Palladium-Catalyzed Borylation of Aryl Sulfoniums with Diborons

Letter Cite This: ACS Catal. 2018, 8, 579−583

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Palladium-Catalyzed Borylation of Aryl Sulfoniums with Diborons Hiroko Minami, Shinya Otsuka, Keisuke Nogi, and Hideki Yorimitsu* Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan S Supporting Information *

ABSTRACT: Palladium-catalyzed borylation of aryl sulfonium salts with bis(pinacolato)diboron has been achieved. Because of the sufficient reactivity of aryl sulfoniums and less catalyst-poisonous property of the leaving dialkyl sulfides, the present borylation proceeded under mild reaction conditions. Various functional groups, such as formyl, acetyl, cyano, nitro, and hydroxy groups, were well-tolerated. Aryl sulfoniums were easily prepared from the corresponding aryl sulfides and methyl triflate, and aryl sulfides could be transformed to the corresponding arylboronate esters in one-pot manners. The methylation selectively activated poorer leaving alkylsulfanyl units to culminate in selective borylation over chloro, tosyloxy, and better leaving arylsulfanyl and trifluoromethylsulfanyl units. KEYWORDS: palladium catalyst, borylation, aryl sulfonium, aryl sulfide, one-pot transformation

T

reaction was not compatible with important and useful functional groups, such as acyl, chloro, and hydroxy moieties. To overcome these problems, we focused on C−S scission of aryl sulfoniums instead of sulfides. In 1997, Liebeskind accomplished palladium-catalyzed Suzuki-Miyaura-type crosscoupling of aryl sulfoniums with arylboronic acids under mild conditions.12 This seminal and elegant work proposed that aryl sulfoniums would have considerable potential as aryl electrophiles. However, transformations of aryl sulfoniums as aryl electrophiles were limited to C−C bond formations.11a,13 To realize more-versatile C−S bond borylation, employment of aryl sulfoniums should bring two advantages: (1) the C−S bonds of aryl sulfoniums would be more reactive than those of aryl sulfides, because of their electron deficiency; (2) neutral sulfur fragments eliminated during the reaction would be less catalyst-poisonous than anionic thiolate species derived from aryl sulfides. With this consideration in mind, here, we report Pd-catalyzed borylation of aryl sulfonium salts with diborons under milder conditions than those of the previous works. Various functional groups such as acyl, cyano, nitro, chloro, and hydroxy groups were well-tolerated to afford the desired arylboronate esters. Borylation of aryl sulfonium 1 with bis(pinacolato)diboron (B2pin2) was chosen as a model reaction. First, we tried the reaction with catalytic amounts of Pd2(dba)3 and a biaryl-based phosphine SPhos, considering the previous successes in Buchwald’s C−Cl borylation4c and our C−Cl borylation,4i in the presence of KOAc as a base. To our delight, the corresponding borylated product 2 was obtained in 47% yield (Table 1, entry 1). However, a ring-opening byproduct 3 was formed in 48% yield. In order to suppress this undesired ring opening, several bases were examined. As a result, K3PO4 was

he great utility of organoboron compounds as synthetic intermediates, functional materials, or pharmaceutical products has prompted researchers to develop new protocols for the synthesis of organoboron compounds.1 Among them, Miyaura-type borylation of aryl (pseudo)halides is one of the most potent tools for synthesizing arylboronate esters.2,3 While aryl iodides, bromides, and triflates have been widely used as substrates, recent research has enabled borylation of inert bonds, such as the C−Cl,4 C−F,5 C−O,6 C−N,7 and C−C8 bonds. Because of the versatility and utility of organosulfur compounds,9 C−S bond borylation should also be an attractive transformation. Recently, our group, as well as Hosoya, reported Pd- and Rh-catalyzed borylation of aryl sulfides, respectively (Schemes 1a and 1b).10,11 However, because of the necessity of strongly basic LiN(SiMe3)2 as an activator, the former suffered from poor functional group compatibility. Although the latter reaction required no external base, the Scheme 1. Borylation of C−S Bonds

Received: November 11, 2017 Revised: December 11, 2017 Published: December 11, 2017 © XXXX American Chemical Society

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ACS Catalysis

of all volatiles under a reduced pressure, one-pot borylation was attempted by adding Pd(OAc)2, XPhos, B2pin2, K3PO4, and tetrahydrofuran (THF). Fortunately, arylboronate ester 5a was obtained in 94% NMR yield and 74% isolated yield over two steps (Scheme 2).

