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Accepted Article Title: Remote C-H Selenylation of 8-Amidoquinolines via Copper-Catalyzed Radical Cross-Coupling
Authors: Mahiuddin Baidya; Harekrishna Sahoo; Anup Mandal; Jayaraman Selvakumar
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201600772 Link to VoR: http://dx.doi.org/10.1002/ejoc.201600772
Supported by
European Journal of Organic Chemistry
10.1002/ejoc.201600772
COMMUNICATION Remote C–H Selenylation of 8-Amidoquinolines via CopperCatalyzed Radical Cross-Coupling Harekrishna Sahoo, Anup Mandal, Jayaraman Selvakumar, and Mahiuddin Baidya*[a]
Abstract: A copper-catalyzed regioselective C–H selenylation of quinolines with readily available diaryl diselenides is developed based on chelation-controlled radical cross-coupling strategy. The reaction is scalable, tolerates a wide spectrum of functional groups, and proceeds with excellent C5-regioselectivity to deliver seleno quinolines in high yields (up to 98%). A single electron transfer (SET) mediated mechanism is proposed.
Thus, development of an efficient protocol for the fabrication of quinoline scaffold with selenium functionality is highly desirable.14 To the best of our knowledge, heretofore, there is no report on catalytic regioselective selenylation at the C5 position of quinolines. Herein, based on chelation-controlled radical cross-coupling strategy, we report an unprecedented copper catalyzed selenylation of 8-amidoquinolines under mild conditions with perfect C5-regioselectivity (Scheme 1b).
Introduction Quinolines are important class of N-heterocycles found in diverse bioactive compounds and functional materials with novel properties (Scheme 1a).1 Consequently, the expeditious functionalization of this privileged scaffold has remained in the focus of general interest. An elegant route would be direct C–H bond functionalization/activation technology. 2 However, implementation of such strategy particularly at the quinoline framework is not straightforward due to uncontrolled regioselectivity issues.3 Nevertheless, taking the beneficial effect of quinoline nitrogen to metal coordination and higher acidic nature of the C–H protons of the heterocyclic ring, a number of protocols have been established for the regioselective functionalization of C–H bonds at C2, C3, C4, and C8 positions of quinolines.4-7 In contrast, functionalization of geometrically inaccessible C–H bonds at C5, C6, and C7 positions is increasingly challenging.3,8 In the pioneering work, Stahl group has demonstrated C5selective chlorination of 8-amidoquinoline under copper catalysis and set the stage for regioselective C5-functionalization of quinolines.9a Afterwards, other groups have extended this strategy for the construction of C5-selective carbon-heteroatom bonds such as halogenation9b,c sulfonylation,10a-e nitration,10f trifluoromethylation,10g and carbon-carbon bonds with copper, cobalt, palladium, and iron catalysts.11 We have also contributed in this field through copper catalyzed C5-amination and -halogenation of quinolines with diazocarboxylates and N-halosuccinimides respectively.12 Similar to quinolines, organoselenides have received continued interest because of their presence as structural motifs in a variety of molecules of pharmaceutical and material interest.13 We envision that molecule encompassing both quinoline framework and selenide entity would be potentially very useful in the material science and drug discovery. In fact, seleno quinolines are known for their antioxidant properties. 13c
[a]
Department of Chemistry, Indian Institute of Technology Madras Chennai 600 036, Tamil Nadu, India E-mail:
[email protected]. Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Scheme 1. Bioactive quinolines and chelation-assisted remote C–H functionalization of quinoline scaffold.
During the preparation of our manuscript, Yin’s group reported regioselective selenylation of 8-amidoquinoline using more than stoichiometric amount of copper salt at elevated temperature (160 oC) with limited substrates.15 Reaction with more challenging aromatic amides, where ortho-selenylation could be a pitfall, was not explored. In fact, when we executed the reaction with aromatic amide 1a under stoichiometric amount of copper salt at lower temperature (100 oC), the regioselectivity was lost leading to a mixture of compounds 3a, 3a′, and 3a′′ (Scheme 2). Yin’s group also observed the formation of C5brominated by-product 3a′′ upon lowering the reaction temperature with aliphatic amide, which is in line with our finding. Nevertheless, this work encourages us to disclose our results.
Scheme 2. Remote C–H selenylation of amidoquinoline 1a with stoichiometric amount of CuBr2.
