Halogenations of substituted 2-alkylquinoline with iodine and halide exchange with AgF₂

Abstract

Halogenations of substituted 2-alkylquinoline with iodine and halide exchange with AgF₂ have been developed. These processes provide facile strategies to construct C–I/F bonds, and expand the methods of fluorination of 2-alkylquinolines. These transformations exhibit good tolerance of functional groups and generate the desired products in moderate to good yields.

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Organic molecules containing halogenated functional groups are recognized as a crucial class of scaffolds, which are widely applied in pharmaceutical chemistry, agrochemistry and functional material science.^1,2 In addition, organohalogen compounds are also used as synthetic precursors for organic synthesis.³ In view of these benefits, the development of carbon–halogen bonds formation has attracted substantial interest and made enormous progress during the past several years.^4,5 Particularly, organic molecules containing fluorine atoms have emerged as star products in many fields due to the unique characteristics of carbon–fluorine bond,⁶ such as steric size effect, strong bond energy, change of lipophilicity, hydrogen bond acceptor, biological activity and high electronegativity. As consequence, continuous efforts have been devoted for the fluorination in the past several decades, as a rule, there are three general strategies for the construction of carbon–fluorine bond: nucleophilic,⁷ electrophilic,⁸ and radical fluorination.⁹ And more recently, success in this area can be attributed to the development of various efficient transition-metal catalysts, such as Pd,¹⁰ Cu,¹¹ Ag,¹² Au,¹³ which have been employed widely in the construction of C–F bond processes. Li,¹⁴ Sanford,¹⁵ Furuya,¹⁶ Hu,¹⁷ Ribas,¹⁸ Doyle,¹⁹ Buchwald,²⁰ Nguyen,²¹ Gouverneur²² et al. have contributed greatly to the transition-metal-catalyzed C–F bond formation. Another important progress has also been investigated in the formation of C–F bonds based on the direct halide exchange. For instance, in 2012, Hartwig's group reported a new strategy of copper-mediated fluorination of aryl iodides²³ (Scheme 1, entry A). Liu²⁴ and Szabó²⁵ respectively reported the fluorination of allylic halides by using Et₃N·HF and (Ph₃P)₃CuF as the fluorine sources in the same year (Scheme 1, entry B). In contrast to recent conspicuous progress in formation of aromatic C(sp²)–F bond,²⁶ the strategies of benzylic C(sp³)–F bond construction of 2-alkylquinolines are relatively limited.²⁷ Therefore, the development of briefness methods to construction of C(sp³)–F bond of 2-alkylquinolines are very desirable and useful.

Scheme 1 C–I/F bonds formation.

To our best knowledge, we found there was very few available literature concerning the methods of C(sp³)–I/F bonds construction of 2-alkylquinolines and their derivatives.²⁸ Inspired by the previous works, herein, we had developed facile methods for C(sp³)–I/F bonds formation of substituted 2-alkylquinolines with iodine and halide exchanging by AgF₂ (Scheme 1).

Firstly, 2-methylquinoline 1a was used as the model substrate to start our research. The expected product 2-(iodomethyl)quinoline 2a was isolated in 33% yield when the reaction was conducted with I₂ (1.5 equiv.) and NaHCO₃ (1.5 equiv.) in THF at 100 °C under N₂ for 1.5 h (entry 1, Table 1). Encouraged by this initial result, we subsequently assessed Lewis bases as additives to improved the yield of this reaction (entry 2–3, Table 1), PPh₃ promoted this reaction efficiently and the yield increased to 51%. The reaction temperature was also screened, 120 °C had been proven to be the efficient temperature in this process (entry 4–5, Table 1). Then several iodination reagents, such as tetrabutylammonium iodide (TBAI) and N-iodosuccinimide (NIS), were evaluated and only trace 2a were detected (entry 6–7, Table 1). Other bases were also examined, lower yields of 2a were obtained (entry 8–10, Table 1). Changing the solvent and the equivalent of I₂ did not improve the yields (entry 11–13, Table 1). Consequently, the optimized reaction conditions should be proceeded with I₂ (1.5 equiv.), NaHCO₃ (1.5 equiv.) and PPh₃ (10 mol%) in THF at 120 °C.

