Expeditious Synthesis of Isoquinolone Derivatives by Rhodium(I

Letter Cite This: Org. Lett. 2019, 21, 185−189

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Expeditious Synthesis of Isoquinolone Derivatives by Rhodium(I)Catalyzed Annulation Reaction through C−C Bond Cleavage Yiyi He,† Chengsha Yuan,† Zeqi Jiang, Li Shuai, and Qing Xiao* College of Pharmacy, Third Military Medical University, Chongqing 400038, China

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ABSTRACT: A Rh(I)-catalyzed intermolecular cyclization between isocyanates and benzocyclobutenols leading to isoquinolin-1(2H)-ones through selective cleavage of a C−C bond has been realized. Exploiting the same strategy, we developed a Rh(I)-catalyzed three-component reaction of benzocyclobutenols, isonitriles, and sulfonyl azides to access isoquinolin-1(2H)-imines. These procedures provide unique and expeditious access to isoquinolone derivatives which are otherwise difficult to prepare in satisfactory yields with excellent functional-group tolerance under mild reaction conditions.

A

s structural motifs widely found in diverse natural products,1 isoquinolones play important roles in pharmaceutical chemistry. Compounds containing the isoquinolone skeleton are famous for their antitumor, antiallergic, and antipsychotic activities.2 Consequently, great efforts of synthetic chemists have been devoted to establishing some efficient procedures for the preparation of isoquinolone derivatives in recent years.3−11 Among these, the most amazing and popular strategy is the transition-metal-catalyzed oxidative annulation of benzamide derivatives with alkynes through C−H/N−H dual activation.4,5 Several hypervalent transition-metal catalysts, such as RhIII,6 RuII,7 CoIII,8 PdII,9 and FeIII,10 can be employed in this transformation (Figure 1).11 Despite considerable advantages, these reactions usually require stoichiometric amounts of external or internal oxidants, and substituent groups on the

nitrogen atoms of products are restricted because of the limitation of amides as directing groups in the transformation. The regioselectivity of alkynes is not good in most cases. Thus, it is still in demand to develop innovative methods to obtain isoquinolone derivatives via distinctive catalytic pathways. Transition-metal-catalyzed “cut and sew”12a transformation through selective C−C single bond cleavage12 and subsequent functionalization has recently emerged as a novel and valuable strategy for the synthesis of complex molecular structures. Mechanistically, most C−C bond cleavage processes are initiated by oxidative addition or β-carbon elimination.12c Due to the release of ring strain via site-selective β-carbon elimination, benzocyclobutenols (BCBs)13−16 have been employed as privileged synthons in the “cut and sew” transformation under mild conditions. In this context, Murakami et al. and He et al. reported the rhodium-catalyzed cyclization of BCBs with alkynes,13a vinyl ketones,13b and allenes.13c A wide array of useful dihydro- or tetrahydronaphthalene derivatives were achieved in high yields with excellent diastereoselectivity. Along this line, Zhu et al. developed an iridium-catalyzed annulation of ring-fused BCBs with alkynes to efficiently prepare polycyclic aromatic hydrocarbons (PAHs).13d These reactions can be considered as formal twocarbon insertion into a C−C bond. Merging the “cut and sew” transformation and carbene chemistry, Wang et al. disclosed a rhodium-catalyzed annulation of BCBs with diazo esters to produce indanol derivatives.14a This reaction can be regarded as a formal carbene (one carbon) insertion into a C−C bond.14 Encouraged by these remarkable findings, we expected to exploit this strategy in the field of heterocyclic synthetic chemistry and

Figure 1. Strategies to synthesize isoquinolone derivatives.

Received: November 15, 2018 Published: December 14, 2018

© 2018 American Chemical Society

185

DOI: 10.1021/acs.orglett.8b03653 Org. Lett. 2019, 21, 185−189

Letter

Organic Letters expand the range of molecular fragments inserting into the C−C bond. Herein, we report our preliminary results: rhodium(I)catalyzed annulation of BCBs with isocyanates and a threecomponent reaction of BCBs, isonitriles, and sulfonyl azides (Figure 1). Both reactions can be considered as a formal CN fragment insertion into a C−C bond. These procedures afford complementary methodologies for the rapid construction of isoquinolone scaffolds in good yields with excellent functionalgroup tolerance. Our initial attempts were aimed at studying the model reaction of 8-MeO BCB 1a and aryl isocyanate 2a (Table 1). Table 1. Effect of Reaction Parametersa

isolated yield (%) entry

variation of standard conditions I

1a

3a

4

.

