Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
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Cobalt(III)-Catalyzed [4 + 2] Annulation of N‑Chlorobenzamides with Maleimides Nachimuthu Muniraj and Kandikere Ramaiah Prabhu* Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012, Karnataka, India
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ABSTRACT: A Co(III)-catalyzed novel [4 + 2] annulation of N-chlorobenzamides with maleimides has been reported. Mostly, maleimides are known to furnish the Michael-type or 1,1-type cyclized products while treating with amides. In this reaction, maleimides furnished [4 + 2] annulated products in good yields at room temperature while being treated with the internal oxidizing N-chlorobenzamide as a directing group. The developed methodology is compatible with a variety of functional groups. The [4 + 2] annulated products obtained are featured in some biologically active molecules.
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Scheme 1. Comparison with Previous Work
aleimide derivatives have their footprints in C−H activation reactions forming the corresponding succinimide derivatives, which feature in numerous biologically and pharmaceutically active molecules and natural products.1 The succinimide moieties can be readily converted into biologically relevant pyrrolidines, γ-lactams, and synthetically useful derivatives.2 We have shown the first successful conjugate addition of maleimides, under C−H activation strategy, to obtain 3-arylated succinimides.3,4 Eventually, other groups5 have also designed a variety of catalytic systems for the conjugate addition reactions of maleimides. Maleimides are not only known for conjugate addition reactions but also known for 1,1-type, 1,2-type cyclization reactions6 and the basemediated Heck-type reaction7 with the substituted aromatics. The amide is one of the most common functional groups, which serves as a weak and versatile directing group in many proximal C−H bond functionalization reactions.8 Generally, the reaction of maleimides with amides under the transitionmetal-catalyzed C−H activation reactions leads to the formation of the corresponding Michael-type (conjugate addition)4a,5f products or 1,1-cyclized (spirocyclic-type)6b,c products, which is dictated by (i) the nature of the directing group (mono or bidentate), (ii) choice of transition metals, and (iii) the reaction conditions (Scheme 1a). Recently, the research group of Zhu has demonstrated the utility of Nchlorobenzamide in the directing group strategy, shedding light on its internal oxidizing power.9 However, there are no reports on the formation of [4 + 2] annulation-type products in the reaction of amides with maleimide (Scheme 1b) to obtain the pyrroloisoquinoline trione moiety, which features in a few biologically active molecules as shown in Figure 1.10 Triggered by these facts and in pursuit of our efforts on the utility of maleimides in the directing group chemistry,3,4 we directed our efforts11 to achieve a hitherto unknown [4 + 2] annulation reaction of maleimides with benzamides. As a result, herein we © XXXX American Chemical Society
report the [4 + 2] annulation reaction of N-chlorobenzamide with maleimides at room temperature in the presence of airstable, highly abundant, and cost-effective Co(III) catalyst.12 The optimization studies, by reacting N-chlorobenzamide (1a, 0.2 mmol) with N-ethylmaleimide (2a, 0.3 mmol), Cp*CoCOI2 (10 mol %), AgOAc (20 mol %), and NaOAc (1.2 equiv) in DCE (1 mL) at room temperature for 24 h, revealed the exclusive formation of a [4 + 2] annulated product 3aa in 40% yield as a single diastereomer (entry 1, Table 1). Generally, the reactions of benzamide and maleimide are known to form the corresponding conjugate addition product4a,5f or 1,1-type cyclic product (spirocyclic-type).6b,c Interestingly, in this reaction, neither the formation of the conjugate addition product nor 1,1-type cyclic product was observed. By solvent screening studies, it was found that TFE is the best solvent for this reaction, which furnished the product 3aa in 82% isolated yield (entries 2−5). Among Received: December 25, 2018
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DOI: 10.1021/acs.orglett.8b04117 Org. Lett. XXXX, XXX, XXX−XXX
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whereas the reaction in the absence of additive or cobalt catalyst resulted either in no reaction or the formation of the product 3aa in trace amounts (entries 17−19). Therefore, the conditions as shown in entry 2 were found to be optimal reaction conditions, which employ the reaction of Nchlorobenzamide (1a, 0.2 mmol) with N-ethylmaleimide (2a, 0.3 mmol), Cp*CoCOI2 (10 mol %), AgOAc (20 mol %), and NaOAc (1.2 equiv) in TFE (1 mL) at room temperature for 24 h, furnishing the product 3aa in 82% isolated yield. Using the optimal conditions, the scope of the reaction with N-chlorobenzamide derivatives has been evaluated (Scheme 2). Thus, the reaction of 4-Me- and 3-Me-substituted NScheme 2. Substrate for N-Chlorobenzamidesa,b
Figure 1. Bioactive pyrroloisoquinoline trione derivatives.
