Letter Cite This: Org. Lett. 2017, 19, 6028-6031
pubs.acs.org/OrgLett
Synthesis of Tetrahydropyridine Derivatives through a Reaction of 1,6-Enynes, Sulfur Dioxide, and Aryldiazonium Tetrafluoroborates Yuanyuan An† and Jie Wu*,†,‡ †
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, 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: Sulfonated tetrahydropyridine derivatives are accessed through a radical reaction of 1,6-enynes, sulfur dioxide, and aryldiazonium tetrafluoroborates under mild conditions. Sulfonated pyrrolidine can be generated when terminal alkyne is used as the substrate. This reaction proceeds efficiently in dichloroethane without the addition of any catalysts or additives, providing sulfonated tetrahydropyridine derivatives in moderate to good yields. During this transformation, aryldiazonium tetrafluoroborates cooperate with DABCO·(SO2)2 leading to sulfonyl radicals, which initiate the radical cyclization process. Two molecules of aryldiazonium tetrafluoroborates are involved in the reaction.
S
Scheme 1. Proposed Reaction of 1,6-Enynes, Sulfur Dioxide, and Aryldiazonium Tetrafluoroborates
ulfonyl-containing compounds are of great importance in pharmaceuticals, agrochemicals, and materials.1 Enormous efforts have been made to develop new and efficient methodologies for the synthesis of sulfonylated compounds.2 Currently, growing interest is focused on the insertion of sulfur dioxide into small organic molecules for the synthesis of sulfonylated compounds.3 Recently, inorganic metal sulfides and DABCO−bis(sulfur dioxide) are used as the source of sulfur dioxide.4−6 So far, aminosulfonylation and arylsulfonylation through the insertion of sulfur dioxide have been achieved. We are also involved in this research.6 For instance, we reported the first example of using potassium metabisulfite as the sulfur dioxide source in palladium-catalyzed aminosulfonylation under mild conditions.6b A catalyst-free approach for the generation of sulfonyl radicals from aryldiazonium tetrafluoroborates and DABCO·(SO2)2 was described recently, affording various sulfonated coumarins6j and other sulfonyl compounds.6k−p 1,6-Enynes have been utilized broadly as convenient building blocks in organic synthesis. They could easily participate in tandem reactions due to the CC and CC units in the structure. For example, transition-metal-catalyzed cycloaddition of 1,6-enynes would result in cyclic compounds via alkyne activation and subsequent addition of double bond.7 Radical cascade cyclization of 1,6-enynes has been developed in the meantime.8 Liang and co-workers described an iodine-mediated radical cyclization of 1,6-enynes and sulfonyl hydrazides (Scheme 1, eq a).8c Various sulfonated tetrahydropyridines were afforded under mild conditions. Although various approaches have been developed for the synthesis of sulfonated tetrahydropyridines, the reactions usually suffered from harsh conditions, and in some cases, high reaction temperatures were © 2017 American Chemical Society
needed.9 Encouraged by the advancement of 1,6-enyne chemistry and our continuous interest in the insertion of sulfur dioxide, we conceived that sulfur dioxide could be incorporated with 1,6-enynes as well and generation of sulfonated tetrahydropyridines would be expected. Herein, we report a radical cyclization of 1,6-enynes, sulfur dioxide, and aryldiazonium tetrafluoroborates (Scheme 1, eq b). Interestingly, two molecular aryldiazonium tetrafluoroborates are participated in the transformation, providing sulfonated tetrahydropyridines in good yields. It is noteworthy that no catalyst and additive are needed in the transformation. Received: October 12, 2017 Published: October 24, 2017 6028
DOI: 10.1021/acs.orglett.7b03195 Org. Lett. 2017, 19, 6028−6031
Letter
Organic Letters In our previous report,6 we discovered that the sulfonyl radicals could be generated from aryldiazonium tetrafluoroborates and DABCO·(SO2)2 in situ under catalyst- and additivefree conditions. The presence of DABCO was important and necessary for the outcome. We therefore began our studies by using 1,6-enyne 1a, DABCO·(SO2)2, and 4-fluorophenyldiazonium tetrafluoroborate 2a as the model substrates without the addition of any catalysts. The preliminary results are shown in Table 1. Initially, the reaction occurred in 1,2-dichloroethane at
Scheme 2. Scope Exploration for the Reactions of 1,6Enynes 1, DABCO·(SO2)2, and Aryldiazonium Tetrafluoroborates 2a,b
Table 1. Initial Studies for the Reaction of 1,6-Enyne 1a, DABCO·(SO2)2, and 4-Fluorophenyldiazonium Tetrafluoroborate 2aa
entry
solvent
temp (°C)
yieldb (%)
1 2 3 4 5c 6d 7c,e 8c 9c 10c
DCE CH3CN 1,4-dioxane toluene DCE DCE DCE DCE DCE DCE
70 70 70 70 70 70 70 50 80 25
40 33 30 trace 63 62 51 75 28 58
a
Reaction conditions: 4-methyl-N-(3-methylbut-2-en-1-yl)-N-(3-phenylprop-2-yn-1-yl)benzenesulfonamide 1a (0.2 mmol), 4-fluorophenyldiazonium tetrafluoroborate 2a (0.24 mmol), DABCO·(SO2)2 (0.4 mmol), solvent (2.0 mL), 4 h, N2 atmosphere. bIsolated yield based on substrate 1a. cIn the presence of 2.0 equiv of 4-fluorophenyldiazonium tetrafluoroborate 2a. d In the presence of 2.5 equiv of 4fluorophenyldiazonium tetrafluoroborate 2a. eIn the presence of 2.2 equiv of DABCO·(SO2)2.
