Letter pubs.acs.org/OrgLett
N‑Radical Initiated Aminosulfonylation of Unactivated C(sp3)−H Bond through Insertion of Sulfur Dioxide Yuewen Li,† Runyu Mao,† 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: N-Radical initiated aminosulfonylation of unactivated C(sp3)−H bond through insertion of sulfur dioxide in the presence of visible light is reported. O-Aryl oximes react with DABCO·(SO2)2 smoothly at room temperature under blue LED irradiation without any metals or photoredox catalysts, generating diverse 5,6-dihydro-4H-1,2-thiazine 1,1dioxides in good yield. Additionally, this approach can be extended to the synthesis of 1H-benzo[d][1,2]thiazine 2,2dioxides. During the reaction process, an N-radical is initiated by the treatment of O-aryl oximes with DABCO·(SO2)2 under visible-light irradiation. It is followed by aminosulfonylation of a nearby C(sp3)−H bond through 1,5-hydrogen atom transfer with accompanying insertion of sulfur dioxide to provide 1,2thiazine 1,1-dioxide derivatives.
S
Selective C(sp 3 )−H functionalization for chemists is important in organic synthesis, medicinal chemistry, and materials chemistry.15 However, activation of C(sp3)−H bonds is not easy because of their high bond energy and selectivity issues. So far, significant advances have been made in the C(sp3)−H functionalization of different compounds using transition metals such as Pd, Rh, Fe, Ni, Co, etc.16 Activation of C(sp3)−H bonds by carbene or nitrene was reported as well.17a−c Additionally, MacMillan developed vinylation and alkylation of C(sp3)−H bonds under visible light irradiation in the presence of a photocatalyst.17d,e Recently, an alternative method to functionalize C(sp3)−H bonds through hydrogen atom transfer (HAT) was developed.18,19 Chen and co-workers reported the photo activation of C(sp3)−H bonds, to obtain allylation and vinyl adducts with excellent regio- and chemoselectivity using alkoxyl radicals from N-alkoxyphthalimides.20 In 2016, alkylation of unactivated C(sp3)−H bonds enabled by amidyl radicals generated from N-alkylamides by hydrogen atom transfer was described by Rovis and Knowles.21 So far, aminosulfonylation of unactivated C(sp3)−H bonds via insertion of sulfur dioxide has not been disclosed. Inspired by recent advances in the formation of N-centered radical species under visible light photocatalysis22−24 and selective C(sp3)−H functionalization through hydrogen atom transfer, we envisioned that the direct aminosulfonylation of unactivated C(sp3)−H bonds with sulfur dioxide would be feasible. A proposed synthetic route is presented in Scheme 1. We hypothesized that in the presence of visible light an iminyl radical would be
ulfonamides are widely present in many pharmaceutical and agrochemical molecules.1 For many years, organic sulfonic acids have served as the main raw material for the synthesis of sulfonamide compounds. They are prepared through a reaction of sulfur trioxide with different nucleophilic organometallic reagents under harsh reaction conditions.2 These transformations usually produce a large amount of toxic waste and need enormous energy. Introduction of the sulfonyl unit into small molecules via insertion of sulfur dioxide is promising and attractive in organic synthesis due to its environment friendly nature and high efficiency.3 Since the development of easily handled DABCO·(SO2)24 and inorganic sulfites5 as a replacement for toxic gaseous sulfur dioxide in organic transformations, rapid progress has been made in the generation of sulfonyl compounds via the insertion of sulfur dioxide. Aryl halides,6 arylboronic acids,7 triethoxysilanes,8 organometallic reagents,9 and others10 have been proven to be good coupling reagents with sulfur dioxide in the presence of transition metals or under metalfree conditions. Recently, our group reported an efficient route to sulfonyl compounds starting from aryldiazonium tetrafluoroborates and sulfur dioxide under mild conditions.11 During the reaction process, the generation of arylsulfonyl radical intermediates, formed in situ from the addition of an aryl radical to sulfur dioxide, was demonstrated as the key step. Other examples of the trapping of sulfur dioxide by a carbon radical intermediate under ultraviolet irradiation, without metals or photoredox catalysts, have also been developed.12,13 Although remarkable advances in the trapping of sulfur dioxide with various partners have been developed, aminosulfonylation of C− H bonds directly with sulfur dioxide is still a challenge, which has attracted our continuous interest.14 © 2017 American Chemical Society