Table 1 Screening of reaction conditions for the iodination of 2-methylquinoline^a

Entry	Iodination reagent	Additive (10 mol%)	Base	Solvent	Yield^b (%)
a Reaction conditions: 1a (0.30 mmol), iodination reagent (1.5 equiv.), NaHCO3 (1.5 equiv.), PPh3 (10 mol%), THF (2 mL), 120 °C, under N2.b Isolated yields.c Reaction temperature was 100 °C.d Reaction temperature was 140 °C.e TBHP (1.5 equiv.).f The amount of I2 and NaHCO3 was increased to 2.0 equiv.
1	I2	—	NaHCO3	THF	33^c
2	I2	PPh3	NaHCO3	THF	51^c
3	I2	1,10-Phenanthroline	NaHCO3	THF	n.r.^c
4	I2	PPh3	NaHCO3	THF	77
5	I2	PPh3	NaHCO3	THF	34^d
6	TBAI/TBHP	PPh3	NaHCO3	THF	—^e
7	NIS	PPh3	NaHCO3	THF	Trace
8	I2	PPh3	K2CO3	THF	—
9	I2	PPh3	Na2CO3	THF	—
10	I2	PPh3	NaOH	THF	48
11	I2	PPh3	NaHCO3	DCE	65
12	I2	PPh3	NaHCO3	Toluene	51
13	I2	PPh3	NaHCO3	THF	72^f

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With the optimized reaction conditions in hand, the scope of substituted 2-alkylquinolines was examined and the results were summarized in Table 2. A wide series of substrates were tolerated in this transformation and gave desired products in moderate yields. Obviously, substituted 2-alkylquinolines with electron-donating groups, such as methyl and alkoxy groups, generated the iodination products in moderate yields. Nevertheless, the substrates with electron-withdrawing groups of fluorine, chlorine and bromine needed longer reaction time and failed to give ideal yields except these groups at C7 position (2j, 2l). 3-Methylbenzo[f]quinoline 1o was employed for this process under the standard conditions and product 2o was isolated in 68% yield. In addition, substrates 1p, 1q and 1r displayed better compatibility and gave desired products in moderate yields. Unfortunately, the benzylic C(sp³)–H of 1s and 1t could not be activated and give the unexpected products 2s and 2t. 2-Methylpyridine 1u and substrate 2v were also failed to give the desired product.

Table 2 Iodination of substituted 2-alkylquinolines^a

a Reaction conditions: 1 (0.30 mmol), I₂ (1.5 equiv.), NaHCO₃ (1.5 equiv.), PPh₃ (10 mol%), THF (2 mL), 120 °C, under N₂, 1.5 h.b Reaction conditions: 1 (0.3 0 mmol), I₂ (1.2 equiv.), TBHP (2.0 equiv.) and THF (2 mL), 120 °C, under N₂.

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In view of the importance of fluorination, we expected that the halide exchange of 2a would be an ideal method for specific position C–F bond formation. To probe the viability of this approach, we started our research with the reaction of 2a and AgF in DMF at 120 °C. To our delight, the desired product 2-(fluoromethyl)quinoline 3a was obtained in 31% yield (entry 1, Table 3). The yield was increased to 54% when AgF₂ was used as fluoride reagent (entry 2, Table 3). Further optimization of solvents demonstrated that toluene was the efficient solvent (entry 4–9, Table 3). After screening other parameters such as reaction temperature and time, the optimized reaction conditions should be performed in the presence of AgF₂ in toluene at 80 °C for 16 h.

