Regioselective Intermolecular Sulfur–Oxygen Difunctionalization

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Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Regioselective Intermolecular Sulfur−Oxygen Difunctionalization (Phenoxysulfonylation) of Alkynes: One-Pot Construction of (Z)‑βPhenoxy Vinylsulfones Mohd Yeshab Ansari,† Navaneet Kumar,† and Atul Kumar*,†,‡ †

Medicinal & Process Chemistry Division, CSIR-Central Drug Research Institute, Sector 10, Jankipuram Extension, Sitapur Road, P.O. Box 173, Lucknow 226031, India ‡ Academy of Scientific and Innovative Research, New Delhi 110001, India

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S Supporting Information *

ABSTRACT: The first regioselective phenoxysulfonylation of alkynes with sodium sulfinate and phenol catalyzed by I2/base has been uncovered. This metal-free, one-pot alkyne difunctionalization process provides various (Z)-β-phenoxy vinylsulfones under mild reaction conditions. A variety of terminal as well as internal alkynes and sodium sulfinates and a wide range of phenols/thiophenols and naphthols are viable in this transformation. Green protocol and wide substrate scope are the remarkable characteristics of this multicomponent procedure. lkynes are one of the most fundamental and flexible building blocks used in modern organic synthesis; as a result, catalytic transformation of these π-systems into other important functionalities is extremely alluring in pharmaceutical and academic research.1 1,2-Difunctionalization of alkynes, enable to furnish the functionalized olefins, has attracted much consideration in recent years. 2 Among these, vicinal difunctionalizations of alkynes triggered by radical provide an efficient pathway for the formation of substituted olefins by interacting with both carbon-3 and heteroatom-possessed radicals,4 with high stereoselectivity.5 Moreover, as an extremely significant functional group, the sulfone functionality is extensively used in organic chemistry and especially in pharmaceutical chemistry.6 Thus, the fabrication of sulfone groups into organic frameworks strongly fosters the synthetic quest of chemists because of their important biological properties and diverse synthetic applications (Figure 1).7 Difunctionalizations of alkynes represent much more convenient strategies that encompass the incorporation of an SO2-embodied group. Thus, efficient protocols for sulfonylations of alkynes with simultaneous formation of C−N,8 C−O,9 C−H,10 C−C,11 C− halide,12 and C−Se13 bonds have been reported.14

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© XXXX American Chemical Society

Figure 1. Some bioactive molecules containing a sulfone group.

In contrast, the concomitant formations of C−SO2, viz., sulfonylation and C−O single bonds, are less abundant and have been mostly explored to intramolecular settings,15 which reduces the scope of diversity. Generally, intermolecular 1,2difunctionalization processes rely on two different catalytic systems, which boundaries their practicality.11 Although good reactivities are accomplished in these procedures, the controlled geometry, particularly Z-selectivity, is still challenging. Received: March 25, 2019

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DOI: 10.1021/acs.orglett.9b01041 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

π−π stacking interaction of sulfur substituents with phenol might be accountable to ameliorate Z-selectivity17 (Scheme 1). Furthermore, the biological activity of these compounds and the use of this methodology for the construction of new biological active pharmacophores are ongoing projects in our group. Based upon the recent updates on sulfonylation reactions,13 we proposed that the multicomponent strategy of alkynes, sodium sulfinates, and phenols would afford (Z)-βphenoxy vinylsulfones. To analyze the viability of our assumption, we commenced the test reaction by employing phenylacetylene 1a, sodium 4methylbenzenesulfinate 2a, and phenol 3a as model substrates under several conditions, and the results are summarized in Table 1.

Recently, Nevado et al. (2017) reported nickel-catalyzed carbosulfonylation of alkynes by means of sulfonyl radical processes (Scheme 1a).12a Bi et al. (2017) also have proposed Scheme 1. Strategies for Intermolecular Radical-Based 1,2Difunctionalization of Alkynes

Table 1. Optimization Study of Phenoxysulfonylation Reactionsa

three components of silver-catalyzed intermolecular aminosulfonylation of alkyne triggered by radical addition (Scheme 1b).13a Most of the above-mentioned strategies have some limitations such as requiring the preparation or use of moisture-incompatible reagents and toxic transition metal catalysts, which reduce the synthetic applicability (Scheme 1a,b). Therefore, conceptually different approaches are in high need. We herein disclosed the hitherto unexplored radical-based metal-free intermolecular vicinal difunctionalization of alkynes via an unrevealed sulfonylation/phenoxide ion addition cascade (Scheme 1). The notable feature for this fruitful conversion is that we communicated an operationally simple and effective protocol to produce sulfonyl radical from sodium salt of sulfinate, thus circumventing the preliminary combative radical reactions to alkynes such as ATRA and terminal alkyne homocoupling.16 Moreover, up to the limit of our knowledge, this is the first report of metal-free, one-pot, intermolecular alkyne phenoxysulfonylation resulting in a stereocontrolled construction of (Z)-β-phenoxy vinylsulfones. Mechanistically, a plausible reaction pathway is observed which is initiated by the addition of radical to alkyne and produces a vinyl radical intermediate, which could further react with another reactant and leads to the formation of generally inaccessible Z-alkene product. However, these vinyl radical species are known to be extremely reactive and rapidly undergo hydrofunctionalization,16 which is a somewhat challenging task to develop radical-based difunctionalization reactions of alkyne. In most of the radical-based difunctionalizations of alkyne, generally E-alkenes are obtained. In contrast, the way of constructing Z-alkene via radical-based difunctionalization of alkyne is at the primitive stage. In the present protocol, the

