Exogenous Photosensitizer-, Metal-, and Base-Free Visible-Light

Letter Cite This: Org. Lett. 2019, 21, 237−241

pubs.acs.org/OrgLett

Exogenous Photosensitizer‑, Metal‑, and Base-Free Visible-LightPromoted C−H Thiolation via Reverse Hydrogen Atom Transfer Ze-Ming Xu,† Hong-Xi Li,*,†,‡ David James Young,§ Da-Liang Zhu,† Hai-Yan Li,† and Jian-Ping Lang*,†,‡ †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, Jiangsu, China State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China § College of Engineering, Information Technology and Environment, Charles Darwin University, Northern Territory 0909, Australia ‡

Org. Lett. 2019.21:237-241. Downloaded from pubs.acs.org by TULANE UNIV on 01/09/19. For personal use only.

S Supporting Information *

ABSTRACT: Visible-light-driven, intramolecular C(sp2)−H thiolation has been achieved without addition of a photosensitizer, metal catalyst, or base. This reaction induces the cyclization of thiobenzanilides to benzothiazoles. The substrate absorbs visible light, and its excited state undergoes a reverse hydrogen-atom transfer (RHAT) with 2,2,6,6-tetramethylpiperidine N-oxyl to form a sulfur radical. The addition of the sulfur radical to the benzene ring gives an aryl radical, which then rearomatizes to benzothiazole via RHAT.

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he utilization of direct C(sp2)−H activation to form intramolecular C−X (X = N, O, S) bonds1−3 is an attractive strategy for the synthesis of a variety of benzoheterocycles.4 Particularly promising is intramolecular C(sp2)−H/S− H cross-coupling for the construction of benzothiazole moieties, which are found in bioactive natural products, pharmaceuticals, and organic optoelectronic materials.5 These intramolecular cyclizations have been achieved using transition metal catalysts with oxidants at high temperatures (Scheme 1).6 Peñeń ̃ory et al. reported the cyclization of thioformanilides with chloranil as a photosensitizer under a medium pressure Hg lamp (λmax = 365 nm) at 80 °C.7 Visible-light-driven aromatic C−H thiolation is of great interest because sunlight is a clean and abundant energy source. Li et al. employed Ru(bpy)3(PF6)2 (bpy = 2,2′-bipyridine) as a photocatalyst for the aerobic, visible-light-driven photoredox synthesis of 2substituted benzothiazoles using molecular oxygen as the terminal oxidant.8a Lei et al. developed a novel ruthenium photoredox and cobalt dual catalytic system for intramolecular C−S coupling to yield 2-substituted benzothiazoles with the release of H2.8b Despite considerable advances in the methodology of oxidative cross-couplings, most procedures suffer from one or more limitations. The use of O2 or inorganic oxidants can lead to the formation of amide byproducts by desulfurization of the thioamide substrate.6−8 The use of transition metal catalysts or noble photocatalyst necessitates the removal of residual metal impurities and thus entails additional processing and cost. The presence of functional groups such as halogen atoms on substrates can result in dehalogenated contaminants under metal-catalyzed, thermal, or light-irradiation conditions.6a,8 For example, the visiblelight-driven photoredox cyclization of N-(2-iodophenyl)benzothioamide only afforded dehalogenated product 2© 2018 American Chemical Society

Scheme 1. C−H Thiolation of Substituted Thioamides

phenylbenzothiazole using Ru(bpy)3(PF6)2 as the photoredox catalyst.8a Hence, developing metal-free methodology operating under mild conditions and with high functional group compatibility would be a valuable extension to the synthetic chemist’s repertoire. Received: November 18, 2018 Published: December 21, 2018 237

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Scheme 2. Cyclization of N-(Aryl)benzothioamides to 2Phenylbenzothiazolesa