Table 1. Optimization of Reaction Conditions

Scheme 2. One-Pot Borylation of Aryl Sulfide entry

catalysta

base

NMR yield (%)

1 2 3 4 5c 6c 7c 8c,e 9 10 11 12

Pd2(dba)3/SPhos Pd2(dba)3/SPhos Pd2(dba)3/SPhos Pd2(dba)3/SPhos Pd2(dba)3/SPhos Pd2(dba)3/XPhos Pd(OAc)2/XPhos Pd(OAc)2/XPhos Pd(PPh3)2Cl2 Pd(PCy3)2Cl2 Pd(dppf)Cl2 Pd-PEPPSI-IPr

KOAc K2CO3 K3PO4 NEt3 K3PO4 K3PO4 K3PO4 K3PO4 K3PO4 K3PO4 K3PO4 K3PO4

47b 71 80 7 89 91 91 (61)d 83 23 20 60 46

By means of this one-pot borylation method, we then examined the scope of aryl methyl sulfides 4 (Scheme 3).15 Aryl

a

Scheme 3. Scope of Aryl Methyl Sulfidesa

b

Ring-opening byproduct 3 was formed in 48% yield. cThe reaction was conducted in THF (0.13 M) at 60 °C, 2 h. dIsolated yield. e1 mol % of catalyst.

found to be effective to provide 2 in 80% yield without the formation of any possible ring-opening products (entry 3 in Table 1). Attempted borylation with NEt3 as a base afforded the corresponding ring-opening product in 61% yield, along with a 7% yield of 3 (entry 4 in Table 1). The desired product 2 was furnished in higher yield at lower reaction temperature under more diluted conditions (entry 5 in Table 1). We then screened a series of biaryl-based phosphine ligands. Eventually XPhos was found to be optimal to afford the product 2 in 91% NMR yield and 61% isolated yields (entries 6 and 7). In our previous borylation,10a >10 mol % catalyst loading was required, because of the leaving catalyst-poisonous thiolate species. On the other hand, the present borylation smoothly proceeded, even with 1 mol % of the palladium catalyst, since less catalystpoisonous dialkyl sulfide was the leaving group (entry 8 in Table 1). Other palladium complexes having triarylphosphine, trialkylphosphine, dppf,12 and an NHC ligand showed lower catalytic activities (entries 9−12 in Table 1).14 Although we found that the borylation of aryl sulfonium proceeded efficiently, preparation of aryl sulfonium salts often suffered from multistep manipulations and difficulty in purification. Therefore, we envisioned one-pot borylation of readily and widely available aryl sulfides through methylation of their S atom, followed by the present palladium-catalyzed borylation. Treatment of methyl p-tolyl sulfide (4a) with MeOTf in 1,2dichloroethane (1,2-DCE) afforded the corresponding aryl sulfonium 4a-Me (confirmed with 1H NMR). After the removal

a

Isolated yield. NMR yields are shown in parentheses. bIsolated as the corresponding phenol derivative after oxidation with H2O2-urea. c SPhos instead of XPhos.

methyl sulfides bearing a methoxy, fluoro, or trimethylsilyl moiety afforded the products 5b−5d uneventfully. Because of the mild reaction conditions, the present borylation accommodated a remarkably wide range of functional groups. Indeed, carbonyl moieties such as formyl and acetyl were compatible. A cyano group also remained intact during the borylation to furnish 5g in good yield. Attempted borylation of nitrosubstituted aryl sulfide 4h gave rise to a low yield of the borylated product 5h, probably because of its low solubility in THF (vide infra). The methylation with MeOTf selectively 580