entry

I2 (equiv)

base (1 equiv)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1 1 1 1 1 1 1 1 0.5 1.5 1 1 1 1 1 1

KOH NaOH CsCO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2C03 K2CO3 K2CO3 K2CO3 K2CO3

17 18c 19d

1 1 1

K2CO3 K2CO3 K2CO3

solvent EtOH EtOH EtOH EtOH EtOH EtOH EtOH EtOH EtOH EtOH H2O DMSO DMF CH3CN THF 1,4dioxane toluene EtOH EtOH

temp. (° C)

time (h)

yield (%)b

60 60 60 60 60 60 80 r.t. 60 60 60 60 60 60 60 60

6 6 6 6 6 8 6 6 6 6 6 6 6 6 6 6

trace 20 25 60 85 82 83 45 50 82 20 68 65 50 50 45

60 60 60

6 6 6

40 85 83

a

Reaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), 3a (0.5 mmol), and I2 (0.5 mmol) in 2 mL of EtOH at 60 °C for 6 h. bIsolated yield. c Under N2 in a sealed tube. dThe reaction was carried out in a dark background.

Our initial exertions were focused on the actual effectiveness of base for the reaction. The model reaction was performed without the use of any base in ethanol at 60 °C, and the desired product was formed in trace amounts (Table 1, entry 1). The obtained results prompted us to screen the reaction with different bases. First, we chose hydroxy base to catalyze the reaction, which resulted in the formation of (Z)-β-phenoxy vinylsulfones (4b) in 20−25% yield (Table 1, entries 2 and 3). Further, we turned our focus toward the use of carbonate bases and acquired a pleasant result. Among the bases evaluated, K2CO3 was superior and provided the best yield of the compound 4b in 6 h at 60 °C (Table 1, entry 5), on B

DOI: 10.1021/acs.orglett.9b01041 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters prolongation of reaction time from 6 to 8 h and temperature from 60 to 80 °C. No enhancement in yield was found (Table 1, entries 6 and 7). The reaction was also performed at r.t., but yield was low (45%) (Table 1, entry 8). The use of less or more than 1 equiv of I2 also resulted in a decrease in product yield (Table 1, entries 9 and 10). Furthermore, a variety of polar and nonpolar solvents like H2O, ethanol, DMSO, DMF, toluene, THF, CH3CN, and 1,4-dioxane were screened in the reaction, and it was observed that ethanol afforded a better yield than the other solvents. Therefore, the survey of solvents with K2CO3 revealed that ethanol was the best choice (Table 1, entry 5), whereas the other organic solvents diminished the reactivity (Table 1, entries 11−17). Thus, the optimal reaction conditions were 1a (1 equiv), 2a (1.2 equiv), 3a (1 equiv), and I2 (1 equiv) in ethanol at 60 °C for 6 h (Table 1, entry 5). The structure of (Z)-β-phenoxy vinylsulfones was unequivocally confirmed by X-ray crystallographic analysis of 4b (Figure 2). With optimized conditions in

Scheme 2. Scope of the Reaction for Synthesis of Various (Z)-β-Phenoxy Vinylsulfones from Phenylacetylenes, Sodium Sulfinates, and Phenolsa,b

Figure 2. X-ray crystal structure of 4b.

hand, a wide range of terminal alkynes, as well as internal alkynes, were employed to test the efficiency of the reaction, and the results are shown in Schemes 2 and 3. Generally, the electron-rich groups (Me and t-Bu) on the phenyl ring gave better reactivity than the electron-deficient groups (Cl and CF3). Furthermore, the aliphatic terminal alkynes as well as internal alkynes were also successfully converted to the corresponding (Z)-β-phenoxy vinylsulfone products, 4y−4ab and 4al−4an, in moderate yields, respectively. Generally, the lower reactivity of aliphatic alkynes in difunctionalization reactions is presumably caused by the lack of a π-conjugation in alkyl-substituted vinyl radical intermediates in comparison with aryl-substituted analogues.18 Moreover, a series of sodium phenylsulfinates, having an electron-rich group (Me) or electron-deficient groups (Cl and Br) on the phenyl ring, furnished the corresponding (Z)-βphenoxy vinylsulfones in good to excellent yields under the optimized conditions. However, electron-deficient sodium 2nitrobenzenesulfinate, sodium 4-nitrobenzenesulfinate, and aliphatic sodium trifluoromethanesulfinate absolutely refused to participate in this transformation due to the instability of the corresponding sulfone radicals. Furthermore, to extend the scope of this protocol, a wide range of phenols, thiophenols, and naphthols were also examined under the same reaction conditions (Table 1) and furnished the corresponding (Z)-β-phenoxy vinylsulfone products in good to excellent yields (Schemes 2 and 3). Generally, for the substituted phenols, the electron-rich groups

