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. 2024 Mar 1;4(3):1073–1080. doi: 10.1021/jacsau.3c00802

Visible Light-Promoted Regioselective Benzannulation of Vinyl Sulfoxonium Ylides with Ynoates

Daksh Singh Davas †, Dinesh Kumar Gopalakrishnan †, Sandeep Kumar †, Anmol †, Tarak Karmakar †,*, Janakiram Vaitla †,*
PMCID: PMC10976606  PMID: 38559716

Abstract

graphic file with name au3c00802_0009.jpg

Herein, we report a highly regioselective [4 + 2]-annulation of vinyl sulfoxonium ylides with ynoates under light-mediated conditions. The reaction proceeds through the new dienyl sulfoxonium ylide, which undergoes photolysis under blue light irradiation to give highly substituted naphthalene scaffolds. The method presented here operates at room temperature and does not require the addition of an external photosensitizer. The in situ-generated dienyl sulfoxonium ylide absorbs light and acts as a photosensitizer for the formation of arenes. The synthetic potential of these benzannulations was further illustrated by various synthetic transformations and a scale-up reaction. Moreover, control experiments and quantum chemical calculations reveal the mechanistic details of the developed reaction.

Keywords: regioselective annulations, sulfur ylides, photochemistry, arenes, alkynes, benzannulation

Introduction

Substituted naphthalenes are important and ubiquitous scaffolds in many natural products,1 bioactive molecules,2 nanomaterials,3 ligands for transition metals,4 and other materials with useful properties (Figure 1).5,6 The synthesis of substituted naphthalenes encompasses a vast number of approaches.7,8

Figure 1.

Figure 1

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Methylated naphthalene core in bioactive natural products.

The dehydro-Diels–Alder (DDA) reaction is one of the most powerful reactions to construct highly substituted arenes.9 The straightforward approach for the naphthalene scaffolds via the intermolecular DDA reaction is challenging because of the high activation energy (Scheme 1a). Since 1963, intramolecular didehydro-Diels–Alder (IMDDA) reactions of styrene-ynes to construct naphthalenes have attracted much attention (Scheme 1b).10 However, the IMDDA reaction required harsh reaction conditions, which often resulted in the polymerization of styrene. The recent renaissance of photocatalysis provides a new space for various styrene-based [4 + 2] annulations.11−14 In 2018, Lei's group developed a visible-light-mediated [4 + 2] annulation reaction between styrene derivatives with alkynes to generate naphthalenes by using an acridinium photosensitizer and cobalt catalyst (Scheme 1c).15 Despite the success of various light-mediated [4 + 2] annulations, these methods encounter certain inherent challenges: (1) The reactions can give the [2 + 2] cycloaddition instead of [4 + 2]-annulation according to the Woodward–Hoffmann rules, (2) to generate the key radical cation intermediate, the reaction usually requires both electron-rich diene and dienophile components, (3) the regio- and stereoselectivities are the main challenges for the intermolecular additions, (4) homodimerization is also another feasible transformation under these conditions, and (5) these methods require expensive metal and organophotoredox catalysts. Thus, it is desirable to develop an efficient, regioselective, catalyst-free intermolecular light-mediated [4 + 2] annulation to address these issues.

Scheme 1. Synthesis of Substituted Naphthalenes from Alkynes and Styrene derivatives.

Scheme 1

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Recently, the catalyst-free generation of carbenes from the diazo compounds (absorption >400 nm) under blue light has proven to be a valuable alternative approach to transition metal catalysis. This approach provides a straightforward and general platform for various carbene transfer reactions with different trapping reagents under mild conditions (Scheme 2a).16−21 Moreover, most of the light-mediated diazo-based transformations follow a reaction pathway similar to that of metal-catalyzed reactions.

Scheme 2. Synthesis of Substituted Arenes from Vinyl Sulfoxonium Ylides.