a
Reaction conditions: 1,6-enyne 1 (0.2 mmol), aryldiazonium tetrafluoroborate 2 (0.4 mmol), DABCO·(SO2)2 (0.4 mmol), DCE (2.0 mL), 50 °C, 4 h, N2 atmosphere. bIsolated yield based on 1,6enyne 1.
70 °C. Gratifyingly, a yellow product was isolated in 40% yield (Table 1, entry 1). After structural illustration, it was identified as (E)-3-(2-((4-fluorophenyl)diazenyl)propan-2-yl)-5-((4fluorophenyl)sulfonyl)-4-phenyl-1-tosyl-1,2,3,6-tetrahydropyridine 3a. Further screening of solvents did not give higher yields (Table 1, entries 2−4). From the structure of compound 3a, it was found that two molecules of 4-fluorophenyldiazonium tetrafluoroborate participated in the reaction. As a result, a higher yield of 63% was obtained when the amount of 4fluorophenyldiazonium tetrafluoroborate was increased to 2.0 equiv (Table 1, entry 5). A similar yield of 62% was observed when 2.5 equiv of 4-fluorophenyldiazonium tetrafluoroborate was added in the reaction (Table 1, entry 6). The result could not be improved when the amount of DABCO·(SO2)2 was increased to 2.2 equiv (Table 1, entry 7). The yield was increased to 75% when the reaction occurred at 50 °C (Table 1, entry 8). However, the reaction did not work well when the temperature was changed to 80 or 25 °C (Table 1, entries 9 and 10). Subsequently, we explored the reaction scope under the optimized reaction conditions shown in Table 1. Various substituted 1,6-enynes 1 and aryldiazonium tetrafluoroborates 2 were evaluated, and the results are shown in Scheme 2. The reactions occurred smoothly and afforded the desired products in moderate to good yields. A range of aryldiazonium
tetrafluoroborates 2 was examined. It was found that aryldiazonium tetrafluoroborates 2 with electron-withdrawing or electron-donating groups attached on the aromatic ring were all compatible under the standard conditions. Different groups such as o-methyl, m-methyl, p-methyl, methoxy, chloro, fluoro, tert-butyl, nitro, and trifluoromethyl were all tolerated, leading to the corresponding tetrahydropyridine derivatives 3. For instance, 4-methoxyphenyldiazonium tetrafluoroborate reacted with DABCO·(SO2)2 and 4-methyl-N-(3-methylbut-2-en-1-yl)N-(3-phenylprop-2-yn-1-yl)benzenesulfonamide 1a, giving rise to the expected product 3d in 75% yield. When 4chlorophenyldiazonium tetrafluoroborate was used as the substrate, the corresponding product 3e was afforded in 58% yield. Meanwhile, the structure of compound 3e was confirmed by X-ray diffraction analysis.10 A thiophene-yl-substituted product 3t could be afforded in 46% yield. Furthermore, reactions of 1,6-enynes 1 bearing functional groups such as methyl, chloro, fluoro, bromo, tert-butyl, nitro, and acetyl in the para-position of the benzene ring were examined with DABCO·(SO2)2 and phenyldiazonium tetrafluoroborate 2b. As expected, all reactions occurred smoothly, providing the corresponding products 3k−s in moderate to good yields. For example, product 3n was afforded in 70% yield when 4-tert-butyl-substituted 1,6-enyne reacted with sulfur 6029
DOI: 10.1021/acs.orglett.7b03195 Org. Lett. 2017, 19, 6028−6031
Letter
Organic Letters dioxide and phenyldiazonium tetrafluoroborate 2b. A lower yield of 41% was obtained when a nitro group was featured on the para-position of benzene ring of the substrate. Reactions of O- or C-tethered 1,6-enynes were also examined, and the corresponding products 3u−w were obtained as expected. Interestingly, vinylsulfonated pyrrolidine 6 was isolated in 42% yield when terminal alkyne 4 was used as the substrate. The structure of compound 6 was identified by X-ray diffraction analysis. During the reaction process, arylsulfonyl radical would attack the terminal position of alkyne, leading to a different result. When a methyl-substituted alkyne 5 was employed in the reaction, the two types of products 7 and 8 were generated in 39% and 20% yields, respectively (Scheme 3).