Hydrogen-atom transfer (HAT) and proton-coupled electron transfer (PCET) are basic mechanisms used in a wide variety of chemical and biological transformations.9 Visiblelight-mediated HAT offers the opportunity for direct C−H/ X−H bond activation, avoiding the use of transition metal catalysts and high temperatures.10 The activated photocatalyst (PC*) generally behaves as a HAT catalyst to abstract a hydrogen atom from a substrate RX−H (X = C, N, O, S) to form a radical RX•, which then reacts to form the desired product.11 Herein, we report a photoinduced C(sp2)−H/S−H activation with intramolecular C−S bond formation through a reverse hydrogen-atom transfer (RHAT). Activated thiobenzanilide (*TBA) formed by visible-light irradiation undergoes a RHAT process with 2,2,6,6-tetramethylpiperidine N-oxide (TEMPO) to form a sulfur radical, which then cyclizes to generate benzothiazole. This photochemical cyclization does not require extra photoredox catalyst, transition-metal catalyst, or base (Scheme 1). We initially chose N-phenylbenzothioamide (1aa) as the substrate for optimization studies. The combination of TEMPO and Ru(bpy)3Cl2·6H2O in CHCl3 provided the desired cyclization product 2aa exclusively in >99% yield under irradiation using a 45 W household fluorescent bulb at room temperature (Table S1, entry 1; Figure S1). To our delight, the reaction without Ru(bpy)3Cl2·6H2O also afforded 2aa in an almost quantitative yield (entry 2). A strong solvent dependence of the coupling reaction was observed with chloroform proving to be the most suitable solvent (entry 2). The same reaction gave lower yields in toluene, THF, CH2Cl2, i-PrOH, EtOH, MeOH, DMF, and DMSO (entries 3−11). Excellent yields could also be obtained with a shorter reaction time of 12 h (entry 12). Product 2aa could be obtained in 90% yield after 8 h of irradiation (entry 13). The additive TEMPO proved more effective (entry 8) than 2,3-dicyano-5,6-dichlorobenzoquinone (DDQ), tetrachloro-p-benzoquinone (p-chloranil), or phenanthrenequinone (entries 14−16). However, di-t-butyl peroxide and diphenylmethanone were almost inactive (entries 17 and 18). When the reaction was carried out in air, the yield of 2aa was decreased to 70% yield and was contaminated by byproducts N-phenylbenzamide (15%) and N-phenylbenzimidic dithioperoxyanhydride (5%) (entry 19). The critical roles of light and TEMPO in this C−H thiolation were demonstrated using control experiments (entries 20 and 21). When the reaction was carried out at 80 °C in the dark, none of the desired product was detected (entry 22). It is noteworthy that 1aa reacted smoothly to afford 2aa in 90% after 8 h of sunlight irradiation (entry 23). Having identified the optimal reaction conditions, we investigated the electronic and steric effects of the N-aryl group. Good to excellent yields could be achieved for a variety of N-(aryl)benzothioamides bearing methyl, methoxy, halide (F, Cl, Br, I), trifluoromethyl, and methylthio groups (Scheme 2). Intramolecular C−H thiolation was satisfactorily accomplished for N-(aryl)benzothioamides (2aa−2ai) with electronrich, -neutral, and -withdrawing groups at the 4-position with over 84% isolated yields. Importantly, iodo-substituted derivative 1ai reacted to provide the desired product 2ai in 88% yield. Successful reaction of similar substrates has rarely been described in the literature.8a N-(2-(Methylthio)phenyl)benzothioamide and N-(2-fluorophenyl)benzothioamide reacted to generate 2ak and 2al in satisfactory yields. Orthopositioned substituents such as methyl and bromo (1aj, 1am) did not hamper this oxidative C−H thiolation, giving the

a

1a (0.2 mmol), TEMPO (0.4 mmol), 6 mL of CHCl3, room temperature, 45 W CFL, N2, and isolated yield.