Table 1. Optimization Studiesa
entry
Ag (20 mol %)
additive (1.2 equiv)
solvent
NMR yield 3aab (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
AgOAc AgOAc AgOAc AgOAc AgOAc AgSbF6 AgOTf AgPF6 Ag2CO3 AgOAc AgOAc AgOAc AgOAc AgOAc AgOAc AgOAc none AgOAc AgOAc
NaOAc NaOAc NaOAc NaOAc NaOAc NaOAc NaOAc NaOAc NaOAc KOAc CsOAc LiOAc NaOPiv NaOAc NaOAc NaOAc NaOAc none NaOAc
DCE TFE MeOH DCM THF TFE TFE TFE TFE TFE TFE TFE TFE TFE TFE TFE TFE TFE TFE
40 85 (82)c nd 25 traces 46 58 68 70 75 60 50 68 55d 20e 64f 48 traces ndg
a
Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), [Cp*Co(CO)I2] (10 mol %), AgOAc (20 mol %), NaOAc (1.2 equiv), TFE (1 mL), rt for 24 h. bIsolated yields. c2 mmol scale. dNMR yield.
chlorobenzamides with N-ethylmaleimide 2a afforded the corresponding annulated products 3ba and 3ca in 75 and 70% yields, respectively. It is noteworthy to mention that 3methyl-substituted N-chlorobenzamide showed reactivity at the sterically free ortho-position. Halogenated N- chlorobenzamides displayed good reactivity with N-ethylmaleimide, providing the corresponding annulated products 3da, 3ea, 3fa, and 3ga in 86, 84, 85, and 80% yields, respectively. 2Fluoro-substituted N-chlorobenzamide furnished the product 3ha in low yield (20%). N-Chlorobenzamide derivatives having electron-withdrawing groups such as −NO2, −CN, and −CF3 at the para-position underwent a smooth reaction with 2a, providing products 3ia, 3ja, and 3ka in 80, 91, and 93% yields, respectively. Naphthyl-derived N-chloroamide also afforded the corresponding annulated product 3la in 82% yield. The reaction of aliphatic N-chloroamide such as N-chloromethacrylamide with 2a furnished the corresponding annulated product 3ma in 30% yield. To showcase the efficacy of this reaction, a 2 mmol scale reaction was performed, which afforded the product 3aa in 80% isolated yield. Next, we turned our attention to explore the scope of this reaction with various maleimide derivatives (Scheme 3). Thus, N-alkylmaleimides such as isopropyl-, tert-butyl-, cyclohexyl-,
a
Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), [Cp*Co(CO)I2] (10 mol %), silver salt (20 mol %), additive (1.2 equiv), solvent (1 mL), rt for 24 h. b1H NMR yield (using terephthaldehyde as an internal standard). cIsolated yield. dReaction at 50 °C. e20 mol % of NaOAc. f5 mol % of [Cp*Co(CO)I2]. gAbsence of cobalt catalyst. nd = not detected. DCE = 1,2-dichloroethane, TFE = 2,2,2trifluoroethanol.
