Visible-Light-Promoted Thiyl Radical Generation from Sodium Sulfinates

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

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Visible-Light-Promoted Thiyl Radical Generation from Sodium Sulfinates: A Radical−Radical Coupling to Thioesters Ganganna Bogonda, Dilip V. Patil, Hun Young Kim,* and Kyungsoo Oh* Center for Metareceptome Research, College of Pharmacy, Chung-Ang University, 84 Heukseok-ro, Dongjak, Seoul 06974, Republic of Korea

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S Supporting Information *

ABSTRACT: A convenient visible-light photoredox catalysis has been developed for the synthesis of thioesters from two readily available starting materials: acid chlorides and sodium sulfinates. The facile generation of acyl radical species under the visible light photoredox conditions allows the formation of thiyl radical species from sodium sulfinates via multiple single electron transfer reactions, where the final acyl radicalthiyl radical coupling has been accomplished. The direct radical−radical coupling strategy offers a mild and controlled photochemical approach to important synthetic building blocks such as thioesters.

O

Acyl radicals are readily generated under various reaction conditions including the photoredox catalysis conditions.14 However, the synthetic utilization of acyl radicals in the radical−radical cross-coupling reactions has been limited to few radical species such as molecular oxygen15 and peroxides.16 In order to discover new radical−radical cross-coupling reactions of thiyl radicals from the parasitic photoredox reaction pathway of sodium sulfinates, the use of acyl radical was envisioned (Scheme 1c). Once developed, the radical− radical cross-coupling strategy allows a direct synthetic access to thioesters,17 a useful building block in the Fukuyama coupling18 and Liebeskind−Srogl coupling reactions.19 To identify the suitable photoredox catalyst conditions to generate the thiyl radicals from sodium sulfinates, a variety of photocatalysts were examined with benzoyl chloride 1a as an acyl radical source (Table 1). Our initial attempts to generate the thiyl radical from sodium p-toluenesulfinate 2a conclusively suggested the critical role of the acyl radical source, benzoyl chloride 1a, where the formation of reduced sulfur compounds such as thioester 3a and disulfide 4a only occurred in the presence of the acyl radical species. Thus, all subsequent optimization conditions utilized 3 equiv of benzoyl chloride 1a. First, the use of 1 mol % fac-Ir(ppy)3 photocatalyst was tested in the presence of reductants such as Hantzsch ester (HE) and i-Pr2NEt (entries 1−2). Due to the slightly better reduction capability of HE (58% yield of 3a and 4a) compared to iPr2NEt (44% yield of 3a and 4a), a series of other photocatalysts were examined in the presence of HE (entries 3−6). The photocatalysts Ir(ppy)2(dtbbpy)PF6 and Ru(bpy)3(PF6)2 provided the preferential formation of disulfide 4a in 26% and 67% yield, respectively (entries 3−4). However, organophotocatalysts such as Eosin Y and Rose Bengal failed

rganosulfur compounds continuously provide new challenges as well as opportunities for chemists thanks to their inherent ability to adopt different sulfur oxidation states.1 Subsequently, there has been an increasing demand of organosulfur compounds in the development of drugs and materials, and the recent advances in the C−S bond formations by transition metal catalysis and Brønsted acid catalysis amply demonstrate the full potential of organosulfur compounds.2 While the oxidation states of sulfur atoms can be readily varied with the help of chemical oxidation and reduction processes, it is highly desirable to identify the reaction conditions that on demand provide the organosulfur compounds with different sulfur oxidation states without separate chemical redox processes. In this context, sodium sulfinates have recently emerged as a dual sulfur source for sulfonylation as well as sulfenylation (Scheme 1a).3 While the transition-metalcatalyzed C−H activation strategy has been predominant in the sulfonylation using sodium sulfinates,4 the sulfonylations of arenes,5 alkenes,6 and alkynes7 have been recently achieved under the photoredox catalysis conditions. In contrast, the electrophilic sulfenylations of alkynes,8 indoles,9 imidazopyridines,10 and isocyanides11 have been achieved with the iodine/phosphine-promoted reactions. While the generation of sulfonyl radicals from sodium sulfinates under the photoredox catalysis conditions can be ensured, the formation of thiyl radicals has been suggested as parasitic reaction pathways (Scheme 1b).5d,e Although no photochemical synthetic utilization of such parasitic thiyl radical species from sodium sulfinates has been investigated, given that the C−H sulfenylations of indoles12 and imidazopyridines13 have been accomplished under the photoredox conditions by using sulfonyl chlorides and sulfinic acids, respectively, we envisioned the photoredox catalysis conditions that could generate the thiyl radicals from sodium sulfinates. © XXXX American Chemical Society