1 2 3 4 5 6 7

none without CF3CO2H HCl instead of CF3CO2H [RhCl(cod)]2 instead of [RhOH(cod)]2 [IrOMe(cod)]2 instead of [RhOH(cod)]2 PhMe instead of DCE room temperature instead of 50 °C

0 0 0 0 0 0 45

84 22 65 32 64 35 34

0 0 0 16 0 55 17

0 56 0 0 0 0 0

a

With 0.2 mmol 1a and 0.24 mmol 2a in 1 mL of DCE.

After a series of experiments, we found the optimal conditions for the reaction: 2 mol % of [RhOH(cod)]2 catalyzed the process at 50 °C in DCE for 5 h, and subsequent dehydration reaction with TFA furnished isoquinolone 3a in 84% isolated yield (entry 1). Control experiments established the importance of [RhOH(cod)]2 and TFA for efficient formation of target molecule 3a (entries 2−5). Without the acid-promoted dehydration process, only 22% 3a and 56% 5 could be obtained (entry 2). Other low valence state metal catalysts, such as [RhCl(cod)]2 and [IrOMe(cod)]2, also enabled the reaction, in low yields (entries 4 and 5) with poor chemoselectivity (entry 4). Replacing DCE with toluene, ring-opening product 4 became the main product of the reaction (entry 6). In addition, transformation required being heated at 50 °C to avoid generation of byproduct 4 (entry 7). We studied the scope of the substrates (Figure 2). Substituent phenyl isocyanates with either an electron-donating or electronwithdrawing group on the benzene ring were able to undergo annulation with BCB 1b to generate corresponding products in good to excellent yields (3b−3l). Reaction conditions were compatible with alkyl, bromide, chloride, fluoride, methoxy, ester, and trifluoromethyl groups on the ortho-, meta-, and paraposition of the benzene ring. Alkyl, benzyl, and naphthyl isocyanates were also proven to be useful starting materials for efficient construction of isoquinolones 3m−3p. Further exploration demonstrated that the reaction proceeded successfully with a variety of combinations of BCBs and isocyanates. Diverse substituents on the phenyl ring of BCB showed essentially no effect on the reaction (3q−3ac). Remarkably, the presence of the Cl, Br, and I in 3e, 3j, 3l, 3t−3w, 3ab, and 3ac

Figure 2. Effect of reaction parameters. All reactions were carried out with 0.2 mmol 1 and 0.24 mmol 2 in 1 mL of DCE.

provides an opportunity for later functionalization of the molecule through cross-coupling reactions. Except for a methyl group, BCBs bearing an ethyl, isopropyl, phenyl, or benzyl group adjacent to the α-carbon of the tertiary alcohol can participate in this reaction smoothly (3w, 3ad−3ah). As analogues of BCB, naphthocyclobutenol and phenanthocyclobutenol can also be effectively used in this reaction (3ai, 3aj, 3ao). Inspired by Zhu’s recent report, we realized the reaction using ring-fused BCBs as substrates to construct isoquinolone[c]-fused cyclic compounds (3ak−3ao). Satisfactory results indicate the value of our method because these products are otherwise difficult to prepare. To produce isoquinolone dimers, we investigated double annulation between BCBs and commercially available diisocyanates. The desired products can be easily generated under the standard conditions in moderate yields (3ap, 3aq). In addition, isoquinolones produced by this method can be further functionalized with no trouble (Figure 3). For instance, a C8-selective arylation (a)17a and a C4-selective alkynylation (b)17b were successfully achieved using hypervalent iodine reagents through C−H activation enabled by iridium and gold catalysts, respectively. A bromo group which is important for many organic transformations was accurately introduced onto 186

DOI: 10.1021/acs.orglett.8b03653 Org. Lett. 2019, 21, 185−189

Letter

Organic Letters

reaction temperature (Figure 5, 12a). A series of isoquinolin1(2H)-imines which are difficult to prepare by other means were

Figure 3. Derivative reactions of the isoquinolone products. Reagents, conditions, and isolated yields: (a) 0.15 mmol 3d, 0.18 mmol Ph2IPF6, 5 mol % of [IrCp*Cl2]2, 20 mol % of AgSbF6, 0.15 mL of AcOH, 100 °C, 24 h, 71%; (b) 0.15 mmol 3o, 0.18 mmol 1-[(triisopropylsilyl)ethynyl]-1,2-benziodoxol-3(1H)-one, 10 mol % of AuCl, 2 mL of MeCN (dry), 50 °C, 24 h, 67%; (c) 0.2 mmol 3d, 0.2 mmol NBS, 1 mol % of AuCl3, 2 mL of DCE, 25 °C, 5 h, 95%; (d) 0.2 mmol 3ao, 0.6 mmol DDQ, 2 mL of DCM, 25 °C, 6 h, 95%.