DOI: 10.1021/acscatal.7b03841 ACS Catal. 2018, 8, 579−583

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ACS Catalysis Scheme 5. Methylsulfanyl-Selective Borylationa

proceeded at the methylsulfanyl moiety, and potentially reactive hydroxy, acetylamido, and carboxy groups remained untouched to afford 5i−5k. Sterically hindered 4l was smoothly borylated by using smaller SPhos, instead of XPhos as a ligand.16 This one-pot method could be also applied to aryl dodecyl sulfides 6 (Scheme 4). Advantageously, aryl dodecyl sulfides 6 Scheme 4. Borylation of Aryl Dodecyl Sulfidesa

a

Isolated yield. NMR yields are shown in parentheses.

Scheme 6. Intermolecular Competition Reactionsa

a Isolated yield. NMR yields are shown in parentheses. bIsolated as the corresponding phenol derivatives.

afforded the desired borylated products in better yields than the corresponding aryl methyl sulfides 4 (compare Scheme 3 vs Scheme 4 for 5e, 5g, and 5h). This is probably because the aryl(dodecyl)methylsulfoniums are more soluble and more reluctant to demethylation than aryldimethylsulfoniums 4-Me. Since aryl dodecyl sulfides that contain electron-withdrawing groups were readily accessible from the corresponding nitroarenes,17 various nitroarenes could be converted to the borylated compounds in a two-pot three-step process. Through the one-pot borylation protocol, a reluctant alkylsulfanyl moiety was selectively activated through methylation and transformed to the boronate moiety under mild reaction conditions. We thus envisioned that chemoselectivity in conventional borylations could be reversed by means of the one-pot borylation protocol. It was reported that the C(sp2)−S bonds of alkyl aryl sulfides are much less reactive toward catalytic borylation than C(sp2)−Cl4c and C(sp2)−OTs18 bonds. In contrast, our one-pot two-step borylation of 4chlorothioanisole (4p) furnished 5p with the chloro group basically untouched (Scheme 5a).19 In a similar fashion, 4tosyloxythioanisole (4q) underwent borylation at the methylsulfanyl moiety with our protocol to afford 5q without deterioration of the tosyloxy moiety (Scheme 5b). These results clearly underline the orthogonality of our C−S borylation via sulfanyl-selective activation and the conventional C−X borylations. Diaryl sulfides are usually more reactive than alkyl aryl sulfides, because the former has the better leaving group (ArS−) and undergoes smoother oxidative addition and transmetalation. Naturally, treatment of a 1:1 mixture of methyl p-tolyl sulfide (4a, 0.50 mmol) and diphenyl sulfide (4r, 0.50 mmol) with 0.50 mmol of B2pin2 under our previous Pd-NHC/ LiN(SiMe3)2 conditions10a led to selective conversions of 4r to yield 5r mainly (Scheme 6a). In contrast, our present

a

NMR yields.

activation/borylation protocol shows the opposite selectivity: The methylation of the same mixture with MeOTf proceeded preferentially with more nucleophilic 4a, and the following borylation afforded 5a with the recovery of 89% of 4r. Similarly, because of the poor nucleophilicity of the trifluoromethylsulfanyl moiety of 4s, treatment of a 1:1 mixture of 4a and 4s furnished 5a predominantly while 5r was not detected (Scheme 6b). In conclusion, we have developed palladium-catalyzed borylation of aryl sulfonium salts with B2pin2. Because of their high reactivity, the borylation proceeded under mild reaction conditions and accommodated a wide range of functional groups, including acyl, cyano, nitro, and hydroxy groups. Since aryl methyl sulfoniums could be prepared in situ from the corresponding sulfides with methyl triflate, aryl sulfides could be transformed into arylboronates in one pot. The SN2 methylation proceeded predominantly at the more nucleophilic sulfanyl moiety that is originally more reluctant to undergo oxidative addition. Hence, the methylation activated poorer leaving sulfur units to culminate in selective borylation over chloro, tosyloxy, and better leaving sulfur units. The present activation/borylation protocol provides an additional useful and reliable way to convert inert molecules to organoboron compounds of prominent importance. 581