several silver salts, AgOAc was found to be better (entry 2), whereas other silver salts such as AgSbF6, AgOTf, AgPF6, and Ag2CO3 did not assist in improvising the yield of 3aa (entries 6−9). As can be seen, NaOAc served as a good additive, while KOAc, CsOAc, LiOAc, and NaOPiv furnished the product 3aa in low yields (entries 10−13). Variation in the reaction conditions such as increasing the reaction temperature to 50 °C, or decreasing the amount of additive (NaOAc) to 20 mol % or decreasing the catalyst loading to 5 mol %, did not bring any significant improvement in the outcome of the reaction (entries 14−16). The reaction in the absence of silver salt resulted in the formation of the product 3aa in low yield, B
DOI: 10.1021/acs.orglett.8b04117 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Scheme 3. Scope for Maleimide Derivativesa,b
Scheme 4. Reactivity of Other Internal Oxidizing DG
undergo oxidation than the N−O bond under the reaction conditions.9 After the scope of the reaction was studied, to gain insight into the reaction mechanism, a few control experiments were performed (Scheme 5). In the reaction of 1a with D2O under Scheme 5. Control Experiments
a
Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), [Cp*Co(CO)I2] (10 mol %), AgOAc (20 mol %), NaOAc (1.2 equiv), TFE (1 mL), rt for 24 h. bIsolated yields.
optimal conditions using TFE as a solvent, the ortho-hydrogens of N-chlorobenzamide were not deuterated, which can be attributed to the rapid deuterium exchange with the solvent TFE. Therefore, we performed the same reaction of 1a with D2O in DCE as a solvent to obtain deuterio-1a in 50% yield with 80% of deuterium incorporation. This reaction suggests that the C−H activation may be a reversible step. The reaction involving 1a/1a-d2 with 2a showed the KIE value of 2.33, indicating the C−H activation step might be a turnoverlimiting step. The competitive experiment between two Nchlorobenzamide derivatives with different electronic natures (1d and 1b) with 2a indicated that the reaction favors the electron-poor 1d against electron-rich 1b in a ratio of 4:1. On the basis of the control experiments and literature precedence,9 a plausible mechanism has been proposed in Scheme 6. The in situ generated catalytically active species A reacts with the anionic form of 1a leading to the cobaltacycle B followed by the oxidation of CoIII-to-CoV to form the intermediate C. Insertion of maleimide to C forming the 7membered intermediate D followed by a reductive elimination forms the CoIII intermediate E. Subsequent protodemetalation of species E leads to the formation of the desired product 3aa along with the regeneration of CoIII catalyst. In conclusion, we have developed a novel Co(III)-catalyzed [4 + 2] annulation reaction of maleimides using an internally oxidizable N-chloroamide as a directing group at room temperature. The highlight of the reaction is the exclusive formation of [4 + 2] annulated products, and the expected conjugate addition product or 1,1-type cyclic product was not observed. This protocol has been applied to a wide range of