corresponding products in high yields. When the metasubstituted thioanilides (1an−1ap) were used, two regioisomeric products were obtained. 2,6- N-(4-Iodo-2methylphenyl)benzothioamide also reacted to afford the expected product 6-iodo-4-methyl-2-phenylbenzothiazole (2aq) in 87% yield. N-(Naphthalen-1-yl)benzothioamide reacted to produce 2ar in 92% yield under the optimized reaction conditions. The cyclization of 1aa was performed on a gram scale (1.07 g, 5 mmol) over 24 h to generate 2aa in 78% yield. Encouraged by the high efficiency for the reaction of N(aryl)benzothioamides described above, we investigated the effect of the substituents on the 2-aryl group (Scheme 3). Both electron-donating and -withdrawing groups on the phenyl ring Scheme 3. Cyclization of N-(Aryl)benzothioamides to 2Phenylbenzothiazolesa

a Reaction conditions: as for Scheme 2, isolated yields. bReaction conditions: as for Scheme 2, but with 5 mol % of 9,10phenanthrenequinone.

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Their excited-state redox potentials were thus estimated to be in the range −1.00 ∼ −1.19 V vs Ag/AgCl based on both the electrochemical and spectroscopic data (Figure S4 and Table S2). The CV of TEMPO had one reversible oxidation wave at E1/2 = +0.57 V vs Ag/AgCl in MeCN and one reduction wave at −1.57 V vs Ag/AgCl (Figure S3). A single electron transfer process from *1aa to TEMPO was unfavorable. Light on/off experiments revealed that the cyclization progressed smoothly under visible-light irradiation, but no further conversion was observed when the light source was removed (Figure S5), indicating that the reaction did not go through a radical chain propagation. The oxidative cyclization of 1aa over time was monitored using HPLC (Figure S6a). Substrate 2aa got accumulated at the same rate as the consumption of 1aa. Substrate 1a was totally converted in 12 h. A logarithmic plot of the percentage of residual substrate (ln c/c0) with irradiation time (t) (Figure S6b) showed that the kinetics of this reaction obeyed a first-order rate law. The kinetic isotope effect (KIE, kH/kD) was measured using monodeuterated 1aa (1aa-d) (Figure S7) and pentadeuterated 1aa (1aa-d5) (Figure S8). The intramolecular KIE value was 0.61 (see Supporting Information) with a 99% combined yield (Figure S9). The reaction of 1aa (0.1 mmol) and deuterated 1aa-d5 (0.1 mmol) for half an hour provided a mixture of the products 2aa and pentadeuterium-2aa (2aa-d5) in 15% overall yield (Figures S10 and S11) and a ratio of 2aa:2aa-d5 of 1.27 (Scheme 4). These intra- and intermolecular KIE values indicated that the cleavage of the C−H bond was not the ratedetermining step. Substrate 1aa was treated with TEMPO under standard conditions for 2 h, and the positive-ion ESI-MS spectrum of the reaction mixture was obtained (Figure S12). The peaks at m/z = 212.0539 and 158.1557 were assigned to be the [2aa + H]+ and [TEMPOH + H]+ ions, respectively. The peak at m/z = 369.1958 was consistent with the [SO-(2,2,6,6-tetramethylpiperidin-1-yl)-N-phenylbenzimido(thioperoxoate) + H]+ ion. Therefore, we proposed that the cyclization involved a sulfurcentered radical. Moreover, reaction of N-phenyl-4(trifluoromethyl)benzimidic dithioperoxyanhydride (3bc, Figure S13) with TEMPO in CHCl3 under visible-light irradiation also gave rise to the desired product 2bc in 90% yield. In the absence of light, the reaction of 3bc and TEMPO in CHCl3 at room temperature or at 80 °C afforded 2bc in about 6% yield. On the basis of this mechanistic data and on literature precedents,7,8,12 we propose the pathway outlined in Scheme 5. Substrate 1aa accepts a photon to generate excited *1aa, which then undergoes a reversible HAT process with TEMPO to form a thiyl radical A and TEMPOH. The resulting A can be trapped by TEMPO to afford the adduct B, which undergoes