Results and Discussion We commenced our investigation using 8-aminoquinoline amide 1a as the model substrate in combination with commercially available diphenyl diselenide 2a as the selenium
European Journal of Organic Chemistry
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COMMUNICATION source (Table 1). Gratifyingly, when the mixture of 1a and 2a in DCE was exposed to 20 mol% Cu(OTf)2 catalyst in the presence of Ag2CO3 at 100 oC, selenylation proceeded cleanly at the remote C5-position delivering product 3a in 55% isolated yield (entry 1). The product was crystalized and the X-ray analysis unambiguously established the C5-regioselectivity (Figure 1).16 Screening of other solvents (entries 2-4) and copper catalysts (entries 5-6) gave inferior results. Improved yields were obtained when fluoride additives were examined (entries 7-12) and the best yield was achieved with selectfluor (0.5 equiv.), providing 3a in 80% isolated yield (entry 11).17 Screening of other oxidants also gave poor yields (entries 13-14). The reaction completely failed in the absence of Cu(OTf)2 and yield also reduced dramatically without Ag2CO3 (entries 15-16). When Ag2CO3 was replaced with K2CO3, desired product was obtained in 47% yield (entry 17). Furthermore, when oxidant Ag2CO3 loadings was
reduced to 30 mol%, 3a was obtained in 58% isolated yield (entry 18). These observations demonstrating that the presence of both the copper catalyst and Ag2CO3 are essential and Ag2CO3 is possibly playing dual role of oxidant as well as base. Table 2. Substrates Scope for the C5-Selective selenylation of Quinolines
a
Table 1. Optimization of C5-Selective Selenylationa
entry
catalyst
oxidant
solvent
additive (equiv.)
yield (%)b
1
Cu(OTf)2
Ag2CO3
DCE
-
55
2
Cu(OTf)2
Ag2CO3
Dioxane
-
40
3
Cu(OTf)2
Ag2CO3
PhCl
-
trace
4
Cu(OTf)2
Ag2CO3
Toluene
-
13
5
CuCl
Ag2CO3
DCE
-
38
6
CuBr2
Ag2CO3
DCE
-
22
7
Cu(OTf)2
Ag2CO3
DCE
(PhSO2)2NF (0.3)
64
8
Cu(OTf)2
Ag2CO3
DCE
KF (0.3)
48
9
Cu(OTf)2
Ag2CO3
DCE
CsF (0.3)
44
10
Cu(OTf)2
Ag2CO3
DCE
Selectfluor (0.3)
67
11
Cu(OTf)2
Ag2CO3
DCE
Selectfluor (0.5)
80
12
Cu(OTf)2
Ag2CO3
DCE
Selectfluor (1.0)
62
13
Cu(OTf)2
Ag2O
DCE
Selectfluor (0.5)
58
14
Cu(OTf)2
K2S2O8
DCE
Selectfluor (0.5)
trace
15
-
Ag2CO3
DCE
Selectfluor (0.5)
0
16
Cu(OTf)2
-
DCE
Selectfluor (0.5)
10
17
Cu(OTf)2
K2CO3
DCE
Selectfluor (0.5)
47
18c
Cu(OTf)2
Ag2CO3
DCE
Selectfluor (0.5)
58
a
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), Cu(OTf)2 (20 mol %), oxidant (0.2 mmol), Solvent (3 mL), 100 oC, 48 h. bIsolated yields. c30 mol % of Ag2CO3 was used. a
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), Cu(OTf)2 (20 mol%), Ag2CO3 (0.2 mmol), DCE (3 mL), 100 oC, 48 h. Yields of isolated compounds are given.
Figure 1. ORTEP diagram of product 3a (50% probability).
With the optimized reaction conditions in hand, we next directed our attention to explore the scope of the C5-selective selenylation reaction (Table 2). The reaction is quite general for a wide range of substituted 8-aminoquinolines. The carboxamides having aromatic (1b-e), aliphatic (1f-j), and vinyl (1k) substitutions furnished C5-phenylselenylated quinolines 3 in
European Journal of Organic Chemistry
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COMMUNICATION good yields (45-83%). The methodology was successfully applied to carboxamides bearing heteroaryl substituents such as furan (67%), benzofuran (62%), and thiophene (62%) to afford desired products 3l-n. Other amides such as Cbz-substituted carbamate 1o and anti-HIV-1 inhibitor sulfonamide 1p also delivered the expected products in 73% and 58% yields, respectively. Substitution at the quinoline moiety was also explored. The substrates with donating substituent such as methoxy in the carbocyclic ring (1q,r) delivered the desired products in excellent yields (86-90%). For methyl (3s), phenyl (3t), and halides (3u,v) substitutions at the heterocyclic ring, moderate yields (48-56%) were accomplished. The reaction is not restricted only to diphenyl diselenide (2a), it worked equally well with various para- (4a-l), ortho- (4m-n), and metasubstituted (4o-q) diaryl diselenides offering corresponding products with very high to excellent yields (up to 98%). It is worth noting that the trifluoromethyl substituted selenides 4o-q synthesized by this protocol are very important for drug discovery. Moreover, various functional groups including halogens are also tolerable and these halogen functionalities are useful synthetic handles for further functionalization.
