Transition-Metal-Free Regioselective Cross-Coupling: Controlled

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

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Transition-Metal-Free Regioselective Cross-Coupling: Controlled Synthesis of Mono- or Dithiolation Indolizines Bin Li,† Zhiyu Chen,† Hua Cao,*,† and Hong Zhao*,† †

School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical University, Zhongshan 528458, P. R. China S Supporting Information *

ABSTRACT: An efficient transition-metal-free regioselective C−H/S−H cross-coupling of indolizines with thiols has been developed for the first time to describe a workable route to indolizine thioethers. This finding provides a new method and more straightforward pathway for controllable synthesis of mono- or dithiolation indolizines that are otherwise difficult to obtain by the literature methods. The reaction exhibits good functional group tolerance and high efficiency and affords the products in good to excellent yields.

I

Additionally, the transformations for the construction of dithioethers are very rare. Only a few routes that employed metal as the catalysts in thiolation of heterocycles have been reported to synthesize dithioethers (Scheme 1a,b,c).16 The methods for the synthesis of dithioethers or monothioethers via transition-metal-free have not been well developed. Therefore, the search for highly efficient approaches, especially for those green processes, for the synthesis of thioethers is still necessary.17

ndolizines are privileged structures that are commonly employed in various small-molecule drug discovery programs.1 Functionalized indolizines have found wide applications in natural products and synthetic pharmaceuticals, which are associated with a broad spectrum of biological activities, such as antifungal, anticancer, antioxidant, phosphatase inhibition, and antagonists.2 In particular, 3-sulfenylindolizines are ligands of the CRTH2 receptor and are valuable in the treatment of various respiratory diseases.3 Moreover, the indolizine derivatives are core frameworks of many fluorescent materials1a,4 Therefore, the development of new approaches for the synthesis of those compounds has received considerable attention due to their potential properties and applications. Generally, pyridine derivatives are used as substrates in the main synthetic methods for the construction of indolizines.3a,4c,5 Many different approaches also have been developed to synthesize these key compounds. Thioether-decorated heterocycles are important structural units that have attracted the interest of chemists as a key building block in synthetic and medicinal chemistry, mainly because they constitute a significant component found in biological molecules6 and drugs.7 Transition-metal-catalyzed direct C−H bond activation is one of the most powerful and useful tools to construct these compounds in the past few years. In particular, transition-metal-catalyzed thiolation of heterocycles is in fact a ubiquitous process for controllable synthesis of biological molecules and is found in drugs that show therapeutic efficacy against diabetes, AIDS, arthritis, and depression.8 Several successful strategies have appeared for the construction of valuable and functionalized monothioethers via direct C−H activation using various sulfenylating reagents, such as disulfides,9 sulfenyl halides,10 N-thioimides,11 thiols,12 S-acetals,13 sulfinic acids,14 and α-acylthione.15 Compared to transition-metal-catalyzed thiolations, few metal-free thiolations for the synthesis of monothioethers have been developed. © XXXX American Chemical Society

Scheme 1. Different Approaches toward C−S Bond Formation

Received: April 12, 2018

A

DOI: 10.1021/acs.orglett.8b01168 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Very recently, our group has developed direct C−H bond activation strategies for the synthesis of heterocycle compounds.18 Herein, we report transition-metal-free regioselective transformation for the construction of mono- or dithioetherdecorated indolizines via direct cross-coupling of indolizines with thiols (Scheme 1d). The advantages of our method are simplicity, high efficiency, and environmental friendliness. We started our investigation by taking 2-phenylindolizine (1a) and propane-1-thiol (2a) as a model substrate to determine the optimized reaction conditions, as summarized in Table 1. Gratifyingly, dithiolation product 2-phenyl-1,3-

Scheme 2. Oxidative Coupling of 2-Phenylindolizine with Thioalcohola

Table 1. Optimization of the Reaction Conditionsa

yieldb (%) entry

cat.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

CuI CuCl CuBr Cu2O Cu(OAc)2 CuCl2

oxidant O2 O2 O2 O2 O2 O2 O2 TBHP DDQ H2O2 TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP H2O2

additive

KI KI KI KI KBr NH4I KI KI KI KI KI

solvent

3a

DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO toluene DMF DCE EtOH dioxane DMSO DMSO

58 19 15 23 trace trace 66 NP 93 32 85 trace 82 NP 54 51 63 trace

5a

a

Isolated yields.

find that aliphatic thiols gave the corresponding dithiolated products 3a−i in 59−93% yields. When aromatic thiols were employed, the corresponding dithiolated products 3j and 3k were also obtained in 72 and 53% yields, respectively. Notably, 2-aminobenzenethiol gave the dithiolated product 3l in 32% yield. The results indicate that both electron-rich groups and electron-poor groups on the benzene ring of thiols could react smoothly and afford the dithiolated products in moderate to excellent yields. Subsequently, various indolizine derivatives were also tested under the standard reaction conditions, and the results are summarized in Scheme 3. Our experiments 67 81c

Scheme 3. Cross-Coupling of Indolizine with Propane-1thiola

a

Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), KI (0.05 equiv), solvent (2 mL), 60 °C, air, 4 h. bIsolated yield. c1.2 equiv or 2.0 equiv of mercaptans, 8 h.

