Photoredox-catalyzed difunctionalization of alkenes with oxime esters and NH-sulfoximines

Abstract

A visible-light-induced one-pot three-component reaction of cycloketone oxime esters, NH-sulfoximines, and alkenes was developed for the difunctionalization of alkenes. As a result, a series of versatile sulfoximido-cyanoalkylative products were synthesized under mild and sustainable reaction conditions (up to 98% yield). The significance of this protocol was highlighted by excellent regioselectivity, broad substrate scope, and excellent synthetic applications.

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Sulfoximines have attracted tremendous attention due to their special structural unit that has widely existed in medicinal chemistry, agricultural science, and organic synthesis in the past few decades.¹ As illustrated in Fig. 1, various bioactive properties were reported in representative compounds with sulfoximine moieties, including an anti-cancer agent, HIV-1 protease inhibitor, anti-inflammatory compound, insecticide, etc.² Additionally, sulfoximines have been extensively used as chiral ligands, organic catalysts, and key intermediates for the construction of heterocyclic compounds.³ Considering their great significance, the exploration of mild and sustainable methodologies for sulfoximidation is important.

Fig. 1 Representative examples of compounds containing sulfoximines.

The one-pot multiple-component reaction has emerged as a powerful strategy for the construction of highly functionalized target molecules due to its convenience and high step economy.⁴ Among the one-pot multiple-component reactions, the difunctionalization of alkenes, installing two functional groups across a double bond, has been regarded as an effective and powerful synthetic method that constructs molecular diversity and complexity.⁵ In the difunctionalization of alkenes, β-selective sulfoximidation of alkenes has been well developed via the sulfoximinyl radical process under photocatalysis or electrocatalysis (Scheme 1a).⁶ However, examples of α-sulfoximidation in the difunctionalization of alkenes via a photocatalytic multicomponent radical cascade reaction are still scarce. In 2021, Zeng and colleagues reported a visible-light-promoted sulfoximido-chalcogenization process of alkenes using K₂S₂O₈ as a radical initiator and tetrabutylammonium iodide (TBAI) as a catalyst, in which the phenylselenyl radical is a key intermediate initiating the radical addition process (Scheme 1b).⁷ However, such a difunctionalization procedure initiated by a carbon-centered radical is currently unknown.

Scheme 1 Difunctionalization of alkenes involving sulfoximines.

On the other hand, cyanoalkyl moieties play a significant and pivotal role in organic synthesis, pharmaceutical chemistry, and bioactive molecules.⁸ Meanwhile, cyanoalkyl compounds, as versatile and general synthons in organic synthetic chemistry, could be transformed into various important compounds, such as amides, carboxylic acids, esters, etc.⁹ Recently, cycloketone oxime esters have been considered effective cyanoalkylating reagents, which undergo N–O and β-C–C cleavage to furnish the corresponding cyanoalkylating compounds under visible-light irradiation,¹⁰ with significant contribution reported by the groups of Chen and Xiao,¹¹ Wu,¹² Studer,¹³ Leonori,¹⁴ and others.¹⁵ Given our ongoing research interest in photochemistry,¹⁶ we have herein developed a photoredox-catalyzed method for cyanoalkylsulfoximidation of alkenes using cycloketone oxime esters as cyanoalkylating reagents and NH-sulfoximines as sulfoximidating reagents via radical–polar crossover (Scheme 1c). Notably, this green photocatalytic strategy without any additives features excellent regioselectivity, broad substrate scope, target products with versatile functional transformation, and good scale-up potential.

Results and discussion

Initially, the model reaction of 1,1-diphenylethylene 1a and cyclobutanone oxime ester 2a with NH-sulfoximine 3a was carried out under blue LED irradiation (Table 1). Fortunately, the desired product 4a was obtained in 67% yield using Ir(dFppy)₂(bpy)PF₆ as a photocatalyst and dichloromethane (DCM) as a solvent (entry 1). Then, various photocatalysts, including Ir(dFppy)₃, Ir(p-Fppy)₂(bpy)PF₆, fac-Ir(ppy)₃, eosin Y, eosin B, and Rose Bengal, were evaluated. The results suggested that the best photocatalyst was Ir(p-Fppy)₂(bpy)PF₆, leading to the formation of 4a in 75% yield (entries 2–7). However, a series of solvents including 1,2-dichloroethane (DCE), tetrahydrofuran (THF), acetonitrile (CH₃CN), dimethyl carbonate (DMC), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) did not improve the yield of the target product (entries 8–13). Next, when the reaction time was reduced to 12 h, product 4a was obtained in 77% yield (entries 14 and 15). Furthermore, the yield of 4a was 90% when the ratio of 1a : 2a : 3a was 4 : 1 : 4 (entry 16), along with the direct cyanoalkylation of 1a as a byproduct (9% yield). It is assumed that excess alkenes and sulfoximines might inhibit the generation of the byproduct, promoting the yield of the major product 4a (for details, see the ESI†). Control experiments did not generate the desired product 4a in the absence of light or a photocatalyst, indicating that light and a photocatalyst are vital factors for the photoredox transformation (entries 17 and 18). Moreover, the reaction failed to perform under air, suggesting that this reaction was sensitive to air. Finally, the optimal reaction conditions were established as follows: 1a (0.4 mmol), 2a (0.1 mmol), 3a (0.4 mmol), Ir(p-Fppy)₂(bpy)PF₆ (1 mol%), and DCM (1 mL) were stirred under N₂ with the irradiation of a 40 W blue LED (456 nm) for 12 h.