Received: April 7, 2019

A

DOI: 10.1021/acs.orglett.9b01218 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 1. Sodium Sulfinates as Versatile Sulfonylating and Sulfenylating Agents

Table 1. Optimization of Thioester Synthesis from Sodium Sulfinatea

to provide the reduced sulfur compounds 3a and 4a (entries 5−6). The screening of solvents revealed that the chlorinated solvents were slightly better for the formation of thioester 3a (entries 10−11). To prevent the formation of disulfide 4a, the use of i-Pr2NEt was reconsidered. While i-Pr2NEt played the role of a reductant,20 it could also capture the inadvertent formation of acid byproducts. A brief survey of bases revealed that tertiary amines significantly reduced the formation of disulfide 4a (entries 12−15). Thus, the use of 3 equiv of iPr2NEt and Et3N led to the exclusive formation of 3a in 76% and 63% yield, respectively (entries 14−15). Next, further finetuning of the reaction conditions employed a higher facIr(ppy)3 photocatalyst loading to 2 mol % (entries 16−17). Thus, the optimal photocatalyst loading was identified as 1.5 mol %, where the product 3a was obtained in 81% yield (entry 16). The control experiments using less optimal use of HE and 1a (entries 18−20), without photocatalyst (entry 21), and in the absence of light (entry 22) all confirmed the visible-lightpromoted photoredox catalyst system for the formation of thioester 3a. Under the optimized photoredox conditions, the substrate scope of acid chlorides was examined (Scheme 2). The employment of benzoyl chlorides with an electron-donating group provided slightly lower yields than benzoyl chlorides with an electron-withdrawing group (3b−3c vs 3d−3e). The aryl bromide moiety was not tolerated, where the debromination product 3a was obtained in 31% yield alongside with the desired product 3f in 25% yield. Benzoyl chlorides with different steric and electronic characters suggested that the

a Reaction using 1a (0.6 mmol), 2a (0.2 mmol), and photocatalyst (1.0 mol %) in 2 mL of solvent (0.1 M) at ambient temperature under an argon atmosphere and blue LED lights for 6 h. bIsolated yields after column chromatography. cUse of photocatalyst (1.5 mol %). d Use of photocatalyst (2.0 mol %). eUse of 1a (2 equiv) and i-Pr2NEt (2 equiv). fWithout light.

ortho-aryl substitution impacts the stability of the corresponding acyl radical species (3k−3l). The optimized photoredox catalyst conditions tolerated other naphthoyl and heteroaroyl chlorides to give good yields of the products 3m−3p. The use of aliphatic acid chlorides was somewhat successful, leading to the corresponding products 3q−3v despite known facile decarbonylation pathways of aliphatic acyl radicals.21 The current photoredox catalyst system did not tolerate the presence of an alkene moiety, where cinnamoyl chloride did not provide the corresponding thioester. The substrate scope of sodium sulfinates is presented in Scheme 3. As for the acid chlorides, a variety of sodium sulfinates provided good yields of the corresponding thioesters 3w−3ag in 50−85% yields. In addition, different acid chlorides and sodium sulfinates were coupled under the optimized photoredox catalysis conditions to give the products 3ah−3al in good yields. However, the use of sodium methanesulfinate B

DOI: 10.1021/acs.orglett.9b01218 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 2. Substrate Scope of Acid Chlorides in the Thioester Synthesis by Photoredox Catalysis

Scheme 4. Control Experiments

although 6a was known to be stable.22 This result implied that the acyl radical species were amply present during the photoredox catalysis conditions. The reaction without reductants such as Hantzsch ester and i-Pr2NEt only led to the formation of 3a in 13% yield and p-tolyl disulfoxide 7a in 8% yield, illustrating the important role of reductants in the generation of thiyl radical species. The reactions without the acyl radical source did not provide the thiyl radical species at all. The possible intermediacy of p-tolyl disulfoxide 7a during the photoredox conditions was confirmed in our control experiment. Also, the potential role of the disulfide 4a as a source of thiyl radical was demonstrated in our control experiment, where the thioester 3a was obtained in 99% yield. Based on the control experiments, a plausible reaction mechanism is proposed in Scheme 5. The irradiation of facIr(ppy)323 with blue visible light generates the photoexcited

a

Reaction at 1 mmol scale.

