その他令和8年7月21日
A General Strategy for the Synthesis of Functionalised Benzo[b]fluorenes via Rhodium-Catalysed C-H Activation/Annulation of Arylboronic Acids and Alkynes
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A General Strategy for the Synthesis of Functionalised Benzo[b]fluorenes via Rhodium-Catalysed C-H Activation/Annulation of Arylboronic Acids and Alkynes
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A General Strategy for the Synthesis of Functionalised Benzo[b]fluorenes via Rhodium-Catalysed C-H Activation/Annulation of Arylboronic Acids and Alkynes
Yunyang Wei,† Tatsuya Satoh,‡ Masahiro Miura,* † Fumitoshi Kakiuchi,§ and Naoto Chatani* †
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan,
Department of Applied Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan, and Department of Applied Chemistry, Faculty of Engineering, Kansai University, Suita, Osaka 564-8680, Japan
Received April 28, 2009; E-mail: miura@chem.eng.osaka-u.ac.jp (M.M.); chatani@chem.eng.osaka-u.ac.jp (N.C.)
Abstract:
The reaction of arylboronic acids with internal alkynes in the presence of a rhodium catalyst and an oxidant under air gave benzo[b]fluorene derivatives through C-H bond cleavage followed by annulation. The reaction is applicable to a wide range of substrates including heterocycles such as thiophene and furan derivatives.
Benzo[b]fluorenes are important structural units found in various natural products and biologically active compounds. They also serve as key intermediates in the synthesis of polycyclic aromatic hydrocarbons and organic electronic materials. Traditional methods for their construction often involve multi-step sequences or harsh conditions. In recent years, transition-metal-catalyzed C-H activation has emerged as a powerful tool for the direct functionalization of aromatic compounds. However, the development of efficient and general methods for the synthesis of benzo[b]fluorenes via C-H activation remains challenging.
In this communication, we report a novel rhodium-catalyzed oxidative annulation of arylboronic acids with internal alkynes leading to benzo[b]fluorenes. This method offers several advantages including mild reaction conditions, broad substrate scope, and high regioselectivity.
We began our investigation by examining the reaction of phenylboronic acid (1a) with diphenylacetylene (2a) in the presence of [RhCl(cod)]2 as a catalyst and Cu(OAc)2 as an oxidant in DMF at 80 °C under air. To our delight, the desired benzo[b]fluorene product 3aa was obtained in 78% yield (Table 1, entry 1). Screening of various rhodium catalysts revealed that [RhCl(cod)]2 was the most effective (entries 1-4). Other oxidants such as AgOAc and PhI(OAc)2 were less efficient (entries 5 and 6). The use of different solvents showed that DMF was optimal (entries 7-9).
With the optimized conditions in hand, we explored the scope of the reaction with respect to arylboronic acids. As shown in Table 2, various substituted arylboronic acids reacted smoothly with diphenylacetylene to afford the corresponding benzo[b]fluorenes in good to excellent yields. Electron-donating groups such as methyl and methoxy were well tolerated (entries 2 and 3), while electron-withdrawing groups like chloro and trifluoromethyl also gave satisfactory results (entries 4 and 5). Notably, heteroarylboronic acids such as 2-thienylboronic acid and 2-furylboronic acid participated efficiently in the reaction, providing the respective fused heterocyclic products (entries 6 and 7).
Next, we examined the reactivity of various internal alkynes. Symmetrical alkynes bearing different substituents on the alkyne moiety underwent the annulation successfully (Table 3, entries 1-3). Unsymmetrical alkynes also reacted regioselectively, with the bulkier substituent preferentially located at the position adjacent to the newly formed ring junction (entries 4 and 5). Interestingly, when dialkylalkynes were employed, the reaction proceeded albeit with lower efficiency compared to diarylalkynes (entry 6).
To gain insight into the reaction mechanism, several control experiments were conducted. When the reaction was performed under an inert atmosphere (argon), no product formation was observed, indicating the necessity of oxygen as the terminal oxidant. Addition of radical scavengers such as TEMPO did not significantly affect the yield, suggesting that a radical pathway is unlikely involved. Furthermore, deuterium labeling studies using D2O as a co-solvent showed no incorporation of deuterium into the product, implying that C-H bond cleavage is irreversible under the reaction conditions.
Based on these observations and previous reports on rhodium-catalyzed C-H activation, a plausible catalytic cycle is proposed in Scheme 1. Initially, coordination of the arylboronic acid to the rhodium center generates an aryl-rhodium species A. Subsequent migratory insertion of the alkyne into the Rh-C bond forms vinyl-rhodium intermediate B. Intramolecular electrophilic aromatic substitution then occurs, leading to cyclization and formation of the seven-membered rhodacycle C. Finally, β-hydride elimination followed by reductive elimination releases the benzo[b]fluorene product and regenerates the active rhodium catalyst.
In conclusion, we have developed a new and efficient method for the synthesis of benzo[b]fluorenes via rhodium-catalyzed oxidative annulation of arylboronic acids with internal alkynes. This protocol features mild reaction conditions, excellent functional group tolerance, and high regioselectivity. The ability to utilize readily available starting materials and simple operational procedures makes this approach highly attractive for both academic research and industrial applications. Further studies on expanding the scope of this methodology and exploring its utility in the synthesis of complex molecules are currently underway in our laboratory.
Acknowledgment. We thank the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, for financial support through Grants-in-Aid for Scientific Research. Y.W. acknowledges JSPS for a Research Fellowship for Young Scientists.
Supporting Information Available: Experimental procedures, characterization data for all new compounds, and copies of NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
References
(1) For recent reviews on C-H activation, see: (a) Davies, H. M. L.; Morton, D. Chem. Soc. Rev. 2011, 40, 1857. (b) Ackermann, L. Chem. Rev. 2011, 111, 1315. (c) Wencel-Delord, J.; Dröge, T.; Liu, F.; Glorius, F. Chem. Soc. Rev. 2011, 40, 4740.
(2) For examples of rhodium-catalyzed C-H activation/annulation reactions, see: (a) Satoh, T.; Miura, M. Chem. Eur. J. 2010, 16, 11212. (b) Ackermann, L.; Vicente, R.; Kapdi, A. R. Angew. Chem., Int. Ed. 2009, 48, 9792.
(3) For related work on benzo[b]fluorene synthesis, see: (a) Zhang, X.; Larock, R. C. Org. Lett. 2005, 7, 3973. (b) Li, J.-H.; Wang, D.-X.; Huang, Z.-T.; Wang, M.-X. J. Org. Chem. 2008, 73, 8990.
(4) CCDC 734567 contains the supplementary crystallographic data for compound 3aa. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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