Received: July 1, 2017 Published: August 15, 2017 4472
DOI: 10.1021/acs.orglett.7b02010 Org. Lett. 2017, 19, 4472−4475
Letter
Organic Letters Scheme 1. N-Radical Initiated Aminosulfonylation of Unactivated C(sp3)−H Bond with Sulfur Dioxide
other solvents but can be dissolved well in DMSO. The yield was enhanced to 56% when the reaction time was prolonged (Table 1, entry 7). Gratifyingly, the desired product 2a could be provided in 75% yield when the amount of DABCO·(SO2)2 was increased to 2.5 equiv (Table 1, entry 9). The reaction was hampered under an air atmosphere or a dark environment (Table 1, entries 11 and 12). Subsequently, we investigated variations of the aminosulfonylation of unactivated C(sp3)−H bonds with sulfur dioxide under the above optimized conditions. As shown in Scheme 2,
formed via single electron transfer accompanied by homolytic cleavage of the N−O bond of oximes.23 Then, aminosulfonylation of an unactivated C(sp3)−H bond through 1,5HAT with the insertion of sulfur dioxide would happen to provide 1,2-thiazine 1,1-dioxide derivatives. Because of the nature of the reaction process, it seemed that no additives would be needed in the transformation. At the outset, the reaction of 4methyl-1-(p-tolyl)pentan-1-one O-2,4-dinitrophenyl oxime 1a and DABCO·(SO2)2 was selected as the model for optimization of the reaction conditions (Table 1). The reaction was initially
Scheme 2. Scope Exploration for the Photoinduced Aminosulfonylation of an Unactivated C(sp3)−H Bond with Sulfur Dioxidea
Table 1. Initial Studies for the Photoinduced Aminosulfonylation of an Unactivated C(sp3)−H Bond with Sulfur Dioxidea
entry
solvent
DABCO·(SO2)2 (equiv)
yield (%)b
1 2 3 4 5 6 7c 8c 9c 10c 11c,d 12c,e
DMSO DCE CH3CN THF 1,4-dioxane DMF DMSO DMSO DMSO DMSO DMSO DMSO
1.5 1.5 1.5 1.5 1.5 1.5 1.5 2.0 2.5 3.0 2.5 2.5
45 trace 11 trace trace 15 56 64 75 74 NR NR
a
Isolated yield based on O-aryl oxime 1.
various O-aryl oximes 1 successfully engaged in the reaction under the photoinduced catalyst-free conditions, giving rise to the desired 1,2-thiazine 1,1-dioxides 2 in moderate-to-good yields in the presence of sulfur dioxide. Various substituted substrates with methyl, methoxyl, fluoro, and chloro groups were compatible with the aminosulfonylation of unactivated C(sp3)− H bonds with sulfur dioxide (compounds 2a−f). It was noteworthy that the reaction of thiophenyl-substituted substrate with sulfur dioxide worked as well, producing the desired product 2g in 35% yield. Variation of the steric environment around the C(sp3)−H bond had a subtle effect on the outcome of the reaction. The products were obtained as expected when substrates containing a six-membered or five-membered carbocycle were employed. We further extended the aminosulfonylation of C(sp3)−H bonds with sulfur dioxide to substrates of structure 3 containing a benzylic carbon. A series of 1H-benzo[d][1,2]thiazine 2,2dioxides 4 was obtained through the strategy of 1,5-HAT. As shown in Scheme 3, different fluoro-containing substrates of 1(o-tolyl)ethanone O-2,4-dinitrophenyl oximes (3b−e) could be converted to target products (4b−e) under the established conditions. During the transformation, the chloro group was not perturbed as well. This visible-light-promoted procedure opens a new avenue for the aminosulfonylation of ubiquitous aliphatic C−H bonds. In order to understand the mechanism of this catalyst-free aminosulfonylation of C(sp3)−H bonds with the insertion of sulfur dioxide, preliminary experiments were carried out. No desired product 2a was detected when 2,2,6,6-tetramethyl-1-
a Reaction conditions: 4-methyl-1-(p-tolyl)pentan-1-one O-2,4-dinitrophenyl oxime 1a (0.1 mmol), DABCO·(SO2)2, solvent (4.0 mL), N2, irradiated under a 18 W blue LED at room temperature for 24 h. b Isolated yield based on 4-methyl-1-(p-tolyl)pentan-1-one O-(2,4dinitrophenyl) oxime 1a. cReaction was performed for 48 h. dReaction occurred under air atmosphere. eReaction was performed in the dark. NR = no reaction.