Scheme 3. (a) Reactions with C5-susbtituted amidoquinolines, (b) aromatic Finkelstein reaction, and (c) reactions with diphenyl disulfide and diphenyl diteluride.
the C5-iodo substituted compound 1y was examined, an exchange reaction, so-called aromatic Finkelstein reaction, took place delivering 3a in 63% yield (Scheme 3b). These observations demonstrate that our protocol operates with an exquisite regioselective profile. Furthermore, no products were obtained with diphenyl disulfide and diphenyl diteluride and only staring materials were recovered in both cases (Scheme 3c). Thus, our reaction condition is highly selective for selenylation process. We have also executed the reaction on a gram-scale with amides 1o and 1q (Scheme 4a). The efficiency of small scale reaction was retained upon scale-up delivering 3o and 3q in 71% and 84% yields respectively. Furthermore, the amide bond can be easily cleaved in excellent yield to obtain free amine 5, which was converted to the high value synthon 6 through Sandmeyer reaction (Scheme 4b). Exposure of the compound 3o to m-CPBA at low temperature gave unsymmetrical selenoxide 7 in very high yield (89%). To gain more insights into the selenylation mechanism, various control experiments were performed (Scheme 5). The reaction was unfruitful for substrates 8-11 and the starting materials were recovered in major quantities (Scheme 5a). This observation infers that the chelation with 8-aminoquinoline is important. In case of free 8-amino quinoline, bis-selenylated product 12 was isolated in poor yield (Scheme 5a). When selenylation was performed in the presence of D 2O under typical reaction conditions, no deuterium incorporation was detected in the recovered starting material or the product, validating the cleavage of the C–H bond is irreversible (Scheme 5b). The intermolecular kinetic isotope effect revealed kH/kD = 3.3, which implies that remote C–H bond breaking may be involved in the rate determining step (Scheme 5c). Further, in the presence of radical quenchers such as TEMPO, butylated hydroxytoluene (BHT), and 1,1-diphenylethylene, the reaction yields dropped significantly (Scheme 5d). These results strongly suggest the involvement of radical species in the reaction pathway. In case of 1,1-diphenylethylene, compound 1318 was isolated in 58% yield, which also imparts a radical phenomenon.
Scheme 4. (a) Gram-Scale synthesis and (b) the post-functionalization of the selenylated product.
Of note, the reaction was completely arrested for the C5substituted substrates 3a and 1x (Scheme 3a). However, when
Scheme 5. Control experiments.
European Journal of Organic Chemistry
10.1002/ejoc.201600772
COMMUNICATION Although comprehensive study detailing the mechanistic underpinning of this reaction will require further investigation, based on preceding discussion (Scheme 5 and Table 1), a plausible reaction mechanism is outlined in Scheme 6. The 8aminoquinoline complex A is converted to B after single electron transfer (SET), which on reaction with diaryl diselenide 2 produces the intermediate C. In the presence of Ag(I), aryl selenium radical is reduced to corresponding anion, which deprotonates to give Cu(I)-complex D. After that, complex D is reoxidized to produce E, which on exchange with another molecule of 1 delivers the product 3 and generate complex A to continue the catalytic cycle.
Scheme 6. Plausible reaction mechanism.
Conclusions In summary, we have unfolded an efficient copper-catalyzed direct selenylation of quinolines with an excellent degree of regioselectivity at the C5-position. This protocol is operationally simple, scalable, displays a broad substrates scope, and uses readily available diaryl diselenides as selenium precursors. Mechanistic studies revealed that the reaction is chelationcontrolled and follows a single electron transfer (SET) mechanism for remote C–H bond functionalization. Additional scope of C–H functionalization with other heteroatoms is ongoing.
Experimental Section General Procedure: The 8-amidoquinolines 1 (0.20 mmol), diselenide derivatives 2 (0.40 mmol), silver carbonate (1 equiv.), Cu(OTf)2 (20 mol%) and selectfluor (0.10 mmol) were taken in a dried schlenk tube with a magnetic stir bar under nitrogen atmosphere. Then DCE (3 mL) was added with a syringe and the resulting mixture was heated at 100 ºC for 48 h. After completion of the reaction (TLC monitored), it was cooled to room temperature and transferred to a round bottom flask after dilution with CH2Cl2. The solvent was evaporated to dryness and the crude reaction mixture was loaded directly onto silica gel column and purified to provide pure selenylation products 3-4.
Acknowledgements We gratefully acknowledge CSIR New Delhi for the financial support (02(0212)/14/EMR-II). H. K. S. thanks IIT-Madras for HTRA and A. M. thanks UGC for a JRF. We also thank the department of chemistry, IIT-Madras for instrumental facilities.
Keywords: quinolines • selenylation • copper • radical homogeneous catalysis • C–H functionalization [1]
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European Journal of Organic Chemistry
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10.1002/ejoc.201600772
European Journal of Organic Chemistry
10.1002/ejoc.201600772
COMMUNICATION COMMUNICATION Key Topic* Harekrishna Sahoo, Anup Mandal, Jayaraman Selvakumar, and Mahiuddin Baidya*
A copper-catalyzed regioselective C–H selenylation of quinolines with readily available diaryl diselenides is developed based on chelation-controlled radical cross-coupling strategy. The reaction is scalable, tolerates a wide spectrum of functional groups, and proceeds with excellent C5-regioselectivity to deliver seleno quinolines in high yields (up to 98%). A single electron transfer (SET) mediated mechanism is proposed.
Page No. – Page No. Remote C–H Selenylation of 8Amidoquinolines via CopperCatalyzed Radical Cross-Coupling