Scheme 5. Plausible Mechanism for the Reaction of 1,6Enynes 1, DABCO·(SO2)2, and Aryldiazonium Tetrafluoroborates 2
Scheme 3. Reactions of 1,6-Enynes DABCO·(SO2)2 and Aryldiazonium Tetrafluoroborates
1,6-enyne 1 to afford radical intermediate C. Then an intramolecular 6-exo cyclization would occur to generate intermediate D, which would be trapped by an additional aryldiazonium cation 2 leading to intermediate E. Then product 3 would be formed via a single-electron transfer between intermediate E and arylsulfonyl radical B or DABCO·(SO2)2.13 If terminal alkyne was used as the substrate, arylsulfonyl radical B would attack the terminal position of triple bond with the formation of a pyrrolidine skeleton. From the results, it seemed that the radicals generated in this system were highly selective leading to the corresponding products. This persistent radical effect could be also observed by others.12 In summary, we have developed an efficient route to sulfonated tetrahydropyridine derivatives through a radical reaction of 1,6-enynes, sulfur dioxide, and aryldiazonium tetrafluoroborates under mild conditions. This reaction proceeds efficiently in dichloroethane without the addition of any catalysts or additives, providing sulfonated tetrahydropyridines in moderate to good yields. During this transformation, two molecules of aryldiazonium tetrafluoroborates are involved. In the reaction process, aryldiazonium tetrafluoroborates cooperate with DABCO·(SO2)2 leading to sulfonyl radicals which initiate the radical cyclization.
Since arylsulfonyl radical would be generated in situ during this tandem annulation, we therefore added 3.0 equiv of 2,2,6,6tetramethylpiperidine-1-oxyl (TEMPO) or 2,6-di-tert-butyl-4methylphenol (BHT) to the reaction, respectively. As a result, the transformation was completely inhibited, which confirmed that the reaction experienced a radical process (Scheme 4, eq a). Additionally, aryl radical and arylsulfonyl radical could be captured when 1,1-diphenylethylene was employed in the reaction (Scheme 4, eq b). Scheme 4. Investigation of the Mechanism
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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.7b03195. X-ray data for compound 3e (CIF) X-ray data for compound 6 (CIF) Experimental procedures, characterization data, and 1H, 19 F, and 13C NMR of products (PDF)
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On the basis of this result and previous reports,6,11 we proposed a plausible mechanism as shown in Scheme 5. We reasoned that aryldiazonium cation 2 would react with DABCO·(SO2)2 to provide a tertiary amine radical cation, nitrogen, sulfur dioxide, and aryl radical A through a homolytic cleavage of N−S bond and a single-electron-transfer process.6j Addition of aryl radical A to sulfur dioxide would produce arylsulfonyl radical B, which would attack the triple bond of
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jie Wu: 0000-0002-0967-6360 Notes
The authors declare no competing financial interest. 6030
DOI: 10.1021/acs.orglett.7b03195 Org. Lett. 2017, 19, 6028−6031
Letter
Organic Letters
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ACKNOWLEDGMENTS Financial support from National Natural Science Foundation of China (No. 21672037 and 21532001) is gratefully acknowledged.
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DOI: 10.1021/acs.orglett.7b03195 Org. Lett. 2017, 19, 6028−6031