Scheme 3. Further Substrate Scope of Sodium Sulfinates and Acid Chlorides

Scheme 5. Proposed Reaction Mechanism and Photoredox Catalysis

and sodium trifluoromethyl sulfinate was not successful under our optimized photoredox conditions. The mechanism of thioester synthesis under the photoredox catalysis system was investigated in order to elucidate the intermediate species (Scheme 4). Thus, in the presence of a radical inhibitor, TEMPO, it was possible to capture the radical species, acyl radical, but not thiyl radical. The acyl-captured TEMPO 5a was isolated in 78% yield; however, the thiylcaptured TEMPO 6a was not observed in the LC-MS analysis C

DOI: 10.1021/acs.orglett.9b01218 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters state of *Ir(ppy)3 that could be reduced by Hantzsch ester (Ered = +0.887 V vs SCE),24 i-Pr2NEt, and sodium sulfinate (Ered = +0.45 V vs SCE in CH3CN)5b to a more reducing Ir(II) species (E1/2III/II = −2.19 V vs SCE in CH3CN)25 and a sulfinate radical A (see the Supporting Information for the Time-Resolved Fluorescence Lifetime studies). The Ir(II) species readily gives one electron to acid chloride to give an acyl radical species. Since a large excess of benzoyl chloride (3 equiv; Ered = −1.42 V vs SCE)26 was used in the reaction, the acyl radical species should be present in sufficient concentration during the photoredox conditions. It is reasonable to speculate the coupling of acyl radical and sulfinate radical to give benzenesulfinic benzoic anhydride B. With the help of the IrIII−IrlI photoredox catalysis the anhydride B generates a sulfinyl radical C that can either couple to give the p-tolyl disulfoxide 7a or undergo another acyl radical coupling to give OS-phenyl benzo(thioperoxoate) D. The fate of the species D should be also controlled by the IrIII−IrlI photoredox catalysis, providing the thiyl radical E. The formation of disulfide 4a during the reaction suggests another productive radical−radical coupling to thioester 3a. To support our proposed reaction mechanism, the LC-MS analysis of the crude reaction mixture by using TEMPO confirmed the presence of the acyl radical (see the Supporting Information for detail). Also, the formations of benzenesulfinic benzoic anhydride B and OSphenyl benzo(thioperoxoate) D were asserted, and the utilization of mixed anhydride instead of benzoyl chloride 1a was successfully demonstrated (see the Supporting Information for detail). The disulfide 4a could serve as a thiyl radical source under the reaction conditions since the use of an authentic sample of 4a in the control reaction did provide the thiolated product 3a. In summary, we have developed a novel radical−radical coupling strategy to an important synthetic building block, thioesters. The formation of thiyl radical species has been successfully achieved from sodium sulfinates with the help of acyl radical species. Given that the sodium sulfinates can serve as a sulfonylating agent under the photoredox catalysis conditions, the current photoredox method fulfils the missing link in the sulfenylation by sodium sulfinates. Such on demand reactivity should find wide synthetic utility in organic synthesis, and our current research efforts are directed to further broadening the photoredox radical−radical couplings.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) grants funded by the Korean government (MSICT) (NRF-2015R1A5A1008958 and NRF2018R1D1A1B07049189).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01218.

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REFERENCES

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Experimental procedures and characterization data for all new compounds (PDF)

AUTHOR INFORMATION

Corresponding Authors

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

Dilip V. Patil: 0000-0003-2103-3840 Hun Young Kim: 0000-0002-8461-8910 Kyungsoo Oh: 0000-0002-4566-6573 D

DOI: 10.1021/acs.orglett.9b01218 Org. Lett. XXXX, XXX, XXX−XXX

Letter

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