Letter Cite This: Org. Lett. 2018, 20, 1011−1014
pubs.acs.org/OrgLett
Cobalt-Catalyzed Remote C‑4 Functionalization of 8‑Aminoquinoline Amides with Ethers via C−H Activation under Visible-Light Irradiation. Access to α‑Heteroarylated Ether Derivatives Tubai Ghosh, Pintu Maity, and Brindaban C. Ranu* Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India S Supporting Information *
ABSTRACT: A cobalt-catalyzed selective remote C-4 alkylation of 8-aminoquinoline amides via C−H activation under irradiation with a CFL lamp in the presence of eosin Y at room temperature has been achieved. A series of pharmaceutically important C-4 quinoline amide-substituted ether derivatives has been obtained by this procedure. The C-4 functionalization of quinoline amides with inert ether is of much significance and was not reported earlier.
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tion, recently the combination of visible light and transition metal has been employed with better outcome.12 With our previous experience of transition-metal-mediated C−H functionalizations13a and visible-light-photocatalyzed transformations,13b,c we report here a remote selective C-4 functionalization of 8-aminoquinoline amides with ethers via C−H activation using a combination of Co(acac)2 and visible light (CFL bulb) at room temperature (Scheme 1).
he functionalization of cyclic ethers, such as tetrahydrofuran and 1,4-dioxane, is a challenging task as they are relatively inert under many conditions. Hence, several of them are used as solvents. However, cyclic ether derivatives are endowed with useful pharmaceutical properties1 and are found in many natural products. Traditionally, functionalization of ether via coupling reaction requires a number of steps.2 Several methods by different routes were reported, although these are not very efficient. Thus, direct functionalization of these molecules is an attractive proposition.3 During the past decade, the direct functionalization of C−H bond has received tremendous interest in organic synthesis as this process eliminates the prefunctionalization step.4 Usually, a heteroatom-containing group is used in the presence of a transition metal for activation of the C−H bond followed by functionalization. The amide group is extensively employed for C−H activation in various molecular frameworks.5 The 8aminoquinoline amide is an excellent directing unit and has been used for functionalization with a variety of moieties.6 Moreover, the interest in the 8-aminoquinoline system is due to the wide occurrence of quinoline scaffolds in bioactive molecules and natural products.7 The transition-metal-catalyzed selective functionalization of 8-aminoquinoline amide via C−H activation is complicated because of the two competing activation sites at C-4 and C-5, and selection is usually determined by the metal co-ordination complex with nitrogen. Although there are several examples of C-5 functionalization,8 efficient functionalization at C-4 is rare.9 This prompted us to investigate the functionalization of 8-aminoquinoline amide system with apparently inert ether moiety with particular attention to selectivity. Visible-light-mediated C−H functionalization has become a powerful tool in organic synthesis. These processes are associated with mild conditions and low energy consumption and are cheaper and easier to apply than UV reactors.10 Although several transition metals such as nickel, palladium, and copper, etc.11 have been reported for C−H functionaliza© 2018 American Chemical Society
Scheme 1. Visible-Light-Mediated Cobalt-Catalyzed Alkylation of Quinoline Amide Derivatives
It may be mentioned that a combination of Co salt and visible light was previously employed for C−S bond formation of aromatic C−H bonds14a and functionalization of amino acid derivatives among others.14b To optimize the reaction conditions, several experiments were performed with a variety of catalysts, oxidants, bases, and solvents for a representative reaction of 4-methyl-N-(quinolin8-yl)benzamide (1a) and tetrahydrofuran (2) (THF) as model substrates. Initially, when the reaction was carried out using 10 mol % of Co(acac)2, 2 mol % of photocatalyst eosin Y, K2S2O8 as an oxidant in the presence of 2 equiv of Cs2CO3 as base in DMSO solvent under the irradiation of 32 W CFL lamp at room temperature, the product 4-methyl-N-(4-(tetrahydrofuran-2-yl)quinolin-8-yl)benzamide was formed in 51% yield (Table 1, entry 1). However, replacement of K2S2O8 with (NH4)2S2O8 or Ag2CO3 (Table 1, entries 2 and 3) failed to produce the corresponding product. The result was dramatiReceived: December 20, 2017 Published: January 31, 2018 1011
DOI: 10.1021/acs.orglett.7b03955 Org. Lett. 2018, 20, 1011−1014
Letter
Organic Letters Table 1. Optimization of Reaction Conditionsa,b