Scheme 2

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Sulfoxonium ylides are alternatives to diazo compounds in carbene transfer reactions due to their advantages of operational safety, and high thermal stability.22−25 However, like diazo-based catalyst-free photochemical transformations, the chemistry of sulfoxonium ylides has not been explored under visible light.26−28

The major challenges associated with the light-mediated conversion of sulfoxonium ylides are the light absorption of ylides as well as the leaving ability of DMSO (Scheme 2b).29 As part of our ongoing research interest in sulfur ylides30−37 and intrigued by metal-free carbene generation from diazo compounds, we would like to explore the reactivity of vinyl sulfoxonium ylide in the presence of blue light to open up a new avenue of sulfoxonium ylide based photochemistry.38−40

Initially, we wanted to perform our previously reported metal-catalyzed vinyl carbene-mediated [4 + 2] benzannulation in the presence of blue light (Scheme 2c, expected pathway).34 Thus, we began our investigation with the reaction of 0.3 mmol of vinyl sulfoxonium ylide 1 (obtained in one step from 1 mmol of ynoate and 1.4 mmol of dimethylsulfoxonium methylide) and 0.4 mmol of methyl propiolate (2) in DCE at 427 nm. Surprisingly, the reaction gave the naphthalene scaffold 3 instead of the expected arene 3′. Initial mechanistic studies reveal that the reaction proceeds through the intermediate dienyl sulfoxonium ylide (int-A), which can act as a sensitizer (see the SI, Figure 5). This serendipitous outcome prompted us to explore the styrene-yne [4 + 2] annulation, which has been difficult to achieve by the traditional intermolecular DDA reaction.41 In this work, we have developed a highly regioselective [4 + 2] annulation of vinyl sulfoxonium ylides with terminal and internal ynoates to generate highly substituted naphthalenes under blue light conditions (Scheme 1d).

Results and Discussion

We optimized the present [4 + 2] annulation by using various protic/aprotic solvents and photocatalysts (for the optimization, see the Supporting Information, Table 1), and the reaction gave good yields of naphthalene 3 in ethanol at rt. Interestingly, the reaction does not require any external photocatalyst or additive.42,43 To explore the substrate scope of the reaction, we examined the different substitutions at the aryl ring of the vinyl sulfoxonium ylide (Scheme 3). A variety of aryl groups bearing electron-donating, electron-neutral, and halogen groups at meta- and para- positions underwent the [4 + 2] annulation with methyl propiolate to afford the corresponding naphthalenes 3a–3h in 58–68% yields. Moreover, the structure of product 3b was confirmed by using X-ray analysis (CCDC 2271065). The substitution at the ortho- position of the aryl group of the ylide gave a low yield of naphthalene 3i due to the lower stability of the ylide. Then, we evaluated the variation in the ester part of ylide 1. The ester group containing 2-(trimethylsilyl)ethyl, benzyl, and cyclohexyl substitutions provided the naphthalenes 3j–3l in 54–60% yields, respectively. The efficiency of this method was further demonstrated through the variations in dienophile 2. A series of ynoates, such as cyclohexyl, allyl, and propargylic substitution at the ester, underwent [4 + 2] annulation and yielded arenes 3m–3p in 34–52% yields.

Scheme 3. Scope of Blue Light Promoted Benzannulation of Vinyl Sulfoxonium Ylide with Terminal and Internal Alkynes,

Scheme 3

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Reaction conditions: 1 (0.4 mmol), 2 (0.5 mmol), in ethanol (0.2 M) under open air conditions at rt and light irradiation from 427 nm Kessil lamp (45W) for 12 h.

Isolated yields of naphthalenes 3.