bis(propylthio)indolizine (3a) was obtained in 58% yield in the presence of 5 mol % of CuI in DMSO using O2 as the oxidant at 60 °C for 4 h (Table 1, entry 1). Different catalysts were then examined, such as CuCl, CuBr, Cu2O, Cu(OAc)2, and CuCl2 (Table 1, entries 2−6), but no further improvement of the yield was obtained. Inspiringly, the product yield increased to 66% when KI was added without a transition-metal catalyst (Table 1, entry 7). Then the reaction of 1a and 2a in DMSO was performed in the absence of KI, and the results indicate that DMSO could not promote the reaction (Table 1, entry 8). To our delight, the product 3a was obtained in 93% yield when the reaction was carried out with TBHP as the oxidant (Table 1, entry 9). No further improvement of the yield was obtained when other oxidants such as DDQ or H2O2 were used (Table 1, entries 10 and 11). With an optimized catalytic system in hand, we next explored the scope and generality of this crosscoupling reaction by examining a variety of thiols in the presence of 5 mol % of KI and TBHP in a test tube at 60 °C for 4 h. The results are outlined in Scheme 2. We were pleased to

a

B

Isolated yields. DOI: 10.1021/acs.orglett.8b01168 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

To understand the reaction pathway, control experiments were carried out as presented in Scheme 5. The reaction of 1a

proceeded smoothly under the optimized conditions in most cases and provided the dithiolated indolizine in moderate to good yields. A variety of substituents, such as 5-Me (4a), were well-tolerated and afforded the desired products in good to excellent yields (84−91%). Interestingly, the results indicate that several important functional groups such as −CN (4k), −NO2 (4l), and halogens −F, −Cl, and −Br (4d, 4h,i, 4e,f) substituted on the benzene ring were also performed under these conditions and led to a beneficial effect on the reaction outcome, which made further functionalization possible to prepare complex compounds. Unfortunately, when 2-(4iodophenyl)indolizine was used as the substrate, only 13% of the desired product 4j was afforded. Furthermore, different groups, such as −Me, −CH2Ph, −OCH3, and −Ph (4g, 4m− p), on the benzene ring worked smoothly and led to the desired products in moderated yields. Additionally, the cross-coupling reaction of 2-(thiophene-2-yl)indolizine with 2a was also performed to give the corresponding product 4s in 41% yield. Interestingly, monothiolation was successfully achieved by removing the catalyst and replacing the oxidant with H2O2 (Scheme 4). The monothiolated product 5a was obtained in

Scheme 5. Control Experiments

with 2a in the presence of radical scavenger TEMPO worked smoothly, generating 3a in 91% yield, suggesting the absence of a radical intermediacy in the reaction (Scheme 5a). However, only a trace amount of product 5a was observed when the reaction was performed in the presence of TEMPO (Scheme 5b). The results show that the reaction might involve a radical process. Further, the reaction of 1,2-bis(4-fluorophenyl)disulfane with 1a was carried out, which proceeded well under the optimized conditions, giving the dithiolation product 3j in 76% yield, suggesting that the transformation may proceed through a disulfide intermediate (Scheme 5c). The reaction of 1a with 2a was carried out in the presence of iodine as a catalyst and DMSO as a solvent (Scheme 5d), generating 3a in 82% yield, suggesting that KI may be converted to I2 during the reaction. On the basis of the above results and several previous references,19 a tentative mechanism is proposed in Scheme 6. First, DMS/I2 or I2 was easily formed with the help of TBHP and DMSO. The disulfide intermediate (I) was then generated in the presence of thiol (2), DMS/I2, or I2, which further reacts

Scheme 4. Scope of the Monothiolation Reaction in Terms of Thioalcohola

Scheme 6. Plausible Mechanism for the Observed Transformation

a

Isolated yields.

81% isolated yield in the presence of 2-phenylindolizine (1.0 equiv) and propanethiol (1.2 equiv) in DMSO (2 mL) under air at 60 °C for 8 h. Various thiols were next examined. We were pleased to find that both aliphatic and aromatic thiols gave the corresponding monothiolated products in moderate to good yields (43−84%, 5a−n). Subsequently, different substituents on the benzene ring of 2-phenylindolizine were examined. It was found that 2-phenylindolizine bearing either an electron-donating or an electron-withdrawing group gave the corresponding monothiolated products in good yields (51− 90%, 5o−u). C

DOI: 10.1021/acs.orglett.8b01168 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

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with DMS/I2 or I2 to give the intermediate (II). Subsequently, nucleophilic displacement of an iodo group by 2-phenylindolizine (1a) occurs to form the intermediate (III), which continues to react with intermediate (II) leads to the dithiolation product 3 (Scheme 6, i). In addition, the radical mechanism is recommended for the synthesis of monothiolation product. Initially, H2O2 reacts with 2 to give RS• radical (A), and then radical (A) addition to 1a forms the intermediate (B). Finally, the C−H bond cleavage forms the monothiolation product 5 (Scheme 6, ii). In summary, we have developed an efficient transition-metalfree regioselective strategy for the controllable construction of mono- or dithiolation indolizines. The transformation could be accomplished under simple and mild conditions with high regioselectivities, which provided chemists an alternative method for designing dithiolation or monothiolation proudcts. Owing to its high selectivity, broad substrate scope, and high efficiency, this cross-coupling reaction should be of high synthetic value.

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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.8b01168. Experimental procedures and spectral data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Hua Cao: 0000-0001-8825-0175 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the National Natural Science Foundation of China (21302023, 21272044), the Innovation and Strong School Project of Guangdong Pharmaceutical University (2015cxqx212), the Science and Technology Planning Project of Guangdong Province (2016A010103039, 201806040009, 201804010349), and Provincial Experimental Teaching Demonstration Center of Chemistry & Chemical Engineering.

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DOI: 10.1021/acs.orglett.8b01168 Org. Lett. XXXX, XXX, XXX−XXX