Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Photocatalytic Reaction of Potassium Alkyltrifluoroborates and Sulfur Dioxide with Alkenes Tong Liu,† Yuewen Li,† Lifang Lai,‡ Jiang Cheng,‡ Jiangtao Sun,‡ and Jie Wu*,†,§ †
Department of Chemistry, Fudan University, 2005 Songhu Road, Shanghai 200438, China School of Petrochemical Engineering, and Jiangsu Province Key Laboratory of Fine Petrochemical Engineering, Changzhou University, Changzhou 213164, China § State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China ‡
S Supporting Information *
ABSTRACT: A three-component reaction of potassium alkyltrifluoroborates, the sulfur dioxide surrogate of DABCO·(SO2)2, and alkenes under photocatalysis in the presence of visible light is developed. This reaction works efficiently at room temperature with the insertion of sulfur dioxide under mild conditions, affording diverse sulfones in good to excellent yields. The alkyl radical and alkylsulfonyl radical are generated as key intermediates, and a reductive single-electron transfer is involved in the reaction process.
U
Scheme 1. Sulfonylation of Olefin
sing sulfur dioxide surrogate of DABCO·(SO2)2 or inorganic sulfites as the source of sulfur dioxide has become a facile pathway for the synthesis of sulfonyl compounds.1−3 In the past few years, we discovered that the combination of DABCO·(SO2)2 with aryldiazonium tetrafluoroborates provided an efficient route to arylsulfonyl radicals,4 thus leading to diverse sulfonyl compounds under mild conditions. Although this strategy is attractive, a drawback associated with this method is the scope limitation since only aryldiazonium tetrafluoroborates could be used in the transformation. The alkyldiazonium tetrafluoroborates are extremely unstable, thus hampering further application of this approach. Therefore, the development of a complementary route for the generation of alkylsulfonyl compounds through alkylsulfonyl radicals is highly desirable and challenging. Under visible-light-induced photoredox catalysis, alkyl radicals could be generated easily from the corresponding organotrifluoroborates.5−7 For example, alkylation of olefins could be achieved through photocatalytic reactions of alkyltrifluoroborates.6 Inspired by these results, we envisioned that sulfur dioxide could be incorporated in the transformation. Thus, alkylsulfonyl radicals would be afforded under photocatalysis in the presence of visible light from alkyltrifluoroborates with sulfur dioxide. For the previous report on the use of DABCO·(SO2)2, aryldiazonium tetrafluoroborates, and alkenes,4 an oxidative single-electron transfer (SET) would occur after the addition of arylsulfonyl radical to alkene (Scheme 1, eq a). However, a reductive SET would occur if alkyltrifluoroborates were employed in the reaction of sulfur dioxide with alkenes (Scheme 1, eq b). Herein, we report the first example for the synthesis of alkylsulfonyl compounds from a three-component reaction of potassium alkyltrifluoroborates, sulfur dioxide surrogate of DABCO·(SO2)2, and alkenes under © XXXX American Chemical Society
photocatalysis in the presence of visible light. During the reaction process, alkyl radical and alkylsulfonyl radical are involved as the key intermediates. The importance of alkylsulfonyl compounds is known since some compounds with alkylsulfonyl units are marketed drugs or biologically active molecules.8 Initial studies were performed for the reaction of 4vinylpyridine 1a, potassium cyclopentyltrifluoroborate 2a, and DABCO·(SO2)2 (Table 1). At the beginning, the reaction was carried out in 1,2-dichloroethane (DCE) in the presence of visible light under catalyst-free conditions (Table 1, entry 1). Under the conditions, no reaction occurred. Several photocatalysts were then introduced in the reaction. The reaction failed to produce the corresponding product when iridium photocatalyst was used in the presence of blue LED (12 W) (Table 1, entries 2 and 3). The desired product 3a could be formed in 92% yield when 9-mesityl-10-methyl acridinium perchlorate [Acr+-Mes]ClO4 was employed as a replacement Received: May 1, 2018
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DOI: 10.1021/acs.orglett.8b01385 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 1. Initial Studies for the Reaction of 4-Vinylpyridine 1a, Potassium Cyclopentyltrifluoroborate 2a, and DABCO· (SO2)2a
entry
photocatalyst
solvent
yieldb (%)
1 2 3 4 5 6 7 8 9 10c 11d 12e
Ir(dfCF3ppy)2bpy]PF6 [Ir(dfCF3ppy)2(dtbbpy)]PF6 Mes-Acr+ Mes-Acr+ Mes-Acr+ Mes-Acr+ Mes-Acr+ Mes-Acr+ Mes-Acr+ Mes-Acr+ Mes-Acr+
DCE DCE DCE DCE MeCN MeOH 1,4-dioxane DMF DMSO DCE DCE DCE
0 trace trace 92 68 27 81 trace trace 77 81 0
Scheme 2. Scope Investigation for the Reaction of Potassium Alkyltrifluoroborates, DABCO·(SO2)2, and Alkenes (Isolated Yield Based on Alkene 1)
a
Reaction conditions: 4-vinylpyridine 1a (0.2 mmol), potassium cyclopentyltrifluoroborate 2a (0.3 mmol, 1.5 equiv), DABSO (0.16 mmol, 0.8 equiv), solvent (3.0 mL), N2, rt, under blue LED irradiation (12 W) for 24 h. bIsolated yield based on 4-vinylpyridine 1a. cIn the presence of Mes-Acr+ (2 mol %). dIn the presence of potassium cyclopentyltrifluoroborate 2a (0.24 mmol, 1.2 equiv). eIn the dark.