performed at room temperature in DMSO in the presence of visible light. To our delight, the expected product 2a was formed in 45% isolated yield (Table 1, entry 1). X-ray crystallography analysis identified the structure of compound 2a.25 Other solvents were then examined. However, no better results were obtained (Table 1, entries 2−6). Only a trace amount of product was observed when the reaction was performed in 1,2dichloroethane (DCE), THF, or 1,4-dioxane. The reaction occurred in MeCN, affording the corresponding product 2a in 11% yield, and a similar result was obtained in DMF. We reasoned that this might be due to the solubility issue of DABCO·(SO2)2 since DABCO·(SO2)2 is hardly dissolved in 4473
DOI: 10.1021/acs.orglett.7b02010 Org. Lett. 2017, 19, 4472−4475
Letter
Organic Letters Scheme 3. Aminosulfonylation of Substrates 3 Containing a Benzyl Carbon with Sulfur Dioxidea
a
Scheme 5. Plausible Mechanism for the Photoinduced Aminosulfonylation of an Unactivated C(sp3)−H Bond with Sulfur Dioxide
Isolated yield based on O-aryl oxime 3.
photoredox catalysts to generate diverse 5,6-dihydro-4H-1,2thiazine 1,1-dioxides in good yield. Additionally, this approach can be extended to the synthesis of 1H-benzo[d][1,2]thiazine 2,2-dioxides.
piperidinyloxy (TEMPO) was added to the reaction of 4-methyl1-(p-tolyl)pentan-1-one O-2,4-dinitrophenyl oxime 1a with DABCO·(SO2)2 under the standard conditions (Scheme 4, eq
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Scheme 4. Investigation of Mechanism
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b02010. Experimental procedures, characterization data, and copies of 1H and 13C NMR of all compounds (PDF) X-ray structure of compound 2a (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
a). A control experiment in the absence of sulfur dioxide was performed as well. Interestingly, 2,2-dimethyl-5-(p-tolyl)-3,4dihydro-2H-pyrrole 5 was obtained in 18% yield when 4-methyl1-(p-tolyl)pentan-1-one O-2,4-dinitrophenyl oxime 1a was treated with DABCO instead of DABCO·(SO2)2, which was similar to Yu’s report (Scheme 4, eq b).19a According to previous reports11−13,19−21 and these experimental results above, we postulate that the reaction proceeds through a radical process in the presence of visible light, as shown in Scheme 5. We reasoned that a photosensitive complex A, revealed by the UV/vis studies (see the Supporting Information), would be formed by treatment of O-2,4-dinitrophenyl oxime 1 with DABCO·(SO2)2.13c Under visible-light irradiation, iminyl radical B would be formed via a single electron transfer (SET) with homolytic cleavage of the N−O bond of the oximes,23 with the release of a cation radical C. Subsequently, the iminyl radical B would undergo 1,5-H atom abstraction to provide radical D. Followed by addition of radical D to sulfur dioxide would yield intermediate E, which would react with cation radical C to produce the desired product 2.19a,c In conclusion, we have described an N-radical initiated aminosulfonylation of unactivated C(sp3)−H bonds through insertion of sulfur dioxide in the presence of visible light. O-Aryl oximes react with DABCO·(SO2)2 smoothly at room temperature under blue LED irradiation without any metals or
Jie Wu: 0000-0002-0967-6360 Notes
The authors declare no competing financial interest.
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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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REFERENCES
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