Transition-Metal-Free Decarboxylative Arylation of 2-Picolinic Acids

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Cite This: Org. Lett. 2018, 20, 7095−7099

Transition-Metal-Free Decarboxylative Arylation of 2‑Picolinic Acids with Arenes under Air Conditions Xitao Zhang,† Xiujuan Feng,*,† Chuancheng Zhou,† Xiaoqiang Yu,† Yoshinori Yamamoto,†,‡,§ and Ming Bao*,† †

State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116023, Liaoning, China Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan § Research Organization of Science and Technology, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan Org. Lett. 2018.20:7095-7099. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/16/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: A facile, transition-metal-free, and direct decarboxylative arylation of 2-picolinic acids with simple arenes is described. The oxidative decarboxylative arylation of 2-picolinic acids with arenes proceeds readily via N-chloro carbene intermediates to afford 2-arylpyridines in satisfactory to good yields under transition-metal-free conditions. This new type of decarboxylative arylation is operationally simple and scalable and exhibits high functional-group tolerance. Various synthetically useful functional groups, such as halogen atoms, methoxycarbonyl, and nitro, remain intact during the decarboxylative arylation of 2-picolinic acids.

T

Scheme 1. Decarboxylative Arylation of 2-Picolinic Acids

he 2-arylpyridine motif is present in many important bioactive molecules and pharmaceuticals (Figure 1).1

Figure 1. 2-Arylated pyridines cores in drug molecules.

The importance of 2-arylpyridines provides the continuous impetus for the synthetic chemists to seek simple and valid methods to construct them. Transition-metal-catalyzed crosscoupling reactions, such as Suzuki−Miyaura coupling, Stille coupling, and Kumada coupling, have been successfully applied for the synthesis of 2-arylpyridines with 2-halogenated pyridines and aryl organometallic reagents.2 However, these reactions need stoichiometric amounts of organometallic coupling reagents and expensive transition-metal catalysts and, thus, produce stoichiometric toxic waste. In the past decade, transition-metal-catalyzed decarboxylative cross-coupling reactions via 2-pyridyl arylpalladium intermediates have also been developed as an effective methodology for the synthesis of 2-arylpyridines with 2-picolinic acid and aryl halides (Scheme 1a).3 Carboxylates are generally inexpensive, stable, nontoxic, and ready available and represent a more ecologically friendly alternative to their organometallic counterparts.4 However, due to the instability of the 2© 2018 American Chemical Society

metallapyridines and their tendency toward protodecarboxylation, these decarboxylative cross-couplings of 2-picolinic acids usually require high temperatures and a bimetallic Cu/Pd catalyst.5 Recently, the decarboxylative arylation of benzoic acids with simple (hetero)arenes by directed C−H cleavage, which takes full advantage of both C−H bond activation and decarboxylation, has received much current interest.6 Such transformations are capable of streamlining organic synthesis and minimizing wasteful byproducts. Not only arenes7 but also various heteroarenes, such as azoles,8 indoles,9 thiophenes,10 and furans,11 can be coupled with benzoic acids smoothly to produce biaryl skeletons in satisfactory to excellent yields. In Received: September 24, 2018 Published: November 2, 2018 7095