(Table 1, entry 4). We further explored the reaction in various solvents, and DCE was found to be the best choice (Table 1, entries 5−9). Inferior results were obtained when the amount of 9-mesityl-10-methyl acridinium perchlorate or potassium cyclopentyltrifluoroborate 2a was decreased (Table 1, entries 10 and 11). No product was obtained when the reaction took place in the dark (Table 1, entry 12). After the optimal conditions were obtained, the scope of this three-component reaction of potassium alkyltrifluoroborates, the sulfur dioxide surrogate of DABCO·(SO2)2, and alkenes under photocatalysis in the presence of visible light was then explored (Scheme 2). At the outset, a range of potassium alkyltrifluoroborates were examined in the reaction of 4vinylpyridine 1a with sulfur dioxide. All reactions worked well, giving rise to the desired products in good to excellent yields. The alkyl groups included cyclohexyl, tert-butyl, and allyl groups. Subsequently, reactions of different alkenes were investigated. It was found that sensitive functional groups including halo, hydroxyl, ester, keto, and nitro groups were all tolerated under the standard conditions. For instance, compound 3x was produced in 83% yield when 4-nitrostyrene was employed as the starting material. Subsequently, the gram scale reaction for the generation of compound 3k was explored. The reaction worked smoothly, leading to the corresponding product in 76% yield. Moreover, furan-2-yl trifluoroborates and pyridin-3-yl trifluoroborates were used as the substrates in this transformation. However, no desired products were obtained and most of the starting materials were recovered. We further extended this approach by using alkene 4 as the starting material. Thus, a reaction of alkene 4, potassium cyclopentyltrifluoroborate 2a, and DABCO·(SO2)2 was exam-
ined under the above conditions (Scheme 3). The reaction proceeded well, giving rise to the corresponding product 5 in 51% yield. This result demonstrated the generality and efficiency of this method. Scheme 3. Reaction of Alkene 4, Potassium Cyclopentyltrifluoroborate 2a, and DABCO·(SO2)2
To gain more insights into this transformation, 2.0 equiv of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was added to the reaction of (E)-1-phenyl-3-(p-tolyl)prop-2-en-1-one (1k), DABCO·(SO2)2, and potassium cyclopentyltrifluoroborate (2a) under the above conditions shown in Table 2 (Scheme 4). No desired product 3k was observed, and cyclopentylTEMPO 6 was detected by GC−MS. This result indicated that alkyl radical might be present in this transformation. Additionally, the reaction of (E)-1-phenyl-3-(p-tolyl)prop-2-en-1-one (1k) and sodium cyclopentanesulfinate 7 in the presence of Mes-Acr+ under blue LED irradiation was examined. However, only 6% yield of the desired product 3k was isolated. Moreover, B
DOI: 10.1021/acs.orglett.8b01385 Org. Lett. XXXX, XXX, XXX−XXX
Organic Letters
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Scheme 4. Investigation of Mechanism
Letter
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01385. Experimental procedures, characterization data, copies of 1 H and 13C NMR of products (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jie Wu: 0000-0002-0967-6360 Notes
no desired product could be observed without the addition of photocatalyst or in the absence of visible light. These observations suggested that the sulfonyl radical might be the key intermediate rather than the corresponding sulfinate salt. On the basis of the above results and previous reports,5−7 we proposed a plausible mechanism, as shown in Scheme 5. Visible
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from National Natural Science Foundation of China (Nos. 21672037 and 21532001) and Jiangsu Province Key Laboratory of Fine Petrochemical Engineering (KF1701) is gratefully acknowledged.
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Scheme 5. Plausible mechanism
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light irradiation of the photocatalyst Mes-Acr+ would produce the excited state of Mes-Acr+* (Ered1/2 = +2.06 V vs SCE),9 which was capable to oxidize alkyltrifluoroborate 2 (Ered1/2 = +1.50 V vs SCE)5c,6b to generate an alkyl radical and the reduced species Mes-Acr• (Ered1/2 = −0.57 V vs SCE). The combination of alkyl radical and sulfur dioxide would provide the alkylsulfonyl radical A, which would react with alkene 1 to produce the radical intermediate B. Further reduction by MesAcr• would lead to the formation of anionic intermediate C, with the regeneration of photocatalyst Mes-Acr+. Subsequently, protonation would happen to deliver the desired product 3. In conclusion, we have described a three-component reaction of potassium alkyltrifluoroborates, sulfur dioxide surrogate of DABCO·(SO2)2, and alkenes under photocatalysis in the presence of visible light. This multicomponent reaction works efficiently at room temperature with the insertion of sulfur dioxide under mild conditions, affording diverse sulfones in good to excellent yields. Preliminary mechanistic studies show that the alkyl radical and alkylsulfonyl radical are generated as key intermediates, and a reductive single electron transfer is involved in the reaction process. C
DOI: 10.1021/acs.orglett.8b01385 Org. Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.orglett.8b01385 Org. Lett. XXXX, XXX, XXX−XXX
