Oxidative C(sp3)–H functionalization of methyl-azaheteroarenes: a facile route to 1,2,4-triazolo[4,3-a]pyridines

Wei-Zhao Wenga, Yin-He Gaoa, Xue Zhanga, Yan-Hua Liua, Ying-Jie Shena, Yan-Ping Zhu*a, Yuan-Yuan Suna, Qing-Guo Menga and An-Xin Wu*b
aSchool of Pharmacy, Key Laboratory of Molecular Pharmacology and Drug Evaluation, Ministry of Education, Collaborative Innovation Center of Advanced Drug Delivery System and Biotech Drugs in Universities of Shandong, Yantai University, Shandong, Yantai, 264005, P. R. China. E-mail: chemzyp@foxmail.com
bKey Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Hubei, Wuhan, 430079, P. R. China. E-mail: chwuax@mail.ccnu.edu.cn

Received 6th January 2019 , Accepted 23rd January 2019

First published on 23rd January 2019


We herein describe an oxidative [4 + 1] annulation used to prepare 1,2,4-triazolo[4,3-a]pyridines in the presence of I2–DMSO. This protocol enables synthesis of triazolo[4,3-a]pyridine–quinoline linked diheterocycles via a direct oxidative functionalization of sp3 C–H bonds of 2-methyl-azaheteroarenes. The reaction shows a wide substrate scope and good functional group tolerance.


In recent years, the direct oxidative functionalization of C–H bonds has drawn considerable attention from the chemical community and industry.1 It has become one of the most significant and powerful synthetic tools for C–C and C–heteroatom bond construction. Despite this significant development, the oxidative functionalization of sp3 C–H bonds still faces many challenges.2 The combination of a metal catalyst with dioxygen or peroxide, such as copper or iron/O2,3 cobalt/O2[thin space (1/6-em)]4 and copper/peroxide,5 is a good catalytic system for direct oxidative functionalization of sp3 C–H bonds. Another efficient approach is to use iodine reagents or S8 with an oxidant, such as I2–DMSO,6 iodine reagents/peroxide,7 hypervalent iodine/azide8 and S8/oxidant.9 However, these systems are usually constrained to special substrates (e.g. aryl methyl ketones and symmetrical alkanes). Thus, developing an efficient method for the oxidative functionalization of diverse sp3 C–H bonds is still challenging and also very desirable.

Triazolopyridines are important heterocycles which are widely present in natural products, synthetic drugs and materials.10 Many triazolopyridines show significant biological activities and pharmaceutical activities, including antifungal, anticonvulsant, antibacterial and antiproliferative activities.11 Given their biological and pharmaceutical activities, extensive synthetic protocols have been reported for triazolopyridine synthesis.

The classical approach is to couple aldehydes/carboxylic acids with 2-hydrazinylpyridines or hydrazones/acylhydrazines with 2-halo pyridines in a multi-step manner, usually using strong acids, high temperatures and transition metal catalysts (Fig. 1a).12 As an alternative, the annulation of 2-hydrazinylpyridines and isothiocyanates with TBAF or iodine is a facile protocol, which can efficiently afford 3-amino-[1,2,4]-triazolo pyridines in good yields with excellent substrate tolerance (Fig. 1b).13 In 2016, Vidavalur and co-workers disclosed a convenient and efficient protocol for carbonylation of aryl iodides and 2-hydrazinylpyridines with molybdenum hexacarbonyl to access [1,2,4]triazolo[4,3-a]pyridines in excellent yields without CO gas or a palladium catalyst (Fig. 1c).14 Multicomponent domino reactions constitute an efficient strategy, which has been widely utilized in organic synthesis.15 Very recently, Subba Reddy and co-workers also developed a graceful multicomponent approach for 1,2,4-triazolo[4,3-a]pyridine synthesis in the presence of iodine/IBX and DMSO from aryl methyl ketones, ethyl benzoylacetate, styrenes, phenylacetylenes and 2-hydrazinylpyridines (Fig. 1d).16 This reaction can efficiently afford the products in good yields from a diversity of substrates via different pathways.


image file: c9ob00033j-f1.tif
Fig. 1 The protocols for 1,2,4-triazolo[4,3-a]pyridine synthesis.

Although many useful methods have been reported for 1,2,4-triazolo[4,3-a]pyridines synthesis, the protocol for direct oxidative functionalization of sp3 C–H bond of methyl group is rare. In addition, the development of a one-pot and metal-free protocol to obtain triazolo[4,3-a]pyridines attached to another heterocycle remains very desirable. Herein, we report an efficient direct oxidative functionalization of sp3 C–H bond of methyl-azaheteroarenes to form triazolo[4,3-a]pyridines in the presence of I2-DMSO under mild conditions.