LED lamp, the product was formed in lower yield (Table 1, entry 15). A change of catalyst, Co(acac)2 to Co(OAc)2 or CoCl2, did not further improve the result (Table 1, entries 16 and 17). It may be mentioned that no product was obtained in the absence of cobalt salts (Table 1, entry 18). The reaction failed to initiate under dark conditions (Table 1, entry 20), and a low yield (22%) of product was formed under an O2 environment (Table 1, entry 21). Use of 5 mol % of Co catalyst led to a lower yield (Table 1, entry 22), whereas use of 15 mol % did not improve the yield further (Table 1, entry 23). Irradiation for 2, 3, and 6 h did not furnish better results (Table 1, entries 24−26). The use of other metal catalysts such as Ni(acac)2 and Fe(acac)2 did not provide further improvement (Table 1, entries 27, 28). Thus, the best result was obtained when the reaction was performed using 10 mol % of Co(acac)2 in the presence of TBHP and Cs2CO3 in DMSO under irradiation with a 32 W CFL lamp using eosin Y (2 mol %) as a photosensizer for 4 h. With these optimized conditions in hand, we now explored the scope of this C(sp3)−C(sp2) bond formation with a series of quinoline amide derivatives and tetrahydrofuran. The results are summarized in Scheme 2. The quinoline amides containing Scheme 2. Reaction of Quinoline Amide Derivatives and Tetrahydrofurana,b
a Reaction conditions: a mixture of of 4-methyl-N-(quinolin-8yl)benzamide (1a, 0.1 mmol), tetrahydrofuran (0.5 mL), catalyst (10 mol %), eosin Y(2 mol %), oxidant, and base (2 equiv) in dry solvent (1 mL) was irradiated with a CFL lamp under nitrogen atmosphere. b Percent yield of the isolated product. c4 h. dIn the presence of 2 mol % of Ru(bpy)3Cl2, 6H2O. eUnder 18W blue LED lamp. fIn the absence of cobalt salt. gReaction under dark. hUnder O2 atmosphere. i Using 5 mol % of catalyst. jUsing 20 mol % of catalyst. k2 h. l3 h. m6 h.
a
Reaction conditions: amide (0.2 mmol), tetrahydrofuran (1 mL), Co(acac)2 (10 mol %), eosin Y (2 mol %), TBHP in decane (4 equiv), Cs2CO3 (2 equiv), DMSO (2 mL), 32 W CFL lamp, nitrogen, 4 h, rt. b Isolated yield.
cally improved when tert-butyl hydroperoxide (TBHP) was used as an oxidant, increasing the yield of product to 74% (Table 1, entry 4). The change of solvent to DCE, toluene, DMF, and CH3CN (Table 1, entries 5, 6, 7, and 8) did not show any promising results. The product was formed in low yield when the reaction was carried out in the absence of solvent (Table 1, entry 9). Use of K2CO3 as base led to lower yield (47%) compared to Cs2CO3 (Table 1, entry 10). However, other bases such as Na2CO3 and NaHCO3 are not much effective (Table 1, entries 11 and 12). The reaction did not proceed at all in the absence of any base (Table 1, entry 13). The product was formed in trace amount using other photoredox catalyst than eosin Y (Table 1, entry 14). When the reaction was carried out under the irradiation of an 18 W blue
both electron donating (−Me, −tBu, −nBu, and −OMe) (1−4) and electron-withdrawing (−F, −Cl, −CF3) groups (6−13) on the benzene ring participated in this reaction and provided the corresponding N-4-tetrahydrofuranyl products as only isolable compounds. The quinoline naphthamides and the heterocyclic amides (thiophene and furan) (14−17) produced the corresponding products without any difficulty under the reaction conditions. Apart from aromatic amides, several aliphatic amides like acetamide, pivalamide, cyclohexane/ cycloproanamide, etc. also led to the corresponding C-4 alkylation (18−21). We then investigated the effect of substituents on the quinoline ring on this reaction. Thus, 21012
DOI: 10.1021/acs.orglett.7b03955 Org. Lett. 2018, 20, 1011−1014
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Organic Letters
corresponding product was observed (see Scheme S1a). On the other hand, the formation of the corresponding tetrahydrofuranyl-trapped compound, 2,2,6,6-tetramethyl-1((tetrahydrofuran-2-yl)oxy)piperidine, was confirmed by 1H and 13C NMR spectroscopic data and HRMS data analysis of the isolated product (see the SI). Thus, it is likely that the reaction is going by a radical pathway involving an α-oxyalkyl radical. A significant kinetic isotope effect (KH/KD = 3.17:1) was observed in the process, providing evidence that C−H bond cleavage of THF molecule may be considered as a ratedetermining step3c (see Scheme S1b). To understand the necessity of the metal-chelation complex for this process, when a few reactions using different quinoline derivatives such as quinoline and 8-aminoquinoline that cannot chelate efficiently, were performed under this condition, no product was formed indicating the requirement of metal (cobalt)-chelation with quinoline derivatives. A recent report8f revealed that among C2, C4, C5, and C7 carbon atoms of 8-aminoquinoline amide, the most electrophilic reactive site is C5, based on natural charge distribution and pz orbital occupancy calculation in a metal (Cu) chelated 8-aminoquinoline amide complex. In this context, in the present reaction the alkyl radical center of ether, being more electron rich,15 is more likely to approach relatively less electrophilic (more nucleophilic) reactive site C4 rather than the more electron-rich center C5. Based on these facts, we proposed a probable mechanism as outlined in Scheme 4. Under irradiation with visible light in the