DOI: 10.1021/acs.orglett.8b03043 Org. Lett. 2018, 20, 7095−7099

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Organic Letters spite of remarkable progress that has been achieved in this area, the decarboxylative arylation of 2-picolinic acid with simple, cheap, and widely available arenes, such as benzene, toluene, and xylene, has remained a challenging goal. Only one example of silver-catalyzed decarboxylative arylation of 2picolinic acids with simple benzene via 2-pyridyl radical intermediates has recently been reported (Scheme 1b).7c This pioneering work demonstrates a broad substrate scope and excellent functional-group tolerance. However, the employment of a noble transition metal catalyst, a strong oxidant, and high temperature (120 °C) was required to complete the target reaction. As well-known, most of the transition metals are toxic, and removal of trace amounts of transition metal residues from desired products is quite costly and challenging for the pharmaceutical industry.12 Therefore, transition-metalfree decarboxylative arylation of 2-picolinic acids with simple arenes as arylating reagents under mild conditions has been desired. Here, we report a new type of transition-metal-free decarboxylative arylation under mild conditions via an Nchloro carbene intermediate (Scheme 1c). It is noteworthy that this protocol represents the first example of a transitionmetal-free direct decarboxylation arylation of 2-picolinic acids with simple arenes to provide an efficient and economical means of building 2-arylpyridines, an attractive structure motif in drug discovery. In our initial study, the decarboxylative arylation reaction of 2-picolinic acid (1a) with benzene (2a) was selected as a model reaction to optimize the reaction conditions. The results are shown in Table 1. Only 24% yield of product 2phenylpyridine (3aa) was observed in the absence of a base (entry 1). The base was then screened using 1.5 equiv of t-

BuOCl as the promoter under air. Among the various bases examined, which included inorganic and organic bases, K2CO3 provided a relatively high yield of the product 3aa (entry 2 vs entries 3−8). No improvement in the yield of 3aa was observed under a pure oxygen atmosphere (entry 9). The inorganic oxidant [potassium persulfate (K2S2O8)] and the organic oxidants [tert-butyl hydroperoxide (TBHP), benzoquinone (BQ), and p-chloranil] were demonstrated to be inefficient (entries 10−13). Only a trace amount of 3aa was observed in the absence of oxidant, indicating that an oxidant is necessary for the processing of the target reaction (entry 14). Oxygen in air was proven to be the best oxidant. Interestingly, the yield of 3aa increased with decreasing K2CO3 loading (entries 15 and 16 vs entry 2); the reason may be attributed to the better stirring with the decreased solid base. The yield of 3aa was further enhanced to 90% by using an increased amount of t-BuOCl from 1.5 to 3 equiv (entry 17 vs entry 16). Therefore, the subsequent decarboxylative arylations of 2picolinic acids (1a−1s) with arenes (2a−2n) were performed in the presence of t-BuOCl (3 equiv) and K2CO3 (0.5 equiv) using oxygen in air as the oxidant at 60 °C for 20 h. With the optimized reaction conditions in hand, a wide range of 2-picolinic acids were investigated for this type of decarboxylative arylation reaction. The results are summarized in Scheme 2. The reactions of 2-picolinic acids 1b, 1c, and 1d bearing methyl (Me), chloro (Cl), or bromo (Br) on the 3positions, respectively, gave the desired products 3ba, 3ca, and 3da in relatively low yields in comparison with the reaction of 1a, a simple substrate; this result was due to steric hindrance of the 3-position substituents. The reactions of 2-picolinic acids Scheme 2. Decarboxylative Phenylation of 2-Picolinic Acids with Benzenea,b

Table 1. Screening of Reaction Conditionsa

entry

base (equiv)

oxidant

yield (%)b

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

none K2CO3 (2) Na2CO3 (2) Li2CO3 (2) K3PO3 (2) NaOH (2) t-BuOK (2) NEt3 (2) K2CO3 (2) K2CO3 (2) K2CO3 (2) K2CO3 (2) K2CO3 (2) K2CO3 (2) K2CO3 (1) K2CO3 (0.5) K2CO3 (0.5)

O2 in air O2 in air O2 in air O2 in air O2 in air O2 in air O2 in air O2 in air O2 in balloon K2S2O8 TBHP BQ p-chloranil none O2 in air O2 in air O2 in air

24 35 32 16 trace 28 trace trace 34 30 5 8 4 trace 55 67 90 (86)d a

Reaction conditions: 2-picolinic acid (1, 0.3 mmol), t-BuOCl (0.9 mmol), K2CO3 (0.15 mmol) in benzene (3 mL) at 60 °C under air for 20 h. bThe isolated yields were calculated based on 2-picolinic acids. cThe reaction was performed at 80 °C. dThe reaction was performed using a reduced amount of t-BuOCl (0.6 mmol) for a shortened time (3 h).

