pyridines with Alcohols through Radical Reaction - ACS Publications

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Metal-Free Site-Specific Hydroxyalkylation of Imidazo[1,2‑a]pyridines with Alcohols through Radical Reaction Shengzhou Jin, Bo Xie, Sen Lin,* Cong Min, Ruihong Deng, and Zhaohua Yan* College of Chemistry, Nanchang University, Nanchang 330031, P.R. China

Org. Lett. Downloaded from pubs.acs.org by ALBRIGHT COLG on 04/17/19. For personal use only.

S Supporting Information *

ABSTRACT: An effective approach to realize the direct hydroxyalkylation of imidazo[1,2-a]pyridines with alcohols promoted by di-tert-butyl peroxide was described without any metal catalyst. It is the first time that the dehydrogenative C(sp3)−C(sp2) coupling of imidazo[1,2-a]pyridines with alcohols occurred regioselectively at the C-5 position of imidazo[1,2-a]pyridines. Multisubstituted imidazopyridine derivatives were smoothly synthesized in moderate to good yields. Through a series of control experiments, a free-radical pathway was proposed to explain the experiment.

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Scheme 1. Reactions of Nitroheterocyclic and Alcohols

uring the past few decades, much effort has been devoted to C−C bond formation by C−H bond activation.1 Previously, this kind of transformation was generally dependent on noble transition-metal catalysis for substrates with functional groups.2 In recent years, as a “new generation” of chemical reaction, free-radical reactions have attracted wide attention.3 In particular, the construction of C−C bonds through free radical triggered C−H bond activation are highly favored by chemical workers.4 In this respect, the C−H bond activation of alcohols and ethers via radical reaction is a typical example.5−7 Initially, hydroxyalkylation of heterocyclic compounds with alcohols and ethers in the presence of oxidants was reported by Minisci.8 Subsequently, similar studies on hydroxyalkylation reactions of heterocycles under metal catalysis were done by Minisci, MacMillan,6 and Li,7 respectively (Scheme 1a,b). As we know, transition-metal catalysis will undoubtedly bring about some limitations, such as toxicity and metal contamination.9 Therefore, the strategy of direct activation of C−H bonds to construct C−C bonds via radical reaction under nonmetallic catalysis or greener conditions is still a challenging and fascinating project to chemists. Imidazo[1,2-a]pyridines have been recognized as a privileged scaffold and attracted great attention from chemists10 because their derivatives are highly bioactive and act such as antiviral, antitumor, antiparasitic, antimicrobial, and fungicidal agents, etc.11 They are also widely used as various drugs, such as zolpidem,12a,b olprinone,12c GSK812397,12d and miroprofen12e (Figure 1). Numerous methods have been developed for © XXXX American Chemical Society

the synthesis of imidazo[1,2-a]pyridines13 and especially for the construction of C−X (X = C, N, O, F, and S) bonds at the C-3 position of the imidazopyridine ring.2e,10d,14,15 It is worth Received: April 5, 2019

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DOI: 10.1021/acs.orglett.9b01212 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

3), and surprisingly, 3aa could be obtained in 48% yield when 3 equiv of H2O2 was used (Table 1, entry 3). In addition, the effect of a variety of oxidants (e.g., DTBP, DCP, CHP, and TBHP) was examined (Table 1, entries 4−7), and we gladly found that DTBP gave the best result with 70% yield of 3aa formed (Table 1, entry 4). Further experiments showed that the yield of 3aa did not increase when the amount of HCl was increased or decreased (Table 1, entries 8 and 9). Moreover, a series of acids were also examined (Table 1, entries 10 and 11), but they were not as effective as hydrochloric acid. On this basis, in order to study the role of oxidants and acids in the reaction, we found that the reaction could not work without oxidants (Table 1, entries 12 and 13). We then switched our attention toward the role of reaction time; gratifyingly, when the time was extended to 48 h, the yield of 3aa increased significantly to 83% (Table 1, entry 14). Meanwhile, we hoped to increase the reaction rate by raising or reducing the temperature. Unfortunately, we did not obtain positive results (Table 1, entries 16 and 17). Furthermore, this reaction can also be performed in an air atmosphere, but the yield of the product 3aa is not as good as that in an argon atmosphere (Table 1, entry 15). It is possible that the side reactions are considerably inhibited in the atmosphere of argon. According to the above experimental results, the optimal reaction conditions were obtained as follows: 3 equiv of DTBP, 1 equiv of HCl, at 120 °C under Ar for 48 h. The structure of hydroxyalkylated product 3aa was further confirmed by X-ray diffraction (Figure 2).

