Visible-light photoredox-catalyzed dual C–C bond cleavage: synthesis of 2-cyanoalkylsulfonylated 3,4-dihydronaphthalenes through the insertion of sulfur dioxide

Abstract

An efficient novel visible-light photoredox-catalyzed dual carbon–carbon bond cleavage of methylenecyclopropanes and cycloketone oximes for the synthesis of 2-cyanoalkylsulfonated 3,4-dihydronaphthalenes through the insertion of sulfur dioxide is established. This dual cleavage of carbon–carbon bonds involves a radical pathway and goes through a sequence of iminyl radical formation, carbon–carbon bond cleavage, sulfur dioxide insertion, sulfonyl radical addition, another carbon–carbon bond cleavage, and intramolecular cyclization.

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Sulfur-containing derivatives are important synthons in organic synthesis and drug synthesis.^1,2 Sulfones, a class of useful organosulfur derivatives, have already been widely used in the chemical industry, agrochemicals, pharmaceuticals, organic synthesis and inflaming retarding materials.^3,4 Due to the extensive application of sulfones, the approaches for the construction of sulfones have attracted organic chemists’ attention and many novel strategies have been established.^5,6 Among them, the insertion of sulfur dioxide was the most convenient way for the synthesis of sulfones. DABSO [DABCO·(SO₂)₂],⁷ sodium sulfites^8a–d and thiourea dioxide^8e,f were usually used as sulfur dioxide sources to prepare sulfones.

Alkylnitriles are an important class of organic skeletons encountered in many nitrile-containing pharmaceuticals and natural products.⁹ Moreover, cyanoalkyl groups could easily be transformed into other functional groups.¹⁰ Thus, the development of efficient and convenient ways to access alkylnitriles has become interesting. Typical strategies to synthesize alkylnitriles include dehydrogenation of amines,^11a–c dehydration of aldoximes^11d–g or amides,^12a–c and cyanation of alkyl halides.^12d–g Recently, the functionalization of α-C–H bonds in inert alkylnitriles has also been developed.¹³ The cleavage of carbon–carbon σ-bonds in cyclobutanone oximes has also emerged as a convenient route to import cyanoalkyl groups, which bear longer aliphatic chains, into organic molecules.^14–16 In 1991, Zard et al.^14a developed the first radical carbon–carbon bond cleavage of cyclobutanone sulphenylimines or carboxymethyl oximes to prepare alkylnitriles. Afterwards, many organic chemists, for example, Uemura's,^14b,c Selander's,^14d,e Xiao's,^14f–j Guo's,^15a–c Yu's,^15d,e Wu's,^15f,g Li's^15h and other groups,¹⁶ have reported similar ring-opening reactions of cyclobutanone oxime derivatives. Most of these methods employed cyanoalkyl radicals, which came from ring-opening of cyclobutanone oximes, to directly react with other reaction partners. However, the methods which used cyanoalkyl radicals to capture SO₂^16e,f and then formed sulfonyl radicals were very few (eqn (1), Scheme 1).

Scheme 1 In situ SO₂-capture reactions of cycloketone oxime esters.

MCPs (methylenecyclopropanes) were usually used to react with diverse free radicals,¹⁷ such as CF₃,^17a SCF₃,^17b alkyl,^17c acyl,^17dα-carbonyl^17e and sulfonyl^17f–h-containing radicals. In the reaction between MCPs and sulfonyl radicals, the sulfonyl radicals usually came from sulfonyl chlorides and sodium sulfinates. However, strategies for ring-opening and cyclization of MCPs with cyanoalkylsulfonyl radicals, which formed from cyclobutanone oximes and sulfur dioxide sources, are lacking.

Visible-light-catalysis, as a convenient and simple tool, is widely used in organic synthesis because of its safety, mild reaction conditions, availability, and high efficiency.¹⁸ Herein, we develop a visible-light photoredox-catalyzed difunctionalization of carbon–carbon σ-bonds to construct 2-cyanopropylsulfonyl-substituted 3,4-dihydronaphthalenes from MCPs, cyclobutanone oximes and K₂S₂O₅, which features the cleavage of two carbon–carbon σ-bonds and the formation of one new carbon–carbon bond and two carbon-sulfur bonds in one-pot (eqn (2), Scheme 1).