Table 1 Optimization of reaction conditions^a

Entry	Photocatalyst	Solvent	Yield^b (%)
a Reaction conditions: 1a (0.3 mmol), 2a (0.1 mmol), 3a (0.3 mmol), photocatalyst (1 mol%), solvent (1 mL), and a 40 W blue Kessil LED lamp (456 nm), under N2 for 24 h. N.D. = no detection. b Isolated yields are given. c Reaction for 12 h. d Reaction for 8 h. e The ratio of 1a : 2a : 3a = 4 : 1 : 4 (0.1 mmol scale). f In the dark. g Under air.
1	Ir(dFppy)2(bpy)PF6	DCM	67
2	Ir(dFppy)3	DCM	60
3	Ir(p-Fppy)2(bpy)PF6	DCM	75
4	fac-Ir(ppy)3	DCM	70
5	Eosin Y	DCM	Trace
6	Eosin B	DCM	Trace
7	Rose Bengal	DCM	Trace
8	Ir(p-Fppy)2(bpy)PF6	DCE	66
9	Ir(p-Fppy)2(bpy)PF6	THF	40
10	Ir(p-Fppy)2(bpy)PF6	CH3CN	55
11	Ir(p-Fppy)2(bpy)PF6	DMC	20
12	Ir(p-Fppy)2(bpy)PF6	DMF	Trace
13	Ir(p-Fppy)2(bpy)PF6	DMSO	N.D.
14^c	Ir(p-Fppy)2(bpy)PF6	DCM	77
15^d	Ir(p-Fppy)2(bpy)PF6	DCM	65
16^e	Ir(p-Fppy)2(bpy)PF6	DCM	90
17^f	Ir(p-Fppy)2(bpy)PF6	DCM	N.D.
18	—	DCM	N.D.
19^g	Ir(p-Fppy)2(bpy)PF6	DCM	N.D.

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With the optimal reaction conditions in hand, the substrate scope of sulfoximines was explored (Scheme 2). The reactions of NH-sulfoximines bearing electron-donating or electron-withdrawing groups including –Me, –OMe, –F, –Cl, –Br, –COCH₃, and –CN with 1a and 2a all proceeded well under the optimal reaction conditions, affording the desired products 4b–4h in good to excellent yields (74%–98%). The results suggested that the reaction was not obviously influenced by the electronic effect of the substituents. Then, meta- and ortho-substituted NH-sulfoximines were also considered as competent substrates to afford the products 4i–4l in 50%–71%. Additionally, when dichloro-substituted NH-sulfoximine was employed to react with 1a and 2a, the target product 4m was obtained in 65% yield. Finally, other aromatic ring substituted NH-sulfoximines, such as naphthalene and pyridine, were compatible with the reaction system, leading to the formation of the corresponding products 4n and4o in 62% and 80% yields, respectively.

Scheme 2 Substrate scope of sulfoximines. Reaction conditions: 1a (0.4 mmol), 2a (0.1 mmol), 3 (0.4 mmol), and Ir(p-Fppy)₂(bpy)PF₆ (1 mol%) in DCM (1 mL) with the irradiation of a blue LED under a N₂ atmosphere for 12 h. Isolated yields are given.

To further investigate the universality of the photoreaction, we next examined the scope of cycloketone oxime esters. As shown in Scheme 3, various cyclobutanone oxime esters bearing different groups (–CO₂Me, –CO₂Et, –CO₂^tBu, and –Ph) could be smoothly converted into the corresponding primary cyanoalkyl target products 4p–4s in 52%–80% yields. To our delight, cyclopentanone oxime esters were also suitable for this transformation, delivering the corresponding products 4t and 4u, of which the secondary oxycyanoalkyl product 4t was synthesized in 54% yield and the tertiary cyanoalkyl three-component product 4u was obtained in 46% yield, showing the excellent compatibility of the substrates. Moreover, the scope of alkenes was also surveyed. The target products 4v–4ac were all synthesized via the green and sustainable photochemical strategy in moderate to good yields (52%–85%) from the corresponding symmetric and nonsymmetric 1,1-diarylalkenes. Unfortunately, when styrene and an aliphatic terminal olefin were employed as the substrates, no desired products were obtained (for details, see the ESI†).