Figure 1. Some pharmaceuticals based on imidazo[1,2-a]pyridine.

mentioning that the alkylation of imidazopyridines at the C-3 position with alcohols was reported by Hajra15 in 2017 (Scheme 1c). To expand the application range of imidazo[1,2a]pyridines, we wonder whether it is possible to achieve reactions at other locations besides C-3? Herein, we report an effective, non-metal-catalyzed, and direct hydroxyalkylation of imidazo[1,2-a]pyridines with alcohols at the C-5 position (Scheme 1d). This novel method provides an easy access to a new type of compounds, 5-hydroxyalkylated imidazo[1,2a]pyridines, which might have potential application in medicine, material, and other fields. To begin, we investigated the optimal reaction conditions by using 2-phenylimidazo[1,2-a]pyridine (1a, 0.3 mmol) as a substrate to react with 2-propanol (2a, 6 mL). The results are shown in Table 1. To our delight, the product 2-(2phenylimidazo[1,2-a]pyridin-5-yl)propan-2-ol (3aa) could be obtained in 36% yield in the presence of 1 equiv of H2O2 and 1 equiv of HCl at 120 °C under Ar for 24 h (Table 1, entry 1). Next, we screened the amount of H2O2 (Table 1, entries 2 and Table 1. Optimization of Reaction Conditionsa

entry

oxidant (equiv)

additive (equiv)

temp (°C)

yieldb (%)

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

H2O2 (1.0) H2O2 (2.0) H2O2 (3.0) DTBP (3.0) DCP (3.0) CHP (3.0) TBHP (3.0) DTBP (3.0) DTBP (3.0) DTBP (3.0) DTBP (3.0) DTBP (3.0)

HCl (1.0) HCl (1.0) HCl (1.0) HCl (1.0) HCl (1.0) HCl (1.0) HCl (1.0) HCl (0.5) HCl (2.0) H2SO4 (1.0) HOAc (1.0)

120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 80 130

36 45 48 70 32 38 trace 64 56 trace 26 60 trace 83c 61d 46 63

DTBP (3.0) DTBP (3.0) DTBP (3.0) DTBP (3.0)

HCl HCl HCl HCl HCl

(1.0) (1.0) (1.0) (1.0) (1.0)

Figure 2. Crystal structure of 3aa.

With the optimized conditions in hand, the substrate scope of imidazo[1,2-a]pyridines (1a) was then explored to react with 2-propanol (2a), and the results are shown in Scheme 2. Initially, a wide range of 2-aryl-substituted imidazo[1,2a]pyridines were investigated. Imidazopyridines with electron-donating substituents such as methyl, methoxyl, ethoxyl, and 2,4-dimethyl on the benzene ring afforded the corresponding hydroxyalkylated products with good yields (3ab−ae). Halogens were also well-tolerated under the present reaction conditions, and substrates with halogens at different positions of benzene ring gave the desired products in moderate to high yields (3af−ak). Notably, the electronwithdrawing groups on the benzene ring gave better yields than the electron-donating groups. To our delight, naphthyland thiophene-substituted imidazo[1,2-a]pyridines also successfully afforded the desired products with good yields (3al and 3am).

a Reaction conditions: 1a (0.3 mmol), 2a (6 mL), oxidant (3 equiv), additive (1 equiv), under Ar, 120 °C, for 24 h, sealed tube. HCl: 37% aqueous. bIsolated yields. cFor 48 h. dUnder air atmosphere.

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DOI: 10.1021/acs.orglett.9b01212 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 2. C-5 Hydroxyalkylation of Imidazopyridines with 2-Propanola

Scheme 3. C-5 Hydroxylkylation of Imidazopyridines with Ethanola

a

Reaction conditions: 1a (0.3 mmol), 2b (6 mL), HCl (1 equiv), DTBP (3 equiv) 120 °C under Ar, 48 h, sealed tube, HCl: 37% aqueous. Isolated yields.

a

Reaction conditions: 1a (0.3 mmol), 2a (6 mL), HCl (1 equiv), DTBP (3 equiv) 120 °C under Ar, 48 h, sealed tube, HCl: 37% aqueous. Isolated yields.

only 26% and 31% (3bn−bo). Under optimal conditions, we also found that when longer chain alcohols such as n-hexyl alcohol (3bp) or aromatic alcohols such as benzyl alcohol were used, the desired products were not formed, but the good news was that the reaction of 1 with 1,4-dioxane yielded the corresponding product 3bq in 28% yield. In order to understand the reaction mechanism, some control experiments were designed to provide insights into the hydroxyalkylation reaction (Scheme 4). The reaction did not proceed at all in the presence of radical scavenger TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy, 3 equiv), which shows that the reaction proceeds through a radical pathway (Scheme 4a). The addition of BHT (butylated hydroxytoluene, 3 equiv) could also suppress the reaction obviously, together with the formation of the compound 4 detected by GC−MS (Scheme 4b). These results imply that the hydroxyalkyl radical might be a key intermediate in this transformation. In addition, we also found that the reaction could not be carried out without DTBP (Table 1, entry 13). Furthermore, when 3.0 equiv of radical scavenger ethene-1,1-diyldibenzene was added under optimized conditions, the reaction was inhibited significantly, and to our surprise, the tert-butoxy radical was captured (compound 5, detected by GC−MS) by ethene-1,1-diyldibenzene (Scheme 4c). Thus, these control experiments can prove that the hydroxyalkyl radical may be formed through the reaction of 2-propanol with DTBP (or tert-butoxy radical from DTBP). Simultaneously, the kinetic isotope effect (KIE), KH/ K D radio 6.96 (Scheme 4d), was described by the

