Triflate Hybrid Benzdiyne Equivalents: Access to

Jun 14, 2017 - The 1,4-benzdiyne equvalent, [2,5-bis(trimethylsilyl)-4-(trifyloxy)phenyl](phenyl)iodonium triflate, was prepared from sodium 2,4 ...
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Hypervalent Iodine/Triflate Hybrid Benzdiyne Equivalents: Access to Controlled Synthesis of Polycyclic Aromatic Compounds Tsugio Kitamura,* Keisuke Gondo, and Juzo Oyamada Department of Chemistry and Applied Chemistry, Graduate School of Science and Engineering, Saga University, Honjo-machi, Saga 840-8502, Japan S Supporting Information *

As the benzdiyne strategy, several approaches have been studied. The representative precursors of benzdiynes are given in Scheme 2. Although Wittig and Härle originally reported

ABSTRACT: The 1,4-benzdiyne equvalent, [2,5-bis(trimethylsilyl)-4-(trifyloxy)phenyl](phenyl)iodonium triflate, was prepared from sodium 2,4,5-trichlorophenoxide. The chemoselective generation of an aryne from the side of the phenyliodonio group was observed after treatment with a fluoride ion. Double cycloaddition of 1,4-benzdiyne with different arynophiles was conducted in one pot, giving bis-cycloadducts in high yields. Similarly, the 1,3benzdiyne equivalent bearing phenyliodonio and triflate groups was prepared from sodium 2,3,6-trichlorophenoxide. The 1,3-benzdiyne equivalent also underwent the chemoselective stepwise generation of arynes and the double cycloaddition with different arynophiles. These hybrid benzdiyne equivalents provided the double cycloadducts in high yields and enabled the convenient one-pot procedure for synthesis of polycyclic aromatic compounds.

Scheme 2. Examples of Benzdiyne Equivalents (Only 1,4Benzdiyne Equivalents Are Shown Here)

synthesis of polycyclic aromatic compounds via the benzdiyne strategy using tetrahalobenzenes, the yield was low.4 Hart and coworkers developed the benzdiyne strategy extensively and synthesized various types of polycyclic aromatic compounds.5 They also explored benzo[1,2-d:4,5-d]bistriazole-1,5-diamine as the 1,4-benzdiyne equivalent.6 Suzuki developed diiodobenzene bis-sulfonates possessing different leaving groups of OTs and OTf.7 Using the different leaving ability of OTs and OTf, two kinds of cycloaddition reactions were performed at different temperatures (−95 and −78 °C), respectively. In the case of halogenated precursors, however, a strong base such as butyllithium or phenyllithium is required for generation of arynes. On the other hand, Wudle and Peñ a reported bis(trimethylsilyl)phenyl ditriflate for double cycloaddition to construct polycyclic compounds.8 Recently, Ikawa and Akai improved the synthesis of bis(trimethylsilyl)phenyl ditriflate and conducted sequential cycloaddition reactions.9 In addition, they reported phenyl ditriflate bearing different silyl groups of trimethylsilyl and tert-butyldimethylsilyl, in which the difference in reactivity between Me3Si and t-BuMe2Si groups enabled the stepwise cycloaddition reactions.9 However, the total yields of polycyclic compounds in the one-pot procedure were not high. Tetrakis-silylated benzene derivatives via 2-silylphenyliodonium salts were developed as the benzdiyne precursors by several researchers. Tetrakis(trimethylsilyl)benzenes10 and benzobisoxadisiloles11 were applied to the synthesis of polycyclic aromatic compounds. However, these approaches involve the