and benzyl-substituted maleimides underwent a smooth reaction with 1a affording the products 4ab, 4ac, 4ad, and 4ae in 81, 77, 75, and 70% yields, respectively. N-Arylated maleimides such as phenyl-, anisole-, tolyl-, and iodobenzenederived maleimides also shown good reactivity with 1a, furnishing the products 4af, 4ag, 4ah, and 4ai in 72, 78, 72, and 68% yields, respectively. Reactive functional groups such as ester and keto groups on the aryl ring of N-arylated maleimides displayed a good reactivity, forming the products 4aj and 4ak in moderate yields of 54 and 67%, respectively. Further, the scope of the reaction has been expanded by attempting the reactions of substituted N-chloroamide derivatives with maleimide derivatives. Thus, the reaction of N-chloro-4-methylbenzamide with N-benzylmaleimide furnished the corresponding cyclic product 4be in 70% yield. Similarly, the reaction of 4-bromo-N-chlorobenzamide with Nphenylmaleimide was also facile to form the product 4ff in 74% yield. The reaction of N-chloro-2-naphthamide with Nisopropylmaleimide was also facile, furnishing the corresponding annulated product 4lb in 80% yield. Unprotected maleimide, maleic ester, and maleic anhydride were found to be unreactive under the reaction conditions. Further, we examined the reactivity of other internal oxidizing directing groups such as N-hydroxybenzamide and N-methoxybenzamide for similar transformations under the optimal conditions (Scheme 4). However, these reactions did not afford the corresponding annulated product 3aa. This observation indicates the N−Cl bond is more prone to C
DOI: 10.1021/acs.orglett.8b04117 Org. Lett. XXXX, XXX, XXX−XXX
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of Succinimide-Derived Negative Allosteric Modulators of Metabotropic Glutamate Receptor Subtype 1 Provides Insight into a Disconnect in Activity between the Rat and Human Receptors. ACS Chem. Neurosci. 2014, 5, 597−610. (2) (a) Hubert, J. C.; Wijnberg, J. B. P. A.; Speckamp, W. N. NaBH4 Reduction of Cyclic Imides. Tetrahedron 1975, 31, 1437. (b) Wijnberg, J. B. P. A.; Schoemaker, H. E.; Speckamp, W. N. A Regioselective Reduction of Gem-Disubstituted Succinimides. Tetrahedron 1978, 34, 179. (c) Katritzky, A. R.; Yao, J.; Qi, M.; Chou, Y.; Sikora, D. J.; Davis, S. Ring Opening Reactions of Succinimides. Heterocycles 1998, 48, 2677. (d) Gali, H.; Prabhu, K. R.; Karra, S. R.; Katti, K. V. Facile Ring-Opening Reactions of Phthalimides as a New Strategy to Synthesize Amide-Functionalized Phosphonates, Primary Phosphines, and Bisphosphines. J. Org. Chem. 2000, 65, 676. (3) (a) Lanke, V.; Bettadapur, K. R.; Prabhu, K. R. Site-Selective Addition of Maleimide to Indole at the C-2 Position: Ru(II)Catalyzed C−H Activation. Org. Lett. 2015, 17, 4662. (b) Bettadapur, K. R.; Lanke, V.; Prabhu, K. R. Ru (II)-Catalyzed C−H Activation: Ketone-Directed Novel 1,4-Addition of Ortho C−H Bond to Maleimides. Org. Lett. 2015, 17, 4658. (4) (a) Keshri, P.; Bettadapur, K. R.; Lanke, V.; Prabhu, K. R. Ru(II)-Catalyzed C−H Activation: Amide-Directed 1,4- Addition of the Ortho C−H Bond to Maleimides. J. Org. Chem. 2016, 81, 6056. (b) Bettadapur, K. R.; Lanke, V.; Prabhu, K. R. A Deciduous Directing Group Approach for the Addition of Aryl and Vinyl Nucleophiles to Maleimides. Chem. Commun. 