Next, we evaluated the scope of the internal alkynes. The reaction of various aryl-substituted vinylsulfoxoniums with diethyl acetylenedicarboxylate gave naphthalene triesters 3q–3t in 10–72% yields. This reaction is also successful with variation at the ester part of the ylide and gave 3u–3x in 42–61% yields. The reaction is also successful with the ynoates containing heterocyclic scaffolds and yielded the products 3y and 3z in 64% and 52% yields, respectively. Moreover, the reaction is also successful with the sulfonyl group-stabilized vinyl sulfoxonium ylide and yielded the naphthalene 3aa and 3ab in 63% and 57% yield, respectively.

Attempts to perform this reaction with other electron-deficient alkynes such as tosyl acetylene were not successful. Notably, this reaction is also successful with the N-methoxy propiolamide and gave the Weinreb amide-substituted naphthalene 3ad in 54% yield. This scaffold can be utilized as a precursor for various transformations to obtain biologically active molecules.44

The robustness of this protocol was demonstrated with structurally complex ynoates and ylides containing steroid and terpenoid derivatives yielding the desired naphthalenes 3ae–3aj in 49–68% yields. Subsequently, we investigated the synthesis of naphthalenes from two different aryl propiolates. The addition of dimethylsulfoxonium methylide generated from the salt (A) on ethyl phenylpropiolate (B) followed by treatment with the methyl 4-nitrophenylpropiolate (C) gave naphthalene 3al, albeit in a lower yield (38%). However, switching the propiolate sequence could not afford desired product 3ak. Interestingly, the reaction between ylide and internal alkynes containing keto and ester groups afforded meta-terphenyls 3am’ as the exclusive product instead of substituted naphthalene 3am (see the SI, Scheme 6.1). When we treated thiophene-derived vinyl sulfoxonium ylide with methyl propiolate under standard conditions, the reaction exclusively afforded benzothiophene derivative 3an in 68% yield. Surprisingly, another possible regioisomer benzo[c]thiophene derivative 3an’ is not observed in this transformation. Moreover, the [4 + 2] annulation of pyridine-derived vinyl sulfoxonium ylide with methyl propiolate is unsuccessful.

Scheme 6. Computed Energy Profile (in kcal/mol) for [4 + 2] Annulation.

Scheme 6

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To demonstrate the potential of this developed method, a scale-up reaction was performed for constructing the important naphthalene scaffold 5, which can be used as a precursor for the polyester fiber (Scheme 4a).45 In the reported approach, product 5 was obtained from the pre-existing naphthalene unit 4 by Friedel–Crafts acylation, which gave possible side products along with the desired product. To our delight, the 5 mmol scale of the present annulation under standard reaction conditions afforded naphthalene 5 in 58% yield in two steps from the ynoate. Subsequently, we synthesized naphthalactone 6 in 82% yield from vinyl sulfoxonium ylide in two steps by three sp2 C–H functionalizations on ylide 1a (Scheme 4b). First, naphthalene scaffold 3ad was generated from the vinyl sulfoxonium ylide with propiolamide. Then, 3ad undergoes a palladium-catalyzed C8-oxygenation/lactonization sequence using Weinreb amide as a directing group to generate naphthalactone 6 (Scheme 4c).46 Next, we performed radical mediated bromination on 3a to generate naphthyl bromide 7 in 97% yield. The reaction of 7 with (S)-phenylethylamine gave lactam 8 in 73% yield. In addition, subjecting naphthyl bromide 7 under silver-mediated conditions led to intramolecular lactonization to give lactone 9. Notably, the present benzannulation was performed in sunlight for 2 days and gave the desired naphthalene 3a in 56% yield (Scheme 4d).

Scheme 4. Synthetic Applicability of the Developed [4 + 2] Annulation.