methyl 8-aminoquinoline amide and 6-methoxy 8-aminoquinoline amide were subjected to this reaction under the same conditions, and the corresponding 4-tetrahydrofuranyl products were obtained (22, 23, and 24). The derivatives of amino acid, such as tert-butyl ((2S,3S)-3-methyl-1-oxo-1-(quinolin-8ylamino)pentan-2-yl)carbamate and tert-butyl (1-oxo-3-phenyl-1-(quinolin-8-ylamino)propan-2-yl)carbamate also produced the respective products (26 and 27). After successful reaction with tetrahydrofuran, we next tried the reaction with other cyclic and acyclic ethers, and the results are summarized in Scheme 3. The reaction with tetrahydropyrScheme 3. Reaction of Quinoline Amides with Other Ether Derivativesa,b
Scheme 4. Possible Reaction Pathway a
Reaction conditions: amide (0.2 mmol), ether (1 mL), Co(acac)2 (10 mol %), eosin Y (2 mol %), TBHP in decane (4 equiv), Cs2CO3 (2 equiv), DMSO (2 mL), 32 W CFL lamp, nitrogen, 4 h, rt. bIsolated yield. c6 h. d1.5 mL of diethyl ether was used.
an also proceeds in the same way producing the 4-functionalized one (28). 1,4-Dioxane also reacted with amides to afford the corresponding C-4 quinoline amide ether derivatives (29− 33). The formation of the C-4-substituted product was confirmed by single crystal XRD analysis of a representative compound 29 (see the ORTEP diagram in the SI). Interestingly, the reaction with 1,3-dioxolane furnished a better yield (81%) of the corresponding product (34). Besides cyclic ether, acyclic dialkyl ether was also investigated, and the respective arylated products were obtained (35−38). In general, the reactions are clean, although a considerable amount (20−25%) of starting quinoline amide remained unreacted and side products are formed after prolonged irradiation. Only one product (C-4-alkylated) was isolated besides the starting material. We did not isolate any C-5substituted product. Thus, this reaction is unique in providing only 4-functionalized quinolinamide in contrast to reported reactions producing 5-substituted quinolinamide.8 We found only one report of C-4 phosphonation in this system,9c and no report of alkylation at C-4 position of 8-aminoquinoline amide via C−H activation is available so far. The use of cobalt salt for this type of remote functionalization is also not reported. The products are purified by simple column chromatography, and many of them are reported for the first time. To understand the reaction mechanism, a series of control experiments were performed (see Scheme S1). To find the possible pathway (ionic/radical) when the reaction was performed in the presence of a radical scavenger, 2,2,6,6tetramethyl-1-piperidinyloxy (TEMPO) formation of no
presence of photocatalyst eosin Y, tert-butyl hydrogen peroxide (TBHP) is dissociated to release tert-butyloxy radical, which abstracts α-proton of THF to generate the corresponding αoxyalkyl radical species. This radical interacts with the cobalt chelated quinoline complex B to form the intermediate C. The intermediate C (radical species) forms the cationic intermediate D by interaction with the excited cationic radical eosin Y. The complex D produced the product by exclusion of H+ and regeneration of cobalt catalyst. In conclusion, we have developed a cobalt-catalyzed selective remote C-4 alkylation of 8-aminoquinoline amides via C−H activation under irradiation with a CFL lamp in the presence of eosin Y at room temperature. This process is unique as it offers C-4 functionalization of 8-aminoquinoline amides in contrast to the usual C-5 functionalization.8 We are not aware of any previous reports of C-4 substitution of the quinoline amide unit by an ether moiety. The other attractive features of this protocol are reaction at room temperature with a CFL lamp irradiation, use of inexpensive cobalt catalyst, and wide scope of reaction with a variety of quinoline amide derivatives and ethers. This protocol provides easy access to pharmaceutically 1013
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Organic Letters important α-heteroarylated ether derivatives. In addition, this method is of potential use for further applications to other systems.
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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.7b03955. Typical experimental procedure and characterization data of all products and 1H and 13C NMR spectra (PDF) Accession Codes
CCDC 1812296 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 data_
[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 Author
*E-mail:
[email protected]. ORCID
Brindaban C. Ranu: 0000-0001-9020-1401 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from Indian National Science Academy, New Delhi, under the offer of an INSA Senior Scientist position to B.C.R. is gratefully acknowledged.
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REFERENCES
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DOI: 10.1021/acs.orglett.7b03955 Org. Lett. 2018, 20, 1011−1014