were tolerated. Good to high yields were obtained under our standard conditions. 2,2-Dimethyl-N-arylpropanethioamide underwent cyclization to yield the corresponding 2-alkylbenzothiazole derivative in a relatively low yield, presumably due to low visible-light absorbance (Figure S2). The yield of 2(tert-butyl)-6 methylbenzothiazole increased to 94% in the presence of 5 mol % of phenanthrenequinone. Some substituted 2-alkylbenzothiazoles could be isolated in high yields under similar reaction conditions. The cyclization of substrates bearing a halogen atom, including iodine, is appealing because of the potential for further functionalizations. Under metal-catalyzed reaction conditions, modest yields of the desired product are usually obtained, accompanied by some level of dehalogenation of the substrate.6−8 In our case, a variety of thiobenzanilides possessing halogen atoms underwent smooth cyclizations to yield the desired products (2af−2ai, 2al−2aq, 2bd−2bg, 2bi− 2bk, 2bn, 2bo, 2br−2bt) without contamination by the corresponding dehalogenated species. The cyclization of N-(2fluorophenyl)-3,4-dimethoxybenzothioamide yielded 2-(3,4dimethoxyphenyl)-4-fluorobenzothiazole (2bv), which is a potent antitumor agent.12 The absorption spectra of substrates 1aa, 1ac, 1ae, and 1bd indicated a photoabsorption range from UV to visible wavelengths with the absorption edge at about 470 nm (Figure S2). Cyclization of 1aa was conducted with various light sources, visible light, blue LED, and green LED and afforded 2aa in >99%, 85%, and 28%, respectively (Scheme 4). Scheme 4. Control and Deuterium Experiments

Scheme 5. Proposed Mechanism

The results correlate with the absorption spectrum of 1aa. A temperature of 80 °C could not promote this reaction in the dark, confirming that this cyclization is a visible-light-driven process. The cyclic voltammogram (CV) measurements of 1aa, 1ac, 1ae, 1ai, and 1bd revealed that these substrates possessed a half wave oxidation potential of 1.35, 1.25, 1.49, 1.43, and 1.39 V (vs Ag/AgCl), respectively, in MeCN (Figure S3). 239

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homolytic cleavage to regenerate A and TEMPO, facilitated by the relative weakness of the S−O bond.12 The 1,5-homolytic radical cyclization of A produces aryl radical C. The aromatized product 2aa is then obtained through the photomediated RHAT process between activated *C and TEMPO under visible light. This mechanism is different from that for TEMPO-catalyzed electrochemical intramolecular C− H thiolation. In the latter, thioamide is oxidized by the electrochemically generated TEMPO+ through an inner-sphere electron transfer to afford a thioamidyl radical, which undergoes homolytic aromatic substitution to form the C−S bond.13 To summarize, we have developed methodology for visiblelight-driven, intramolecular C−S bond formation of aromatic substrates that does not require addition of a photosensitizer, metal-based catalyst, or base. Thioamide derivatives in the presence of TEMPO smoothly cyclize to give benzothiazoles through two RHAT events. This cyclization is compatible with a wide range of functional groups and will therefore be suitable for constructing a variety of other aromatic heterocyclic compounds.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03679. Experimental procedures, 1H and 13C NMR spectra, and characterization data for all products (PDF) Accession Codes

CCDC 1874831 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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AUTHOR INFORMATION

Corresponding Authors

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

Jian-Ping Lang: 0000-0003-2942-7385 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21471108, 21531006, 21771131, and 21773163), the Natural Science Foundation of Jiangsu Province (BK20161276), the State Key Laboratory of Organometallic Chemistry of Shanghai Institute of Organic Chemistry (2018kf-05), and the “Priority Academic Program Development” of Jiangsu Higher Education Institutions and Scientific and Technologic Infrastructure of Suzhou (SZS201708). We are grateful to the useful comments of the editor and the reviewers. 240

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