Scheme 3 Substrate scope of oxime esters and alkenes. Reaction conditions: 1 (0.4 mmol), 2 (0.1 mmol), 3a (0.4 mmol), and Ir(p-Fppy)₂(bpy)PF₆ (1 mol%) in DCM (1 mL) with the irradiation of a blue LED under a N₂ atmosphere for 12 h. Isolated yields are given.

Subsequently, a gram-scale reaction was performed on a 4 mmol scale under the standard conditions (Scheme 4a). To our delight, the target compound 4a was obtained in 84% isolated yield (1.35 g), indicating that this meaningful protocol has good practical potential. Considering the synthetic versatility of the cyanoalkylsulfoximidation of alkenes, a variety of transformations were further performed (Scheme 4b). For example, the hydrolysis–esterification of 4a using concentrated H₂SO₄ as a catalyst and ethanol as a solvent under reflux conditions delivered the desired product 5a in 78% yield. The hydrolysis–acidification of 4a with concentrated H₂SO₄ in acetic acid and water furnished 1,5-enoic acid 5b in 65% yield. To prove the generality of the photocatalyzed protocol, N-methylindole 3ad and indole 3ae could be smoothly applied to the reaction system, delivering the corresponding products 4ad and 4ae in 57% and 48% yields, respectively.

Scheme 4 Synthetic applications.

In order to investigate the reproducibility of this sustainable protocol, sensitivity experiments were designed (for details, see the ESI†). It could be intuitively observed from the radar figure that the photoreaction was sensitive to air and tolerated other factors including concentration, water level, photocatalyst level, light intensity, and scale (Fig. 2).

Fig. 2 Sensitivity assessment.

Afterward, some fluorescence quenching experiments were conducted. As shown in Fig. 3, a linear relationship between the fluorescence intensity I₀/I and concentration of 2a was observed, indicating that a single electron transfer (SET) process between the excited photocatalyst and 2a might take place.

Fig. 3 Fluorescence quenching experiments.

To gain deeper insight into the photoreaction mechanism, a series of radical trapping experiments were carried out (Scheme 5). The radical scavenger 2,2,6,6-tetramethylpiperidin-1-yl-oxidanyl (TEMPO) or 2,6-di-tert-butyl-4-methylphenol (BHT) was added into the model reaction which resulted in a dramatic decrease in the yield of the target product, suggesting that the photocatalytic transformation may involve a radical pathway. Furthermore, the radical adducts A and B were detected by high-resolution mass spectroscopy (HRMS), which proved the generation of cyanoalkyl radicals. Surprisingly, the radical adduct C was also detected by HRMS, reminding us that a reaction pathway involving the sulfoximinyl radical was also possible.

Scheme 5 Radical trapping experiments.

The plausible mechanism was proposed based on the above detailed experimental results and previous reports (Scheme 6).^7,17 Initially, the ground-state photocatalyst PC was excited to the excited-state PC* under the irradiation of a blue LED. Subsequently, cyclobutanone oxime ester 2a and PC* underwent a SET process to form the radical cation PC˙⁺ and the unstable N-centered radical 5, which rapidly underwent β-C–C cleavage to afford cyanoalkyl radical 6. The radical 6 was added to 1a to produce intermediate 7. Then, 7 was oxidized by PC˙⁺ to afford the cationic intermediate 8, which was attacked by nucleophile 3 to furnish the target product 4 (pathway A). Since the sulfoximinyl radical was detected by HRMS, pathway B involving the direct coupling of radical intermediate 7 and the in situ generated sulfoximinyl radical 10 could not be ruled out.

Scheme 6 Proposed mechanism.

Conclusions

In conclusion, we have developed a green and sustainable one-pot three-component strategy for the cyanoalkylsulfoximidation of alkenes with cycloketone oxime esters and NH-sulfoximines via radical-polar crossover under irradiation of visible light. Various target products can be synthesized with excellent regioselectivity in moderate to excellent yields under mild conditions. Additionally, the superior synthetic applications of this photocatalytic protocol are reflected in the scalability and the formation of the cyanoalkylsulfoximidative product with versatility, which can be converted into a variety of important organic molecules.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the financial support from the National Key R&D Program of China (2021YFB4001100 and 2021YFB4001101), the National Natural Science Foundation of China (21971224, 22071222, and 22171249), and the Science & Technology Innovation Talents in Universities of Henan Province (23HASTIT003).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qo01575k

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