Scheme 4

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Several mechanistic experiments were conducted to elucidate the present benzannulation (Scheme 5). The reaction of vinyl sulfoxonium ylide 1 with ynoate 2 in the absence of light gave the dienyl sulfoxonium ylide 10.47,48 The structure of ylide 10 was confirmed using X-ray analysis (CCDC 2271105). Next, the reaction of 10 in the presence of blue light gave arene 3a in 75% yield (Scheme 5a). The light irradiation is essential to the transformation because 3a could not be detected when the reaction is performed in the dark at room temperature or heated at 70 °C. The UV–vis absorption spectra of vinyl sulfoxonium ylide 1 showed absorption around the 330 nm region. However, dienyl sulfoxonium ylide 10 showed appreciable absorption around the 400 nm region. This indicates that the π–conjugation extended ylide is the key component in the light-mediated reaction. To investigate the involvement of radicals in the mechanism, we performed EPR studies using N-tert-butyl-α-phenylnitrone (PBN) as a radical trap in the present reaction, which clearly showed the generation of radical species (Scheme 5b).

Scheme 5. Mechanistic investigation for [4 + 2] Benzannulation.

Scheme 5

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Next, we performed the reaction with radical trapping agents such as TEMPO. The gradual decrease in the yield of 3a is observed by increasing the concentration of TEMPO (Scheme 5c). Moreover, the TEMPO-captured intermediates are detected by HRMS, which further supports that a radical pathway is involved in the catalytic cycle. When the reaction was performed under argon, the reaction was completely shut down, suggesting the role of O2 (Scheme 5d). Additionally, the introduction of DABCO, an inhibitor of 1O2, suppressed the formation of product 3a, indicating the presence of singlet oxygen species in the reaction process (Scheme 5e). To further prove the presence of photoexcited singlet oxygen, trapping experiments were performed using triphenylphosphine under the standard conditions and afforded the triphenylphosphine oxide as a side product, which formed through a peroxide intermediate, while the corresponding oxidation was not detected in the dark (Scheme 5f).49 This reaction clearly indicates that 1O2 is produced from air under irradiation. The fluorescence emission quenching study showed that the emission intensity of excited ylide 10 was remarkably affected by O2 (Scheme 5g). In addition, the cyclic voltammetry (CV) analysis reveals that the ylide 10 (−1.46 V vs SCE) can reduce the oxygen (−0.75 V vs SCE) to generate the radical ion 10a (see Supporting Information, Figure 12). Next, the reaction was conducted in CD3OD instead of EtOH. The deuteration took place at the ylidic carbon suggesting that the solvent could be a proton source for this reaction (Scheme 5h).

To further understand the mechanistic details, density functional theory (DFT) calculations were carried out at the B3LYP-D3(BJ)/6-311++G(d,p)/SMD (ethanol) | B3LYP/6-31G(d) level of theory using Gaussian 09 Revision D.01. software. In the present annulation (Scheme 6), the radical intermediate int-1A cyclizes via TS-1A with an energy barrier of 21.5 kcal/mol and generates the int-2A. Energetics shows that this is the slowest step in the reaction mechanism. Int-2A undergoes H-radical abstraction promoted by the singlet oxygen to form int-3A via TS-2A (12.9 kcal/mol). Transferring the abstracted H-radical to the ylide carbon of int-3A leads to int-4A, which shows significant stability as the generated radical is at the benzylic and allylic positions. Subsequently, the DMSO radical cation is released from int-4A by homolytic cleavage forming int-5A. This step does not require high energy, as it entails the production of a neutral molecule that aromatizes directly into the final product, 3.

Based on the experimental and theoretical investigations and previous reports on singlet oxygen-mediated photochemical reactions,50 the plausible mechanism for the [4 + 2] annulation is proposed in Scheme 7. The excited sulfoxonium ylide 10* is capable of converting 3O2 to 1O2, which can oxidize either 10 or 10* via the SET process to generate int-1A. The radical-mediated annulation of int-1A gives int-2A, which can undergo sequential hydrogen atom transfer to give int-4a. Then elimination of the DMSO radical cation from int-4a gives int-5A, which further aromatizes to produce the arene 3. The released DMSO radical cation oxidizes the superoxide ion to the resulting ionic reagent, O2.