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ighly efficient, chemoselective hybrid benzdiyne equivalents composed of hypervalent iodine and triflate (OTf) are demonstrated for synthesis of polycyclic aromatic compounds. Although phenyliodonio and OTf groups have been used as an excellent leaving group for generation of arynes,1 they have not been used together until now. Polycyclic aromatic compounds possessing linear and angular structures have attracted much attention because of their potent applications to materials and pharmaceutical uses. As the synthetic methods, cycloaddition reactions with benzynes have been studied for many years.2 One of the most efficient approaches to such polycyclic aromatic compounds is the double cycloaddition reaction of benzdiynes with arynophiles, which can directly construct cyclic skeletons as shown in Scheme 1. Since 1,3- and 1,4-benzdiynes are fairly high in energy and isomerize,3 it is difficult to use them as the intermediates in organic synthesis. Therefore, a sequential, stepwise process as the formal benzdiyne strategy has been conducted. Scheme 1. Benzdiyne Strategies for Linear and Angular Polycyclic Aromatic Compounds

Received: May 2, 2017 Published: June 14, 2017 © 2017 American Chemical Society

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DOI: 10.1021/jacs.7b04483 J. Am. Chem. Soc. 2017, 139, 8416−8419

Communication

Journal of the American Chemical Society repeated preparations of 2-silylphenyliodonium triflates and their reactions. Here we wish to demonstrate hitherto unknown, novel hybrid benzdiyne equivalents (synthetic equivalents) bearing different leaving groups of hypervalent iodine and triflate. Although the OTf group is recognized as a good leaving group, the leaving ability of the phenyliodonio group is much higher than that of the OTf group (more than 106).12 Since there is a large difference in leaving ability between the phenyliodonio group and OTf group, aryne A can be generated chemoselectively and undergo sequential cycloaddition reaction through aryne B, as shown in Scheme 3. Namely, the first generation of aryne A and the second generation of aryne B can be controlled simply by the reaction temperature.

Scheme 4. Chemoselective Generation of an Aryne from 4

Since the chemoselective generation of aryne A even under the different conditions was confirmed, we examined the double cycloaddition reactions of 4. The results are given in Table 1. First, double cycloaddition reactions of 4 with 2,5-dimethylfuran were conducted. A solution of 4 in MeCN was treated with CsF (3 equiv) in the presence of 2,5-dimethylfuran (3 equiv) at 0 °C for 10 min, and then the reaction mixture was warmed to 45 °C. After stirring at that temperature for 15 h, double cycloadduct 6a

Scheme 3. One-Pot Procedure for Synthesis of Polycyclic Aromatics via Sequential Cycloaddition Reactions by Hybrid Benzdiyne Equivalents

Table 1. Double Cycloaddition of 4 with Arynophilesa

Synthesis of hybrid 1,4-benzdiyne equivalent 4 was conducted as follows. Trimethylsilylation of sodium 2,4,5-trichlorophenoxide was conducted according to our reported method.13 The reaction of sodium 2,4,5-trichlorophenoxide with Me3SiCl in the presence of Mg, CuCl, and LiCl in DMI gave [2,4,5tris(trimethylsilyl)phenyl](trimethylsilyl)ether (1) in 80% yield. Desilylation of 1 with an ethanolic KOH solution provided 2,4,5-tris(trimethylsilyl)phenol (2) quantitatively. Treatment of 2 with Tf2O in the presence of pyridine afforded 2,4,5tris(trimethylsilyl)phenyl triflate (3) in 88% yield. Finally, [2,5bis(trimethylsilyl)-4-(trifyloxy)phenyl](phenyl)iodonium triflate (4) was obtained in 75% yield by the reaction of 3 with PhI(OAc)2/TfOH. The hybrid 1,4-benzdiyne equivalent 4 is in the form of nonhygroscopic and stable crystals and can be stored for a long period of time without any special cautions. Before conducting the double cycloaddition of hybrid 1,4benzdiyne equivalent 4, we examined the generation and trapping of aryne A (Scheme 3) in order to confirm the chemoselective generation of aryne A derived only from the side of the phenyliodonio group. When the reaction of 4 with 2,5dimethylfuran (1 equiv) was conducted in the presence of CsF (3 equiv) at 0 °C for 10 min, a single adduct 5a was obtained in 93% yield (Scheme 4). Similarly, the desilylation of 4 using a THF solution of Bu4NF (1 equiv) occurred selectively to give a single adduct 5b with tetraphenylcyclopentadienone in 81% yield. This method is suitable when an arynophile is difficult to dissolve in MeCN. These results indicate that the phenyliodonio/Me3Si part exhibits high reactivity toward the fluoride ion to generate aryne A predominantly under the present conditions.

