rearomatization had occurred (Scheme 4a). This result indicates that the carboxylate-directed C−H olefination of 1g with methyl acrylate occurred under standard conditions during the early stages of the reaction. Since decarboxylation of benzoic acids by Pd or Ag metals has been reported,16,7c we then investigated which metal is responsible for the decarboxylation steps in regard to the olefinated cyclohexa-2,5-dienyl-1-carboxylic acid 6. Substrates 1g and 6 were treated (no Pd) with a stoichiometric amount of Ag2CO3 (3.0 equiv), and decarboxylation followed by rearomatization of 1g occurred much faster than 6 (Scheme 4b). On the other hand, when a mixture of 1g and 6 was treated with 10 mol % of Pd(TFA)2, selective decarboxylation followed by rearomatization of 6 occurred but with low conversion (Scheme 4c). We then submitted a mixture of 1g and 6 to the optimized conditions; both of them were efficiently converted to the corresponding decarboxylation followed by rearomatization products (Scheme 4d). These control experiments strongly suggest that the Pd/Ag bimetallic metal is the system that cooperatively catalyzes the decarboxylation followed by rearomatization of the olefinated cyclohexa-2,5-dienyl-1-carboxylic acid 6. Based on the experiments performed as described in Schemes 1−4, a plausible mechanism was proposed for DCHORE and is shown in Scheme 5. Coordination of the Ag−carboxylate substrate A to Pd(II) undergoes carboxylatedirected C−H activation generating the five-membered ring cyclopalladated intermediate B. Because of the large steric effect of the methyl substituent, palladation of the sterically hindered
a
Reaction carried out with 2 (0.5 mmol) and 4 (0.25 mmol); the yield was calculated on the basis of 4. bCombined yields and ratio of two regioisomers.
styrenes with electron-donating groups such as methyl, methoxy, and benzyloxy proceed in better yields than the electron-withdrawing, methyl ester, nitro, and cyano groups (5c−g and 5l). It is worth noting that substrates bearing chloro or fluoro substituents are also compatible with DCHORE and proceeded in high yields, enabling further cross-coupling to be performed (5h and 5i). 1330
DOI: 10.1021/acs.orglett.8b00064 Org. Lett. 2018, 20, 1328−1332
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Organic Letters
104-2113-M-390-004-MY2 and MOST 106-2113-M-390-001MY2) for financial support. We also thank Prof. J. R. Carey at the Department of Applied Chemistry, National University of Kaohsiung, Kaohsiung, Taiwan, for helpful discussions.
Scheme 5. Proposed Mechanism
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(1) (a) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402. (b) Grimsdale, A. C.; Leok Chan, K.; Martin, R. E.; Jokisz, P. G.; Holmes, A. B. Chem. Rev. 2009, 109, 897. (c) Hua, X.; Fu, Y.-J.; Zu, Y.-G.; Wu, N.; Kong, Y.; Li, J.; Peng, X.; Efferth, T. J. Pharm. Biomed. Anal. 2010, 52, 273. (d) Rukachaisirikul, V.; Rodglin, A.; Sukpondma, Y.; Phongpaichit, S.; Buatong, J.; Sakayaroj, J. J. Nat. Prod. 2012, 75, 853. (e) Hassam, M.; Taher, A.; Arnott, G. E.; Green, I. R.; Van Otterlo, W. A. L. Chem. Rev. 2015, 115, 5462. (2) (a) Friedel−Crafts and Related Reactions, Vol. II, Part I; Olah, G. A., Ed.; Wiley-Interscience: New York, 1964. (b) Roberts, R. M.; Khalaf, A. A. Friedel−Crafts Alkylation Chemistry: A Century of Discovery; Marcel Dekker: New York, 1984. (c) Bandini, M.; UmaniRonchi, A. In Catalytic Asymmetric Friedel−Crafts Alkylations; WileyVCH: Weinheim, Germany, 2009. (d) Prades, A.; Corberán, R.; Poyatos, M.; Peris, E. Chem. - Eur. J. 2009, 15, 4610. (3) (a) Metal-Catalyzed Cross-Coupling Reactions; De Meijere, A., Diederich, F., Eds.; Wiley-VCH: Weinheim, 2004. (b) Modern Arylation Methods; Ackermann, L., Ed.; Wiley-VCH: Weinheim, 2009. (c) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457. (d) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. Rev. 2004, 104, 6217. (e) Fürstner, A.; Krause, H.; Lehmann, C. W. Angew. Chem., Int. Ed. 2006, 45, 440. (f) Martin, R.; Buchwald, S. L. Acc. Chem. Res. 2008, 41, 1461. (g) Cahiez, G.; Foulgoc, L.; Moyeux, A. Angew. Chem., Int. Ed. 2009, 48, 2969. (4) (a) March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed.; Wiley: New York, 1992. (b) Maryanoff, B. E.; Reitz, A. B. Chem. Rev. 1989, 89, 863. (5) Beletskaya, I. P.; Cheprakov, A. V. Chem. Rev. 2000, 100, 3009. (6) Recent reviews on directed C−H olefinations: (a) Satoh, T.; Miura, M. Synthesis 2010, 2010, 3395. (b) Le Bras, J.; Muzart, J. Chem. Rev. 2011, 111, 1170. (c) Engle, K. M.; Mei, T.