Scheme 7. Plausible Mechanism for [4 + 2] Benzannulation.

Scheme 7

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Conclusions

In conclusion, we have developed the first catalyst and additive-free blue-light-mediated transformations of sulfoxonium ylides. The reaction affords various highly substituted naphthalene scaffolds via [4 + 2] annulation of vinyl sulfoxonium ylides with ynoates. In this approach, the in situ-generated novel dienyl vinyl sulfoxonium ylide can act as a photosensitizer. This method offers several key advantages, including mild reaction conditions, broad substrate scope, operational simplicity, high regioselectivity, high scalability, and the ability to facilitate late-stage modifications. Detailed computational and experimental studies reveal that the reaction proceeds through a ylide-based radical cation intermediate. Sulfoxonium ylides are frequently employed as synthetic alternatives to diazo compounds in metal-catalyzed transformations owing to their similar reactivity. Sulfoxonium ylides are regarded as synthetic equivalents of diazo compounds and have been widely used as carbene surrogates in organic transformations under transition-metal catalysis. Unlike diazo compounds, it was observed that the excited dienyl sulfoxonium ylide did not undergo photolytic cleavage of DMSO to generate a carbene under visible light. However, the ylide involved in an energy transfer event with O2 led to a completely different reaction path. This difference could be attributed to the leaving tendency of DMSO compared to N2. Similar to the catalyst and additive-free visible light-mediated diazo transfer reactions, the present alkenyl sulfoxonium ylide can open up new windows in the photochemistry of ylide-based transformations to generate novel organic transformations.

Methods

To an oven-dried, 10 mL reaction vial equipped with a magnetic stir bar are added vinyl sulfoxonium ylide 1 (1 equiv, 0.4 mmol) and ynoate (1.2 equiv, 0.5 mmol) in 5 mL of ethanol as solvent. The vial was then irradiated using a 427 nm Kessil lamp at 6 cm from the light for 12 h. Then evaporate the solvent under reduced pressure. Then, the crude reaction mixture was loaded directly on silica (Biotage SNAP Ultra Samplet 10 g) and purified by flash column chromatography.

Acknowledgments

This work was supported by the Science and Engineering Research Board (CRG/2023/000584). T.K. acknowledges IIT Delhi for the seed grant. D.S.D., D.K.G., and Anmol thank MHRD (Govt. of India) for a PMRF Fellowship. S.K. thanks the University Grants Commission of India for the UGC fellowship. We thank IIT Delhi HPC facility for computer resources and CRF for providing the instrumentation facility. We also thank Prof. Sudipta Raha Roy, Department of Chemistry IIT Delhi for the discussions on photochemistry. We gratefully acknowledge Aishwarya Chauhan and Shubham Kumar for assisting in X-ray crystallography and fluorescence spectroscopy and Vikas Tiwari for discussions and assistance while carrying out DFT calculations. The crystal structures for 3b (CCDC 2271065) and 10 (CCDC 2271105) have been deposited with the Cambridge Structural Database.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.3c00802.

  • Representative experimental procedures, characterization and spectral data (NMR, HRMS), crystallographic data for 3b and 10, and DFT calculations (PDF)

  • Crystallographic data for 3b and 10 (CIF)

  • Crystallographic data for 10 (CIF)

Author Contributions

D.S.D., D.K.G, and S.K., conducted the experimental investigation under J.V.’s supervision. D.K.G. and Anmol performed computational investigation under T.K.’s supervision. D.S.D, D.K.G., T.K., and J.V. wrote the manuscript. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

au3c00802_si_001.pdf (10.6MB, pdf)
au3c00802_si_002.cif (1.6MB, cif)
au3c00802_si_003.cif (2MB, cif)

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Associated Data

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Supplementary Materials

au3c00802_si_001.pdf (10.6MB, pdf)
au3c00802_si_002.cif (1.6MB, cif)
au3c00802_si_003.cif (2MB, cif)

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