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Hosoya, T. J. Am. Chem. Soc. 2015, 137, 14313−14318. (e) Zhou, J.; Kuntze-Fechner, M. W.; Bertermann, R.; Paul, U. S. D.; Berthel, J. H. J.; Friedrich, A.; Du, Z.; Marder, T. B.; Radius, U. J. Am. Chem. Soc. 2016, 138, 5250−5253. (f) Niwa, T.; Ochiai, H.; Hosoya, T. ACS Catal. 2017, 7, 4535−4541. (6) For borylation of aryl carbamates and esters, see: (a) Huang, K.; Yu, D. G.; Zheng, S. F.; Wu, Z. H.; Shi, Z. J. Chem.Eur. J. 2011, 17, 786−791. (b) Kinuta, H.; Hasegawa, J.; Tobisu, M.; Chatani, N. Chem. Lett. 2015, 44, 366−368. For borylation of aryl ethers, see: (c) Kinuta, H.; Tobisu, M.; Chatani, N. J. Am. Chem. Soc. 2015, 137, 1593−1600. (d) Zarate, C.; Manzano, R.; Martin, R. J. Am. Chem. Soc. 2015, 137, 6754−6757. (e) Tobisu, M.; Zhao, J.; Kinuta, H.; Furukawa, T.; Igarashi, T.; Chatani, N. Adv. Synth. Catal. 2016, 358, 2417−2421. For boron insertion into endocyclic C−O bonds of benzofurans, see: (f) Saito, H.; Otsuka, S.; Nogi, K.; Yorimitsu, H. J. Am. Chem. Soc. 2016, 138, 15315−15318. (7) For borylation of N-aryl amides, see: (a) Tobisu, M.; Nakamura, K.; Chatani, N. J. Am. Chem. Soc. 2014, 136, 5587−5590. (b) Hu, J.; Zhao, Y.; Liu, J.; Zhang, Y.; Shi, Z. Angew. Chem., Int. Ed. 2016, 55, 8718−8722. For borylation of ammonium salts, see: (c) Zhang, H.; Hagihara, S.; Itami, K. Chem.Eur. J. 2015, 21, 16796−16800. (d) Hu, J.; Sun, H.; Cai, W.; Pu, X.; Zhang, Y.; Shi, Z. J. Org. Chem. 2016, 81, 14−24. (8) For borylation of aryl cyanides, see: (a) Tobisu, M.; Kinuta, H.; Kita, Y.; Rémond, E.; Chatani, N. J. Am. Chem. Soc. 2012, 134, 115− 118. For decarbonylative borylation of aryl esters, see: (b) Pu, X.; Hu, J.; Zhao, Y.; Shi, Z. ACS Catal. 2016, 6, 6692−6698. For decarbonylative borylation of aryl thioesters, see: (c) Ochiai, H.; Uetake, Y.; Niwa, T.; Hosoya, T. Angew. Chem., Int. Ed. 2017, 56, 2482−2486. (9) (a) Cremlyn, R. J. An Introduction to Organosulfur Chemistry; Wiley: New York, 1996. (b) Ruano, J. L. G.; de la Plata, B. C. In Organosulfur Chemistry I; Page, P. C. B., Ed.; Springer: Heidelberg, Germany, 1999; p 1. (c) Furukara, N.; Sato, S. In Organosulfur Chemistry II; Page, P. C. B., Ed.; Springer: Heidelberg, Germany, 1999; p 89. (d) Rayner, C. M.; Advances in Sulfur Chemistry, Vol. 2; JAI Press: Greenwich, CT, 2000. (e) Sulfur Compounds: Advances in Research and Application; Acton, A. Q., Ed.; Scholarly Eds.: Atlanta, GA, 2012. (f) Feng, M.; Tang, B.; Liang, S. H.; Jiang, X. Curr. Top. Med. Chem. 2016, 16, 1200−1216. (10) (a) Bhanuchandra, M.; Baralle, A.; Otsuka, S.; Nogi, K.; Yorimitsu, H.; Osuka, A. Org. Lett. 2016, 18, 2966−2969. (b) Uetake, U.; Niwa, T.; Hosoya, T. Org. Lett. 2016, 18, 2758−2761. (11) For reviews on catalytic C−S bond transformations, see: (a) Dubbaka, S. R.; Vogel, P. Angew. Chem., Int. Ed. 2005, 44, 7674− 7684. (b) Prokopcová, H.; Kappe, C. O. Angew. Chem., Int. Ed. 2008, 47, 3674−3676. (c) Wang, L.; He, W.; Yu, Z. Chem. Soc. Rev. 2013, 42, 599−621. (d) Modha, S. G.; Mehta, V. P.; Van der Eycken, E. V. Chem. Soc. Rev. 2013, 42, 5042−5055. (e) Pan, F.; Shi, Z.-J. ACS Catal. 2014, 4, 280−288. (f) Gao, K.; Otsuka, S.; Baralle, A.; Nogi, K.; Yorimitsu, H.; Osuka, A. J. Synth. Org. Chem. Jpn. 2016, 74, 1119− 1127. (g) Ortgies, D. H.; Hassanpour, A.; Chen, F.; Woo, S.; Forgione, P. Eur. J. Org. Chem. 2016, 2016, 408−425. (12) Srogl, J.; Allred, G. D.; Liebeskind, L. S. J. Am. Chem. Soc. 1997, 119, 12376−12377. (13) (a) Zhang, S.; Marshall, D.; Liebeskind, L. S. J. Org. Chem. 1999, 64, 2796−2804. (b) Lin, H.; Dong, X.; Li, Y.; Shen, Q.; Lu, L. Eur. J. Org. Chem. 2012, 2012, 4675−4679. (c) Vasu, D.; Yorimitsu, H.; Osuka, A. Angew. Chem., Int. Ed. 2015, 54, 7162−7166. (d) Vasu, D.; Yorimitsu, H.; Osuka, A. Synthesis 2015, 47, 3286−3291. (e) Wang, S.M; Song, H.-X.; Wang, X.-Y.; Liu, N.; Qin, H.-L.; Zhang, C.-P. Chem. Commun. 2016, 52, 11893−11896. (f) Tian, Z.-Y.; Wang, S.-M.; Jia, S.J.; Song, H.-X.; Zhang, C.-P. Org. Lett. 2017, 19, 5454−5457. (14) Other ethereal solvents, such as 1,4-dioxane and cyclopentyl methyl ether (CPME), were less effective than THF to afford the product in 71% and 51% yields, respectively. See Figure S1 for more details. (15) Isolated yields of some of the borylated products were much lower than the corresponding NMR yields, because of difficulty in