We chose 1-(benzyloxy)-2-(cyclopropylidenemethyl)benzene 1a and cyclobutylidenemethyl 4-(trifluoromethyl)benzoate 2a as model substrates to clarify the standard reaction conditions. To our surprise, the reaction employing Ru(bpy)₃Cl₂ as a photocatalyst, K₂S₂O₅ as a sulfur dioxide source, 2,6-lutidine as base, and CH₃CN as solvent at 80 °C and irradiated with a 5 W blue LED light source for 18 h could afford the product 4-((8-(benzyloxy)-3,4-dihydronaphthalen-2-yl)sulfonyl)butanenitrile 3aa in 81% yield. Various sulfur dioxide sources, cyclobutanone O-benzoyl oximes, photocatalysts, solvents, visible-light sources, temperatures and reaction times were successively examined, and the optimized conditions were as follows: 1-(benzyloxy)-2-(cyclopropylidenemethyl)benzene 1a (0.2 mmol), cyclobutylidenemethyl 4-(trifluoromethyl)benzoate 2a (0.3 mmol, 1.5 equiv.), Ru(bpy)₃Cl₂ (0.01 mmol, 5 mol%), 2,6-lutidine (0.6 mmol, 3 equiv.), K₂S₂O₅ (0.4 mmol, 2 equiv.), CH₃CN (2 mL) at 80 °C and irradiated with a 5 W blue LED light source for 18 h (see Table S1 in the ESI†).

Based on the optimal conditions, we initially investigated the scope of MCPs 1 and cycloketone oxime esters 2 (Table 1). A series of substituted cycloketone oxime esters 2 were examined in the presence of MCPs 1a under the standard conditions. The results revealed that 3-substituted cyclobutanone oximes 2b–j bearing benzyl, benzyloxy, aryl and naphthalen-1-ylmethyl groups were all suitable substrates. The corresponding products 3ab–aj could be obtained in moderate yields. Employing 3,3-disubstituted cyclobutanone oximes 1k and 1l in the reaction could provide the sulfonated products 3ak and 3al in 79% and 72% yields, respectively. The 2-substituted cycloketone oxime ester 2m was examined and the target product 3am could be obtained in 69% yield. Then, our attention was turned to research the scope of MCPs 1 in the presence of cycloketone oxime esters 2a, Ru(bpy)₃Cl₂, K₂S₂O₅, 2,6-lutidine and CH₃CN. A variety of mono-substituted MCPs, which bear one substituent at the ortho-, meta- or para-position on the aryl ring, were examined. All of them could undergo the dual carbon–carbon bond cleavage/sulfonylation reaction smoothly and afforded the corresponding 2-cyanoalkylsulfonylated 3,4-dihydronaphthalenes 3 in moderate yields (3aa–na). The reaction yields indicated that both the hindrance and electronic effect of the substituents had obvious influences on the transformation: the reactivity order is ortho- < meta- < para- and electron-withdrawing < electron-donating. However, the meta-substituted MCP 1e gave products 3ea and 3ea′ (3 : 1) in 83% yield. Subsequently, di-substituted MCPs 1o–r and poly-substituted MCP 1s were also explored and all of them were suitable for this reaction (products 3oa–sa). Halogen substituted MCPs 1p–s were suitable for this transformation, which provided opportunities for further modification of the products (products 3pa–sa). Additionally, phenyl-substituted MCP 1t was also a good candidate for this reaction. However, the four-membered substrate 1u could not install the target product 3ua. Substrate 1v, which connected a phenyl group and a methyl, could react well with 2a to deliver the sulfonylated product 3va in 72% yield. The naphthyl-substituted MCP 1w could also deliver product 3wa under the standard conditions (product 3wa). Finally, a series of MCPs 1x–aa connecting two aryl rings at the terminal of the carbon–carbon double bond also worked well and afforded the ring-opening and cyclization products 3xa–3aaa in good yields. However, 1-Cbz-3-azetidinone, oxetan-3-one and cyclopentanone derived O-acyl oximes 2n, 2o and 2p could not give the target products 3 under the standard conditions.