2017, 53, 6251. (c) Muniraj, N.; Prabhu, K. R. Cobalt(III)-Catalyzed C−H Activation: Azo Directed Selective 1,4-Addition of Ortho C−H Bond to Maleimides. J. Org. Chem. 2017, 82, 6913. (d) Muniraj, N.; Prabhu, K. R. Co(III)-Catalyzed C−H Activation: A Site-Selective Conjugate Addition of Maleimide to Indole at the C-2 Position. ACS Omega 2017, 2, 4470. (5) Selected examples: (a) Han, S.; Park, J.; Kim, S.; Lee, S. H.; Sharma, S.; Mishra, N. K.; Jung, Y. H.; Kim, S. Rhodium(III)Catalyzed C(sp3)−H Alkylation of 8-Methylquinolines with Maleimides. Org. Lett. 2016, 18, 4666. (b) Han, S. H.; Kim, S.; De, U.; Mishra, N. K.; Park, J.; Sharma, S.; Kwak, J. H.; Han, S.; Kim, H. S.; Kim, I. S. Synthesis of Succinimide-Containing Chromones, Naphthoquinones, and Xanthones under Rh(III) Catalysis: Evaluation of Anticancer Activity. J. Org. Chem. 2016, 81, 12416. (c) Yu, W.; Zhang, W.; Liu, Y.; Liu, Z.; Zhang, Y. Cobalt(III)-Catalyzed CrossCoupling of Enamides with Allyl Acetates/Maleimides. Org. Chem. Front. 2017, 4, 77. (d) Zhang, Z.; Han, S.; Tang, M.; Ackermann, L.; Li, J. C−H Alkylations of (Hetero)Arenes by Maleimides and Maleate Esters through Cobalt(III) Catalysis. Org. Lett. 2017, 19, 3315. (e) Liu, S.-L.; Li, Y.; Guo, J.-R.; Yang, G.-C.; Li, X.-H.; Gong, J.-F.; Song, M.-P. An Approach to 3-(Indol-2-yl)succinimide Derivatives by Manganese Catalyzed C−H Activation. Org. Lett. 2017, 19, 4042. (f) He, Q.; Yamaguchi, T.; Chatani, N. Rh(I)-Catalyzed Alkylation of ortho-C−H Bonds in Aromatic Amides with Maleimides. Org. Lett. 2017, 19, 4544. (g) Mandal, A.; Sahoo, H.; Dana, S.; Baidya, M. Ruthenium(II)-Catalyzed Hydroarylation of Maleimides Using Carboxylic Acids as a Traceless Directing Group. Org. Lett. 2017, 19, 4138. (h) Mandal, R.; Emayavaramban, B.; Sundararaju, B. Cp*Co(III)-Catalyzed C−H Alkylation with Maleimides Using Weakly Coordinating Carbonyl Directing Groups. Org. Lett. 2018, 20, 2835. (i) Pan, C.; Wang, Y.; Wu, C.; Yu, J.-T. Rhodium-Catalyzed C7-Alkylation of Indolines with Maleimides. Org. Biomol. Chem. 2018, 16, 693. (j) Yu, J.-T.; Chen, R.; Jia, H.; Pan, C. Rhodium-Catalyzed Site-Selective ortho-C−H Activation: Enone Carbonyl Directed Hydroarylation of Maleimides. J. Org. Chem. 2018, 83, 12086. (6) (a) Lv, N.; Liu, Y.; Xiong, C.; Liu, Z.; Zhang, Y. CobaltCatalyzed Oxidant-Free Spirocycle Synthesis by Liberation of Hydrogen. Org. Lett. 2017, 19, 4640. (b) Miura, W.; Hirano, K.; Miura, M. Copper-Mediated Oxidative Coupling of Benzamides with Maleimides via Directed C−H Cleavage. Org. Lett. 2015, 17, 4034. (c) Manoharan, R.; Jeganmohan, M. Cobalt-Catalyzed Oxidative Cyclization of Benzamides with Maleimides: Synthesis of Isoindolone
Scheme 6. Plausible Mechanism
substrates, and this transformation is compatible with a variety of functional groups. The obtained pyrroloisoquinoline trione moieties feature in a few biologically active molecules.
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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.8b04117. Experimental procedures, characterization data, and spectra for all compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Nachimuthu Muniraj: 0000-0003-4728-6141 Kandikere Ramaiah Prabhu: 0000-0002-8342-1534 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by SERB (EMR/2016/006358), New-Delhi, CSIR (No. 02(0226)15/EMR-II), New-Delhi, Indian Institute of Science, and R. L. Fine Chem. We thank Dr. A. R. Ramesha (R. L. Fine Chem) for useful discussions. N.M. thanks UGC, New Delhi, for a fellowship.
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REFERENCES
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DOI: 10.1021/acs.orglett.8b04117 Org. Lett. XXXX, XXX, XXX−XXX