a

Reaction conditions: 1a (0.3 mmol), t-BuOCl (1.5 equiv, 0.45 mmol), base, and oxidant at 60 °C for 20 h; 2.0 equiv of K2S2O8, TBHP, BQ, and p-chlornail were examined. bYields were determined via 1H NMR analysis using dibromomethane as an internal standard. c t-BuOCl (3.0 equiv) was used. dIsolated yield. 7096

DOI: 10.1021/acs.orglett.8b03043 Org. Lett. 2018, 20, 7095−7099

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Organic Letters 1e−1g bearing a substituent (Me, Cl, or Br) on the 4-position proceeded smoothly to produce the products 2-phenylpyridines 3ea−3ga in 62%−75% yields. Products 3ha and 3ia were isolated in almost the same yield (69% and 68%, respectively) from the reactions of 5-methyl 2-picolinic acid (1h) and 5-phenyl 2-picolinic acid (1i). The reaction of 5methoxyl 2-picolinic acid (1j) required a higher temperature to complete; the reason might be attributed to the strong electron-donating ability of methoxyl (MeO) group. The reactions of 2-picolinic acids 1k and 1l also needed higher temperatures to complete even though an electron-withdrawing group such as CO2Me or NO2 was linked to the pyridine ring. However, the reason behind this behavior remains unclear. Similarly, a satisfactory yield (71%) of product 3ma was obtained from the reaction of 6-methyl 2picolinic acid (1m) compared with the reactions of 1b and 1e. A moderate yield of 46% was obtained again when 3,5dichloro-2-picolinic acid (1n) was examined as observed in the reaction of 1c. Moreover, the reactions of quinoline-2carboxylic acid (1o) and quinoxaline-2-carboxylic acid (1p) having fused heterocycles proceeded smoothly to produce the desired products 3oa and 3pa in 87% and 65% yields, respectively. The substrate isoquinoline-1-carboxylic acid (1q) exhibited a higher reactivity than others; its reaction was completed in a short time in the presence of reduced amounts of t-BuOCl to produce the product 3qa in 86% yield. Substrates pyrimidine-4-carboxylic acid (1r) and pyrazine-2carboxylic acid (1s) were finally examined at relatively high temperature, and the desired products 3ra and 3sa were obtained in 62% and 47% yields, respectively. These results indicated that the reactivities of 1r and 1s were lower than that of 1a. The scope of the arene substrate was also explored using 1a as the reaction partner, and the results are summarized in Scheme 3. The reactions of monosubstituted arenes toluene (2b), chlorobenzene (2c), bromobenzene (2d), fluorobenzene (2e), and methyl benzoate (2f) produced a mixture of orthoisomer, meta-isomer, and para-isomer. The ortho-isomers o3ab, o-3ac, and o-3ad were separated in 38%, 16%, and 23% yields, respectively. Two isomers could be separated from the reaction of 1a with o-xylene (2g) or 1,2-dichlorobenzene (2h) (o-3ag: 24%, m-3ag: 41%; o-3ah: 25%, m-3ah: 32%). A mixture of o-3ai and m-3ai was isolated in 61% total yield with ratio of 3.1:1 when 1,2-difluorobenzene (2i) was used. A sole product was obtained from the reaction of 1a with p-xylene (2j) or 1,4-difluorobenzene (2k) (3aj, 53%; 3ak, 67%). A mixture of ortho-isomer and meta-isomer was again generated in the reaction of 1a with 1-fluoro-4-methylbenzene (2l) or 1chloro-4-methylbenzene (2m); the isomers o-3al and m-3al as well as o-3am and m-3am were successfully separated. The reaction of 1a with naphthalene (2n) was examined, and αisomer (α-3an) and β-isomer (β-3an) were obtained in 40% and 23% yields, respectively. To gain insight into the mechanism of this type of decarboxylative arylation reaction, control experiments were conducted (Scheme 4). No reaction was observed when the mixture of 1a and 2a was treated under standard conditions in the presence of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) (Scheme 4, eq 1). This result suggests that the target reaction may involve a radical process.13 No reaction was observed again when the mixture of nicotinic acid (4) and 2a was treated under standard conditions (Scheme 4, eq 2). This phenomenon indicates that the carboxyl group must be