Table 3 Reaction conditions for the fluorination of 2-methylquinoline^a

Entry	Fluorination reagent	Solvent	Yield^b (%)
a Reaction conditions: 2a (0.30 mmol), AgF2 (1.2 equiv.), toluene (1.5 mL) at 80 °C under N2, 16 h.b Isolated yields.c Reaction temperature was 120 °C.d Reaction temperature was 40 °C.
1^c	AgF	DMF	31
2	AgF2	DMF	54
3^d	AgF2	DMF	43
4	AgF2	DMF	51
5	AgF2	CH3CN	51
6	AgF2	DCE	65
7	AgF2	THF	—
8	AgF2	Toluene	89
9	AgF2	DMSO	Trace

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Under the optimized reaction conditions, we then proceeded to investigate the scope of 2-(iodoalkyl 1,10-phenanthroline quinolines) (Table 4). To our delight, various substituted iodoalkylquinolines reacted with AgF₂ well and generated the desired products in moderate to high yields. In this process, the presence of electron-donating and electron-withdrawing groups had very little influence on this transformation (3a–3n).

Table 4 Fluorination of substituted 2-methylquinoline^a

a Reaction conditions: 2a (0.30 mmol), AgF₂ (1.2 equiv.), toluene (1.5 mL) at 80 °C under N₂, 16 h.

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As shown in Table 4, substrate 2o was also tolerated in this process and the target compound 3o was obtained in excellent yield. When 2-(1-iodoethyl)quinoline 2p and 4-iodo-1,2,3,4-tetrahydroacridine 2r reacted with AgF₂ under the optimized conditions, the desired products 3p and 3r were afforded in moderate yields. Additionally, substrate 2q failed to give target product 3q.

In order to gain further information about the reaction mechanism, several control experiments were set up. First, to determine whether 2-methylquinoline 1a rendered into 2-(iodomethyl)quinoline 2a through a sequence of radical pathway in this transformation, radical scavengers 2,2,6,6,-tetramethylpiperidyl-1-oxyl (TEMPO) and 2,6-di-tert-butyl-4-methylphenol (BHT) were respectively performed with 2-methylquinoine 1a under the standard conditions (Scheme 2, entry A), the corresponding product 2a were separately isolated in 71% and 53% yields, which ruled out that the iodination process might proceed through a radical pathway. Then, further experimental study was carried out to confirm the mechanism for fluorination of 2a. TEMPO was added into the reaction system of (iodomethyl)quinoline 2a and AgF₂ in toluene at 80 °C under N₂, and gave the corresponding product 3a in 85% yield (Scheme 2B), which indicated that the (iodomethyl)quinoline 2a transformed to the target product 3a without a radical procedure. Based on the above experiment results, a possible reaction mechanism of iodination was proposed and summarized in Scheme 2. Firstly, substrate 1a equilibrated to generate intermediate A through tautomerization under the optimized conditions. Then, iodine underwent heterolytic cleavage in presence of PPh₃ to form cation Ph₃PI⁺. Then Ph₃PI⁺ attacked A to form the intermediate B through electrophilic addition. Finally, the product 2a was achieved by the procedures of proton elimination. Meanwhile, we also proposed the plausible mechanism of fluorination between 2a and AgF₂ (Scheme 2). Incipiently, substrate 2a would involve coordination with AgF₂ to produce the pyridine–AgF₂ complex C.²⁹ Then, the complex C went through a consecutive state of intermediate D, afforded the corresponding product 3a via halide exchange, and Ag(II)-intermediate was also generated at the same time (Scheme 3).

Scheme 2 Control experiments.

Scheme 3 Proposed mechanism.

In summary, we have developed simple methods of iodination/fluorination of substituted 2-alkylquinolines. The substrates with different functional groups, such as methyl, methoxyl, bromo, chloro and fluoro, were performed smoothly in the process of iodination/fluorination and formed target products in good yields under mild conditions.

Acknowledgements

This work was supported by National Natural Science Foundation of China (21672086, 21202067), Gansu Province Science Foundation for Youths (1606RJYA260) and the Fundamental Research Funds for the Central Universities (lzujbky-2016-39).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, spectroscopic data. See DOI: 10.1039/c6ra17410h

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