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Conditions: 4 (0.5 mmol), arynophile 1 (0.5 mmol), CsF (1.5 mmol); arynophile 2 (1.5 mmol). bConditions: 4 (0.5 mmol), arynophile 1 (0.5 mmol), Bu4NF (0.5 mmol) instead of CsF, CH2Cl2 (5 mL) instead of MeCN; arynophile 2 (1.5 mmol), Bu4NF (1.0 mmol). 8417

DOI: 10.1021/jacs.7b04483 J. Am. Chem. Soc. 2017, 139, 8416−8419

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Journal of the American Chemical Society

chemoselective formation of cycloadduct 11a was observed, as shown in Scheme 6. In addition, the similar trapping reaction

was obtained in 90% yield (entry 1). Next, sequential cycloadditions were demonstrated. The first cycloaddition with 2,5-dimethylfuran (1 equiv) was carried out in the presence of CsF (3 equiv) in MeCN at 0 °C for 10 min, followed by the second cycloaddition with tetraphenylcyclopentadienone at 45 °C for 15 h. Double cycloadduct 6b derived from different arynophiles was obtained in 85% yield (entry 2). Similarly, the sequential cycloaddition reaction using 2,5-dimethylfuran/ anthracene gave double cycloadduct 6c in 77% yield (entry 3). In the arynophile combinations of tetraphenylcyclopentadienone/2,5-dimethylfuran and 2,6-diphenylisobenzofuran/2,5dimethyfuran, the sequential cycloaddition of 4 was conducted in CH2Cl2 using Bu4NF as the desilylating agent. Double cycloadducts 6b and 6d were obtained in 77% and 70% yields, respectively (entries 4 and 5). These double cycloaddition reactions are noteworthy, as synthesis of polyacenes since cycloadducts 6 can be readily converted into polyacene derivatives by deoxygenation.14 Next, we focused on 1,3-benzdiyne equivalent 10. First, according to the procedure for synthesis of 4, the synthesis of the 1,3-benzdiyne equivalent from sodium 2,3,6-trichlorophenoxide was examined. Trimethylsilylation of sodium 2,3,6-trichlorophenoxide with Me3SiCl in the presence of Mg, CuCl, and LiCl in DMI gave [2,3,6-tris(trimethylsilyl)phenyl](trimethylsilyl)ether (7) in 98% yield. However, the desilylation of 7 with an ethanolic KOH solution resulted in the formation of 2,5bis(trimethylsilyl)phenol.15 Then, we relinquished the procedure similar to the synthesis of 1,4-benzdiyne equivalent 4 to adopt the synthetic method for iodonium triflates from the corresponding iodoarenes.16 The outline of the synthesis is shown in Scheme 5.

Scheme 6. Chemoselective Generation of an Aryne from 10

with benzylazide gave benzotriazole 11b selectively. Thus, high chemoselectivity for generation of an aryne was observed even in the case of 1,3-benzdiyne equivalent 10. We examined briefly the sequential generation of arynes and their trapping reactions with arynophiles. The results are given in Table 2. Treatment of 10 with CsF in the presence of 2,5Table 2. Double Cycloaddition of 10 with Arynophilesa

Scheme 5. Synthesis of Hybrid 1,3-Benzdiyne Equivalent 10

First, we conducted iodination of 7 with I2/PhI(OAc)2 followed by desilylation with an ethanolic KOH solution and obtained 2-iodo-3,6-bis(trimethylsilyl)phenol (8) in 97% yield. After converting 8 to the corresponding triflate 9 with Tf2O in the presence of pyridine, we conducted the reaction of triflate 9 with anisole in the presence of mCPBA and TsOH·H2O in a mixed solution of TFE and CH2Cl2 followed by treatment with aqueous NaOTf to give 1,3-benzdiyne equivalent 10 in 76% yield. To confirm the chemoselective generation of an aryne, 1,3benzdiyne equivalent 10 was allowed to react with CsF in the presence of 2,5-dimethylfuran as a trapping agent and the

a

Conditions: 10 (0.5 mmol), arynophile 1 (0.5 mmol), CsF (1.5 mmol); arynophile 2 (1.5 mmol).