Initially, we evaluated various reaction conditions using 2-methyl quinoline (1a) and 2-hydrazinylpyridine (2a) as model substrates (Table 1). Gratifyingly, the reaction of 1a with 2a occurred smoothly with 1.0 equivalent of iodine at 100 °C to afford the product 3aa in 58% yield. On increasing the amount of iodine to 1.5 and 2.0 equivalents, the reaction yield increased to 65% and 62%, respectively. However, the yield significantly decreased when the amount of iodine was reduced to 0.8 or 0.5 equivalents. Subsequently, we investigated the influence of temperature on the reaction, and the results showed that the reaction worked well at 110 °C. A higher or lower temperature would lead to a low yield. Because the acid HI should be generated from I2 and DMSO in this reaction, using a base as an acid binding agent would probably improve the yield. Next, a series of bases, such as Na2CO3, K2CO3, Cs2CO3 and KOH, were screened for use in the reaction. The bases Na2CO3 and K2CO3 did not show an influence on the yield. But the yield significantly decreased when Cs2CO3 or KOH was added. In addition, a blank experiment was conducted without any iodine, and the reaction did not give the desired product. This result indicates that iodine is essential for this transformation. Different reaction solvents were also investigated. The reaction cannot occur in other solvents, such as MeOH, dioxane, MeCN and toluene.

Table 1 Optimization of the reaction conditions for the preparation of 3aa

image file: c9ob00033j-u1.tif

Entry I2 (equiv.) Base Temp. (°C) Yieldb (%)
a Reaction conditions: 2-methyl quinoline 1a (0.5 mmol), 2-hydrazinylpyridine 2a (0.6 mmol) and I2 were heated in DMSO (3 mL).b Isolated yields.c MeOH instead of DMSO.d Dioxane instead of DMSO.e MeCN instead of DMSO.f Toluene instead of DMSO.
1 1.0 100 58
2 1.5   100 65
3 2.0   100 62
4 0.8   100 48
5 0.5   100 35
6 1.5   110 71
7 1.5   120 60
8 1.5   90 54
9 1.5   70 20
10 1.5 Na2CO3 110 68
11 1.5 K2CO3 110 69
12 1.5 Cs2CO3 110 40
13 1.5 KOH 110 15
14 0   110 0
15c 1.5 110 Trace
16d 1.5 110 0
17e 1.5 110 0
18f 1.5 110 0


With the optimized conditions in hand, we firstly investigated the reaction scope using substituted methyl quinolines and other methyl azaheterocycles (Scheme 1). The results showed that various substituted methyl quinolines were tolerated in the reaction. For example, 2-methyl quinolines bearing electron-rich substituents on the aryl ring (e.g. 6-Me, 6-OMe, 6-OEt) did not affect the reaction efficiency, and the desired products 3ba–3da could be obtained in 73%, 75% and 72% yield, respectively. When the aryl rings had halogen atom substituents (e.g. 6-Cl, 7-Cl, 6-Br, 6-F, 7-F), the reactions proceeded smoothly to give the corresponding products 3ea–3ia in moderate to good yields, which could be used for further transformations. Moreover, the two heteroatom-containing substrates 2-methylquinoxaline (1k) and 2-methylbenzo[d]thiazole (1l) were also suitable for the reaction, affording the corresponding products 3ka and 3la in 57% and 46% yield, respectively. It is noteworthy that 4-methyl quinoline (1m) was also tolerated in the reaction to generate the desired product 3ma (64% yield). Unfortunately, 3-methyl quinoline (1n) and 8-methyl quinoline (1o) did not react under the standard conditions. However, 2-picoline could undergo the reaction to afford the desired product 3pa in 25% yield. The substrates 4-picoline, 2,6-dimethyl pyridine and 2,4,6-trimethyl pyridine could not undergo the reaction.


image file: c9ob00033j-s1.tif
Scheme 1 Scope investigation using substituted methyl quinolines and derivatives. Reaction conditions: 1 (0.5 mmol) and I2 (0.75 mmol) were stirred in DMSO (3 mL) at 110 °C for 4–6 h, then 2a (0.6 mmol) was added and the mixture was stirred at 110 °C until the disappearance of 2a. Isolated yields are shown. n.r. = no reaction. aThe reaction was performed for 48 hours.

Encouraged by the obtained results, we turned our attention to the scope with respect to 2-hydrazinylpyridines (Scheme 2). Agreeably, a variety of substituted 2-hydrazinylpyridines were suitable for this reaction and afforded the corresponding products. For example, 2-hydrazinylpyridines with halogen atom substituents (e.g. 3-Cl, 5-Cl, 6-Cl, 5-Br, 6-Br, 2-Me-5-Br, 3-F) were tolerated in the reaction, affording the corresponding products 3ab–3ah in 61–71% yield. Moreover, 2-hydrazinylpyridines attached to the strong electron-withdrawing group CF3 (2i–2k; 3-CF3, 5-CF3, 6-CF3) could also undergo the reaction smoothly to give products 3ai–3ak in 57–61% yield. 2-Hydrazinyl-5-nitropyridine (2l) was also tolerated; however, the reaction yield was significantly decreased (41%). Moreover, 2-hydrazinylpyridines attached to electron-donating groups were also investigated. 5-Bromo-2-hydrazineyl-3-methylpyridine (2g), 2-hydrazineyl-6-methylpyridine (2m) and 2-hydrazineyl-5-methylpyridine (2n) could couple with 2-methyl quinoline and 4-methyl quinoline to afford the corresponding products (3ag, 3mg, 3am, 3mm and 3mn) in moderate yields. Both 2-hydrazinylpyridines and 2-methyl quinolines attached to bromine atoms were also tolerated to afford 3ge and 3gf (65% and 71%), which could be used for further functionalization. In addition, 4-methyl quinoline (1m) could also couple with 5-bromo-2-hydrazinylpyridine (2e) and 6-bromo-2-hydrazinylpyridine (3f) to deliver the corresponding products 3me and 3mf in 59% and 63% yield, respectively.