a

Reaction condition: 1a (0.5 mmol), 2a (0.6 mmol), 3a (0.5 mmol), and I2 (0.5 mmol) in 2 mL of EtOH at 60 °C for 6 h. bIsolated yield.

(Me, MeO, and Et) gave better yields compared to electronwithdrawing groups (NO2, Br, CF3, Cl, and Br). However, the positional variation of the same substituents in the phenyl ring C

DOI: 10.1021/acs.orglett.9b01041 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 3. Scope of the Reaction for Synthesis of Various (Z)-β-Phenoxy Vinylsulfones from 1-Phenyl-1-propyne, Sodium Sulfinates, and Phenolsa,b

Scheme 6. Possible Reaction Mechanism

a Reaction condition: 1a (0.5 mmol), 2a (0.6 mmol), 3a (0.5 mmol), and I2 (0.5 mmol) in 2 mL of EtOH at 60 °C for 6 h. bIsolated yield.

vinylsulfone intermediate (5) is formed, followed by nucleophilic addition of a phenoxide ion to generate intermediate (D). This resultant intermediate (D) finally delivered the desired product (Z)-β-phenoxy vinylsulfone (4) with the elimination of iodide. To evaluate the efficacy of our strategy, the closely connected, important green metrics and parameters (yield: 85%, atom economy: 91.36%, mass intensity: 1.38, reaction mass efficiency: 72.02, carbon efficiency: 79.68, E-factor: 0.38, atom efficiency: 77.65) were calculated and represented in a radar chart (Figure 1; see SI).20 In summary, we have reported an operationally simple multicomponent reaction of alkynes, sodium sulfinates, and phenols which is catalyzed by I2/base. This protocol shows tolerance with a broad range of functional groups and is environmentally benign. The reaction takes place through the sequence of an alkyne sulfonylation and phenoxide ion addition cascade. This approach represents an attractive way to attain alkyne phenoxy functionalization using eco-friendly reaction conditions; we are further exploring with other nucleophilic and radical species and results will be disclosed in the future.

did not alter the yields of the corresponding (Z)-β-phenoxy vinylsulfone products, as 4c and 4d were obtained in 85% and 84% yields, respectively, with Z-selectivity, despite unfavorable steric hindrance especially in the case of an ortho-substituent. Moreover, with the aim of evaluating the practicality of this strategy, a 20 mmol scale reaction was carried out under optimal conditions by using 20 mmol of phenylacetylene (1a), 24 mmol of sodium p-toluene sulfinate (2a), and 20 mmol of 4-bromophenol (3a). Product 4b was obtained in 82% yield (Scheme 4), demonstrating its great potential for large-scale synthesis in the industry (Scheme 4). Scheme 4. Large-Scale Experiment

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To explore the mechanistic pathway, some control experiments were carried out (Scheme 5). On quenching the reaction with radical scavengers like TEMPO and BHT, the reaction was inhibited, which showed that the current transformation should involve a radical pathway.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01041. Experimental procedures and compound characterization data, including the 1H/13C NMR spectra (PDF)

Scheme 5. Control Experiments

Accession Codes

CCDC 1873821 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Finally, based on the control experiment outcomes and literature precedents,19 a plausible mechanism for metal-free, one-pot (Z)-β-phenoxy vinylsulfonylation reaction is proposed in Scheme 6, which is initiated by the interaction of sodium sulfinate with molecular iodine to generate a sulfonyl radical (A) that further reacts with alkyne to generate vinyl sulfone radicals (B) or (C). Subsequently, molecular iodine, iodine radical, or sulfonyl iodide may react with a vinyl sulfone radical, and β-iodo

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. ORCID

Atul Kumar: 0000-0002-4886-0355 Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acs.orglett.9b01041 Org. Lett. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS We are thankful to CSIR-UGC for research fellowships. We thank SAIF division CSIR-CDRI for the analytical facility. Financial support by CSIR-Network project BSC0102/ BSC0108 is gratefully acknowledged. We thank Dr. Tejender S. Thakur, MSB Division, CSIR-CDRI, for supervising the Xray data collection and structure determination.

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