Table 1 Scope of cycloketone oxime esters (2) and MCPs (1)^a

a Reaction conditions: 1 (0.2 mmol), 2a (0.3 mmol, 1.5 equiv.), K₂S₂O₅ (0.4 mmol, 2 equiv.), Ru(bpy)₃Cl₂ (5 mol%), 2,6-lutidine (0.6 mmol, 3 equiv.), CH₃CN (2 mL) at 80 °C under an argon atmosphere and 5 W blue LED irradiation for 18 h. b Most of the substrate 1 was decomposed.

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To gain further understanding of the reaction mechanism, three radical trapping experiments were carried out. The yields of MCPs 1a with cycloketone oxime esters 2a and K₂S₂O₅ were very low when these reactions were carried out by using TEMPO, hydroquinone and 1,1-diphenylethene as radical inhibitors (eqn (1)–(3), respectively, Scheme 2). Additionally, using TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) in the transformation could afford 4-((2,2,6,6-tetramethylpiperidin-1-yl)oxy)butanenitrile 4 (eqn (1), Scheme 2). The trapping product 4 could be detected by GC-MS. Using 1,1-diphenylethene in the sulfonylation reaction could give cyanoalkyl group-trapping product 6,6-diphenylhex-5-enenitrile 5 and cyanoalkylsulfonyl group-trapping product 4-((2,2-diphenylvinyl)sulfonyl)butanenitrile 6 in 11% and 27% yields, respectively (eqn (3), Scheme 2). These experimental results suggested that this transformation definitely contained a radical process. Additionally, the cyano group could be transformed into a series of other functional groups, such as carboxyl,^14g,15b,16a amide,^16a ester,^16a and tetrazole^15b groups. The cyano group in the sulfonylated product 3aa could be converted into an ester group in the presence of ZnCl₂ and n-butyl alcohol (eqn (4), Scheme 2).

Scheme 2 Control experiments.

According to the literature^14–18 and above results, we proposed a mechanism for the visible-light photoredox-catalyzed dual cleavage of C–C bonds (Scheme 3). Initially, [Ru(bpy)₃]²⁺ was transformed into [Ru(bpy)₃]²⁺* photoexcited under irradiation with visible-light.^17f,18c Subsequently, the reduction of cycloketone oxime 2a by [Ru(bpy)₃]²⁺* provided iminyl radical A, which underwent cleavage of the carbon–carbon bond to deliver cyanoalkyl radical B. Next, intermediate B trapped sulfur dioxide, which came from K₂S₂O₅, to give cyanoalkylsulfonyl radical C. Both the cyanoalkyl radical B and cyanoalkylsulfonyl radical C could be trapped by 1,1-diphenylethene to give products 5 and 6, respectively. Then, the addition of cyanoalkylsulfonyl radical C to the carbon–carbon double bond in substrate 1a afforded radical D, which went through another cleavage of the carbon–carbon bond to install radical E. The intramolecular cyclization of intermediate E assembled radical F. Finally, radical F underwent SET by [Ru(bpy)₃]³⁺ and deprotonation by 2,6-lutidine to provide the target product 3aa, and [Ru(bpy)₃]³⁺ was reverted to ground-state [Ru(bpy)₃]²⁺.

Scheme 3 Possible reaction mechanism.

In summary, we have developed a facile and convenient route to diverse 2-cyanoalkylsulfonated 3,4-dihydronaphthalenes via visible-light photoredox-catalyzed dual C–C bond cleavage of methylenecyclopropanes and cycloketone oxime esters by the insertion of sulfur dioxide. This reaction follows a radical pathway and goes through a sequence of iminyl radical formation, C–C bond cleavage, sulfur dioxide insertion, sulfonyl radical addition, another C–C bond cleavage, and cyclization. Both the cyanoalkyl radical and cyanoalkylsulfonyl radical were trapped by radical inhibitors. Further research and application of this dual C–C bond cleavage reaction are currently underway in our laboratory.

We thank Hunan Provincial Innovation Foundation for Postgraduates (CX20190921), the Natural Science Foundation of Hunan Province (No. 2018JJ3208) and the National Natural Science Foundation of China (No. 21602056, 51874132 and 21805081) for financial support.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cc10057a

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