image file: c9ob00033j-s2.tif
Scheme 2 Scope investigation using substituted 2-hydrazinylpyridines and methyl quinolines. Reaction conditions: 1 (0.5 mmol) and I2 (0.75 mmol) were stirred in DMSO (3 mL) at 110 °C for 4–6 h, then 2 (0.6 mmol) was added and the mixture was stirred at 110 °C until the disappearance of 2 (monitored by TLC). Isolated yields are shown. a2-Hydrazineyl-5-methylpyridine hydrochloride (1.2 equiv.) and K2CO3 (2.0 equiv.) were used for this reaction.

To gain insight into the reaction mechanism, some control experiments were performed (Scheme 3). When 2-methylquinoline was placed under the standard conditions for 4 hours without 2-hydrazinylpyridine, it absolutely transformed into quinolin-2-carboxaldehyde with a 86% isolated yield, whereas this reaction could not proceed without I2 (Scheme 3a). If the reaction of 2-methylquinoline with iodine was performed at 110 °C in DMSO for only 50 min, 2-(iodomethyl)quinoline and quinolin-2-carboxaldehyde could be detected in 52% and 38% yield, respectively. The above results indicate that 2-(iodomethyl)quinoline and quinolin-2-carboxaldehyde are potential intermediates in the reaction, and I2 is essential for this transformation. Given the potential intermediate of 2-(iodomethyl)quinoline, reactions of 2-methylquinoline with iodine were conducted in MeOH, dioxane, MeCN and toluene. Interestingly, 2-(iodomethyl)quinoline could be observed, and quinoline-2-carbaldehyde was not detected in these reactions (see ESI). 2-(Iodomethyl)quinoline was also reacted with 2-hydrazinylpyridine under the standard conditions, and the desired product 3aa was obtained in 67% yield (Scheme 3b). Subsequently, a reaction of quinolin-2-carboxaldehyde with 2-hydrazinylpyridine was conducted under the standard conditions, and the product 3aa was generated in excellent yield (Scheme 3c). However, the reaction could not give the desired product without I2. These results have uncovered that 2-(iodomethyl)quinoline and quinolin-2-carboxaldehyde are potential intermediates, and I2 may play an important role in the condensation step of the reaction of quinolin-2-carboxaldehyde with 2-hydrazinylpyridine.


image file: c9ob00033j-s3.tif
Scheme 3 Control experiments.

On the basis of the above results and previous reports,17 a plausible reaction mechanism was proposed and is shown in Scheme 4. The substrate 1a exists in equilibrium between 2-methylpyridine 1a and its enamine counterpart 1a′ bearing an exocyclic double bond. Initially, the enamine counterpart 1a′ undergoes an iodination reaction with I2 to afford the intermediate 2-(iodomethyl)quinoline (1aa), which is further oxidized to quinolin-2-carboxaldehyde (1ab) through a subsequent Kornblum oxidation with DMSO. Subsequently, quinolin-2-carboxaldehyde (1ab) undergoes condensation and annulation reactions with 2-hydrazinylpyridine to generate intermediate 1ad. Finally, intermediate 1ad undergoes oxidation and aromatization processes in the presence of I2 to form the end product 3aa.


image file: c9ob00033j-s4.tif
Scheme 4 A plausible reaction mechanism for the preparation of 1,2,4-triazolo[4,3-a]pyridines.

Conclusions

In summary, we have developed an oxidative [4 + 1] annulation to prepare 1,2,4-triazolo[4,3-a]pyridines in the presence of I2–DMSO. This protocol enabled the synthesis of triazolo[4,3-a]pyridine–quinoline linked diheterocycles via direct oxidative functionalization of the sp3 C–H bonds of methyl-azaheteroarenes without any metal catalysts. The reaction occurred through a sequential iodination, Kornblum oxidation, condensation, oxidation and aromatization process. A mechanism investigation uncovered that 2-(iodomethyl)quinoline and quinolin-2-carboxaldehyde are potential intermediates. The reaction showed a wide substrate scope and good functional group tolerance for methyl-azaheteroarenes and 2-hydrazinylpyridine derivatives.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21702091, 21472056 and 21772051) and the Key Research & Development Project of Shandong Province (2018GGX109014). This work was also supported by the Yantai “Double Hundred Plan”. The National College Students Innovation and Entrepreneurship Training Program (201811066005) is gratefully acknowledged (for Y.-H. Gao, X. Zhang, Y.-H. Liu, and Y.-J. Shen).

Notes and references

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ob00033j
These authors contributed equally to this work.

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