dimethylfuran (3 equiv) in MeCN was conducted at 0 °C followed by heating at 45 °C for 15 h. Double cycloadduct 12a was obtained in 72% yield (entry 1). Next, the first cycloaddition of 10 with 2,5-dimethylfuran (1 equiv) was conducted in the presence of CsF (3 equiv) in MeCN at 0 °C for 10 min, which was then followed by the second cycloaddition with tetraphenyl8418

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Journal of the American Chemical Society cyclopentadienone at 45 °C for 15 h. The double cycloadduct 12b was obtained in 74% yield (entry 2). Similarly, the sequential cycloaddition reactions of 10 with 2,5-dimethylfuran and anthracene afforded double cycloadduct 12c in 75% yield (entry 3). Furthermore, the cycloadditions with benzylazide and 2,5-dimethylfuran gave heterocyclic adduct 12d in 51% yield (entry 4). Thus, angular polycyclic aromatic compounds including heterocycles can be prepared easily using 1,3benzdiyne equivalent 10. In conclusion, we have demonstrated a new type of benzdiyne equivalents which are composed of phenyliodonio and OTf groups as the leaving group, together with a trimethylsilyl group. The hybrid benzdiyne equivalents can be prepared in four steps from commercially available sodium trichlorophenoxides in good yields. Each aryne can be generated by controlling the reaction temperature and allowed to perform the sequential cycloaddition with different arynophiles. Especially, the combination of phenyliodonio and OTf groups enables the chemoselective generation of an aryne by convenient temperature control at 0 and 45 °C. Double cycloadducts with different arynophiles are obtained in high yields via the one-pot procedure. The present strategy using the hybrid benzdiyne equivalents provides the quick construction of functionalized polycyclic aromatic compounds. Therefore, we believe that the hybrid benzdiyne equivalents are very useful reagents to construct polycyclic aromatic compounds. Further synthetic applications are currently underway.

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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/jacs.7b04483. Experimental procedures and spectral data of products (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Tsugio Kitamura: 0000-0001-5592-5228 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by JSPS KAKENHI Grant Number 16K05703. REFERENCES

(1) For a phenyliodonio group, see: (a) Kitamura, T.; Yamane, M. J. Chem. Soc., Chem. Commun. 1995, 983−984. (b) Kitamura, T.; Yamane, M.; Inoue, K.; Todaka, M.; Fukatsu, N.; Meng, Z.; Fujiwara, Y. J. Am. Chem. Soc. 1999, 121, 11674−11679. For an OTf group, see: Himeshima, Y.; Sonoda, T.; Kobayashi, H. Chem. Lett. 1983, 12, 1211− 1214. For a review of these groups, see: Kitamura, T. Aust. J. Chem. 2010, 63, 987−1001. (2) (a) Hoffmann, R. W. Dehydrobenzene and Cycloalkynes; Academic Press: New York, 1967. (b) Hoffmann, R. W. Dehydrobenzene and Cycloalkynes; Academic Press: New York, 1967. (c) Gilchrist, T. L. In The Chemistry of Triple-bonded Functional Groups, Supplement C; Patai, S., Rappoport, Z., Eds.; John Wiley & Sons: Chichester, 1983; Chapter 11, pp 383−419. (d) Kessar, S. V. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: New York, 1991; Vol. 4, pp 483−515. (e) Hart, H. In The Chemistry of Triple-bonded Functional 8419

DOI: 10.1021/jacs.7b04483 J. Am. Chem. Soc. 2017, 139, 8416−8419