-S.; Wasa, M.; Yu, J.-Q. Acc. Chem. Res. 2012, 45, 788. (d) Arockiam, P. B.; Bruneau, C.; Dixneuf, P. H. Chem. Rev. 2012, 112, 5879. (e) Kozhushkov, S. I.; Ackermann, L. Chem. Sci. 2013, 4, 886. (f) Zhou, L.; Lu, W. Chem. Eur. J. 2014, 20, 634. (g) Shi, G.; Zhang, Y. Adv. Synth. Catal. 2014, 356, 1419. (h) Chen, Z.; Wang, B.; Zhang, J.; Yu, W.; Liu, Z.; Zhang, Y. Org. Chem. Front. 2015, 2, 1107. (i) Dey, A.; Agasti, S.; Maiti, D. Org. Biomol. Chem. 2016, 14, 5440. (j) Ma, W.; Gandeepan, P.; Li, J.; Ackermann, L. Org. Chem. Front. 2017, 4, 1435 and references cited therein. (7) (a) Maehara, A.; Tsurugi, H.; Satoh, T.; Miura, M. Org. Lett. 2008, 10, 1159. (b) Mochida, S.; Hirano, K.; Satoh, T.; Miura, M. Org. Lett. 2010, 12, 5776. (c) Cornella, J.; Righi, M.; Larrosa, I. Angew. Chem., Int. Ed. 2011, 50, 9429. (d) Kumar, N. Y. P.; Bechtoldt, A.; Raghuvanshi, K.; Ackermann, L. Angew. Chem., Int. Ed. 2016, 55, 6929. (e) Huang, L.; Biafora, A.; Zhang, G.; Bragoni, V.; Gooßen, L. J. Angew. Chem., Int. Ed. 2016, 55, 6933. (8) Recent reviews on decarboxylative couplings: (a) Rodríguez, N.; Gooßen, L. J. Chem. Soc. Rev. 2011, 40, 5030. (b) Cornella, J.; Larrosa, I. Synthesis 2012, 44, 653. (c) Dzik, W. I.; Lange, P. P.; Gooßen, L. J. Chem. Sci. 2012, 3, 2671. (d) Xuan, J.; Zhang, Z.-G.; Xiao, W.-J. Angew. Chem., Int. Ed. 2015, 54, 15632. (e) Pichette Drapeau, M.; Gooßen, L. J. Chem. - Eur. J. 2016, 22, 18654. (f) Wei, Y.; Hu, P.; Zhang, M.; Su, W. Chem. Rev. 2017, 117, 8864. (g) Font, M.; Quibell, J. M.; Perry, G. J. P.; Larrosa, I. Chem. Commun. 2017, 53, 5584. (h) Patra, T.; Maiti, D. Chem. - Eur. J. 2017, 23, 7382 and references cited therein. (9) (a) Vorndran, K.; Linker, T. Angew. Chem., Int. Ed. 2003, 42, 2489. (b) Krüger, T.; Vorndran, K.; Linker, T. Chem. - Eur. J. 2009, 15, 12082. (c) Bramborg, A.; Linker, T. Eur. J. Org. Chem. 2012, 2012, 5552.
position is unfavorable (cyclopalladated hindered intermediate C). Subsequent olefin coordination to B delivers intermediate D, which is followed by a 1,2-migratory insertion to give intermediate E. β-Hydride elimination and reductive elimination occurs to deliver the olefinated Ag-carboxylate adduct F, along with the formation of a Pd(0) species that can be further oxidized by Ag(I), regenerating the active Pd(II) species and completing the catalytic cycle. Meanwhile, the olefinated Agcarboxylate F readily undergoes decarboxylation in the presence of a Pd catalyst and is subsequently rearomatized to produce the desired ortho-alkylated vinylarenes. In conclusion, we have reported a practical and straightforward method for preparing a series of ortho-alkylated vinylarenes from readily available benzoic acid derivatives using Birch reductive alkylation followed by a tandem decarboxylative C−H olefination/rearomatization. Moreover, we have also demonstrated that the Pd/Ag bimetallic system is critical for performing decarboxylative C−H olefination of cyclohexa-2,5-dienyl-1-carboxylic acids 1 under standard conditions. Further studies to explore the regioselective palladium-catalyzed decarboxylative γ-arylation10a of the olefinated cyclohexa-2,5-dienyl-1-carboxylic acid 6 and related catalytic C−H functionalizations of the cyclohexa-2,5-dienyl-1carboxylic acids 1 are ongoing in our laboratory.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00064. Experimental details and NMR spectra (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chih-Ming Chou: 0000-0002-9894-5365 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully thank the National University of Kaohsiung and the Ministry of Science and Technology of Taiwan (MOST 1331
DOI: 10.1021/acs.orglett.8b00064 Org. Lett. 2018, 20, 1328−1332
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