Scheme 3. Decarboxylative Arylation of 2-Picolinic Acid with Various Arenesa,b

a

Reaction conditions: 2-picolinic acid (1a, 0.3 mmol), t-BuOCl (0.9 mmol), K2CO3 (0.15 mmol) in arene (3 mL) at 60 °C for 20 h. bThe isolated yields were calculated based on 2-picolinic acid; the ratio of regioisomers was calculated via 1H NMR spectra. cNaphthalene (30 equiv) was used in CCl4 (1 mL).

Scheme 4. Control Experiments

linked on the position nearby a nitrogen atom. A deuterium kinetic isotope effect was investigated by conducting an intermolecular competition reaction between 2a and 2a-d6; a 1.06 ratio of 3aa to 3aa-d5 was observed demonstrating that 7097

DOI: 10.1021/acs.orglett.8b03043 Org. Lett. 2018, 20, 7095−7099

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Organic Letters

mild reaction conditions (60 °C, under air), the experimental simplicity (single step), and the absence of a transition-metal catalyst make the present methodology highly useful in organic synthesis, especially in drug molecule synthesis. A new strategy for generating carbene intermediates from 2-picolinic acids was established in the current study. Further investigations into the synthetic applications of the carbene intermediates are ongoing.

the cleavage of the C−H bond in 2a is not involved in the ratedetermining step (Scheme 4, eq 3). A plausible reaction mechanism is depicted in Scheme 5 on the basis of the mechanistic studies described above. Initially, Scheme 5. Proposed Mechanism for the Decarboxylative Arylation Reaction of 2-Picolinic Acids

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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.8b03043. Experimental procedures and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Xiaoqiang Yu: 0000-0001-9396-3882 Ming Bao: 0000-0002-5179-3499 radicals Cl· and t-BuO·, generated from t-BuOCl under heating conditions,14 were subsequently transformed to Cl+ and tBuO− in the presence of oxygen as the oxidant. The reaction of potassium picolinate (B) with Cl+ and t-BuO− would produce intermediate N-chloro potassium picolinate (C). The intermediate C would release CO2 and t-BuOK, resulting in an ylide, N-chloro pyridinium D, which would transform into a carbene intermediate E.15 When benzene is reacted with the carbene intermediate E, it would produce intermediate F containing a cyclopropane motif. The ring-opening reaction would take place in the presence of base t-BuOK to generate the desired product 3aa. The formation of byproducts t-BuOH and KCl was verified by gas chromatography−mass spectrometry and aqueous solution of silver nitrate (AgNO3), respectively. The decarboxylative arylation reaction of quinoline-2carboxylic acid (1o) was effected on the gram scale to further explore the practicability of our methodology; the result is shown in Scheme 6. When 1.04 g of 1o was treated in benzene

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (Nos. 21573032 and 21773021) for their financial support. This work was also supported by the Fundamental Research Funds for the Central Universities (DUT17ZD212).

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REFERENCES

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Scheme 6. Gram-Scale Synthesis of Product 3oa

at standard conditions, 1.00 g of product 3oa was obtained (81% yield). This yield was slightly lower than that obtained in a small-scale experiment. In conclusion, we have developed a new and efficient method for synthesizing 2-arylpyridines under transition-metalfree conditions. The decarboxylative arylation reaction of 2picolinic acids with arenes proceeded smoothly in the presence of oxygen as the oxidant via a carbene intermediate that produced 2-arylpyridines in satisfactory to good yields. The wide availability and simplicity of the starting materials, the 7098

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