Subsequently, the reaction of the substrates with substituents at different positions on the pyridine ring were investigated under optimal reaction conditions. Interestingly, we found that both the electron-withdrawing and electrondonating groups at the C-7 position of imidazopyridine can provide medium yields of products (3an−ao). However, disappointingly, substrates with groups at the C-6 position on imidazopyridine ring are not suitable for this transformation (3ap−aq). Finally, a gram-scale reaction of 1a (1 mmol) with 2a (15 mL) was explored, and the product 3aa could be produced in 52% yield. Inspired by the above results, we continued to explore the scope of the regioselective hydroxyalkylation of imidazo[1,2a]pyridine substrates with ethanol and a few primary alcohols. The results are shown in Scheme 3. It is worth noting that although we used a primary alcohol (EtOH), we found that the yield (3ba−bi) of the desired products did not decrease significantly. On the contrary, satisfyingly, by comparison with 2-propanol, the reaction of thiophene-substituted imidazo[1,2a]pyridine with ethanol showed higher efficiency, giving 84% yield of product (3bj). Next, a series of aliphatic primary alcohols such as methanol, n-propanol, n-butanol, and nhexanol were used to investigate the reactivity and regioselectivity. The reaction of 2-arylimidazo[1,2-a]pyridines with n-propanol provided products in good yields (3bk−bm), but unfortunately, when we used methanol and n-butanol to react with imidazopyridines the product yields were very poor, C

DOI: 10.1021/acs.orglett.9b01212 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 4. Mechanistic Studies

Scheme 6. Applied Experiment of (5(Hydroxyethyl)imidazo[1,2-a]pyridine

with alcohols without any metal catalysis or light initiation. Multisubstituted imidazopyridine derivatives were thus smoothly synthesized in moderate to good yields. This system has good compatibility with 2-phenylimidazo[1,2-a]pyridine, regardless of whether the substituents on the phenyl ring are electron-withdrawing or electron-donating groups. Previously, it has been reported that the coupling reactions of imidazo[1,2a]pyridines with partners mostly occurred at the C-3 position of this heterocyclic ring.2e,10d,14,15 The distinct characteristic of our work is that, for the first time, the hydroxyalkylation occurred at the C-5 position of imidazo[1,2-a]pyridines. Furthermore, a method of constructing the C(sp2)−C(sp3) bond by activating the C(sp3)−H bond without any metal catalysis was successfully developed.

intermolecular competition experiments of CH3OH and CD3OD. The KIE experiment indicated that the cleavage of C(sp3)−H might be the decisive step of the reaction system. On the basis of the control experiments and previous studies, a plausible mechanism for the radical hydroxyalkylation has been outlined in Scheme 5. At first, the reaction is

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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.9b01212. Detailed experimental procedures, characterization data, and copies of 1H and 13C NMR spectra for the products (PDF)

Scheme 5. Possible Reaction Mechanism

Accession Codes

CCDC 1876584 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].

initiated by homolysis of DTBP under heating conditions, affording tert-butoxy radical A. Then A abstracts the α proton of 2-propanol to give the α-hydroxyisopropyl radical B. Next, the nucleophilic carbon radical B reacts at the most electrophilic C-5 positon on the protonated imidazo[1,2a]pyridine ring to form C−C-bonded intermediate C. Subsequently, the reaction between C and tert-butoxy produces a promoted C−C-bonded imidazo[1,2-a]pyridine derivative D. Finally, D was neutralized in a saturated sodium bicarbonate solution to obtain the product 3aa. To illustrate the application of the product (5(hydroxyethyl)imidazo[1,2-a]pyridines), the conversion of hydroxyl to halogen was successfully achieved after referring to relevant literature16 (Scheme 6). This also proves that the hydroxyalkylation reaction is of great significance in organic synthesis. In conclusion, we have developed an effective DTBPpromoted radical-coupling reaction of imidazo[1,2-a]pyridines

ORCID

Sen Lin: 0000-0002-7996-7526 Zhaohua Yan: 0000-0002-9422-3963 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (No. 21362022) for financial support. REFERENCES

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