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b03841. Detailed experimental procedures, and full spectroscopic data for all new compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shinya Otsuka: 0000-0001-5598-9707 Keisuke Nogi: 0000-0001-8478-1227 Hideki Yorimitsu: 0000-0002-0153-1888 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Nos. JP16H01019, JP16H04109, JP16H06887, as well as JST ACTC Grant No. JPMJCR12ZE, Japan. S.O. acknowledges a JSPS Predoctoral Fellowship.

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REFERENCES

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ACS Catalysis separation from B2 pin2 or boron-containing byproducts. To demonstrate catalytic activity of the present system, the NMR yields are shown together. (16) Although we tried the one-pot borylation of heteroaryl sulfides such as 2-pyrazyl and 2-benzothiazolyl sulfide, methylation with MeOTf proceeded selectively at the nitrogen atoms and no borylated products were obtained. (17) Kondoh, A.; Yorimitsu, H.; Oshima, K. Tetrahedron 2006, 62, 2357−2360. (18) (a) Chow, W. K.; So, C. M.; Lau, C. P.; Kwong, F. Y. Chem. Eur. J. 2011, 17, 6913−6917. (b) Murata, M.; Oda, T.; Sogabe, Y.; Tone, H.; Namikoshi, T.; Watanabe, S. Chem. Lett. 2011, 40, 962− 963. (19) In this case, the use of XPhos resulted in unselective formation of a 1:1 mixture of 5p and 5p′ in 70% yield.

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