the C4 position of the isoquinolone scaffold in the presence of N-bromosuccinimide (c).17c Isoquinolone[c]-fused cyclic compound 3ao was almost quantitatively oxidized to a N-containing PAH under mild conditions (d).13d To further demonstrate the synthetic utility of this “cut and sew” transformation, we applied this method to the synthesis of a biologically interesting 2,3-diaryl isoquinolinone derivative 9, which exhibits significant antiproliferative and antiangiogenesis activities against human breast cancer cells.1 Through the Rh(I)catalyzed annulation reaction and Pd-catalyzed C−O coupling reaction, the ERα binding reagent and VEGFR-2 inhibitor 8 were prepared. The most potential dual inhibitor 9 can be produced from 8 in one step via a literature procedure (Figure 4).1

Figure 5. Three-component reaction with BCBs, isonitriles, and sulfonyl azides. All reactions were carried out with 0.2 mmol 1, 0.3 mmol 10, and 0.3 mmol 11 in 4 mL of PhMe.

efficiently produced in satisfactory yields with excellent regioselectivity from the combinations of various isonitriles, BCBs, and sulfonyl azides (12a−12l). Furthermore, the structure of 12e (CCDC1866730) is confirmed by X-ray crystallography. Although other azides including acyl, aryl, and alkyl ones have not been smoothly applied in this reaction, it provides a facile method to formally insert an amidino motif [−(CNR′)−NR−] into the C−C bond for construction of N,N′-disubstituted cyclic amidine compounds. Finally, we investigated the mechanism of our reaction (Scheme 1). The observation of byproduct 4 (Table 1, entries 4, 6, and 7) indicated that an aryl−rhodium species might be generated through a β-carbon elimination mechanism. The deuterium labeling experiment A further proved this process in the Rh(I) catalytic cycle (Scheme 1I, I → II).20 Deuterium labeling experiments B and C ruled out another possible route to the aryl−rhodium species from byproduct 4 via C−H activation. Experiment D indicated that cyclization product 5 could be partially converted to isoquinolone 3a in the absence of acid. It was in accordance with the experiment result of the model reaction of 1a and 2a (Table 1, entry 2). Unexpectedly, the annulation reaction of 1a and prepared carbodiimide 1319d failed under standard conditions II (Scheme 1E). In other words, the three-component reaction cannot be described as an intermolecular cyclization of BCBs with carbodiimides generated in situ. The results of experiments F and G revealed that the isoquinolin-1(2H)-imines cannot be obtained through a tandem reaction of BCBs with azides and then with isonitriles. Experiment H might be explained by the insertion of isonitrile 10a into the aryl−rhodium(I) bond of intermediate II being a fast and reversible process (Scheme 1I). Moreover, to our

Figure 4. Synthesis of antibreast cancer agents.

Through the annulation of BCBs with isocyanates, an amide motif (−CONR−) can be inserted into a C−C bond efficiently. Encouraged by this finding, we searched for other nitrogenous molecular motifs that can be introduced into the corresponding heterocyclic products by exploiting the same “cut and sew” strategy.12a Unfortunately, the analogues of isocyanates, such as thioisocyanates and carbodiimides, cannot be used driectly in this transformation after multiple attempts. In consideration of the fact that the inserted molecular segment is derived from not only one reactant but also several substrates, the main emphasis of our research shifted to developing mutilcomponent reactions in this field. As we know, carbodiimide intermediates, especially some unstable ones, which were recently used in coupling reactions, can be conveniently generated from transition-metalcatalyzed nitrene transformation of azides with isonitriles.18,19 Hence, we investigated the three-component reaction of BCB 1a, 1,3-dimethylphenyl isonitrile 10a, and tosyl azide 11a. The reaction was performed successfully by the same Rh(I) catalysis, but replacing the DCE solvent with toluene and increasing the 187

DOI: 10.1021/acs.orglett.8b03653 Org. Lett. 2019, 21, 185−189

Letter

Organic Letters Scheme 1. Mechanistic Studies and Discussion

Detailed experimental procedures, characterization data, and copies of 1H and 13C NMR spectra for all new products (PDF) Accession Codes

CCDC 1866730 (12e) 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 [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

Qing Xiao: 0000-0002-9019-937X Author Contributions †

Y.H. and C.Y. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by The National Key Research and Development Program of China (Grant No. 2018YFA0507900), the Scientific Research Foundation for High-Level Talent Introduction Plan (Grant No. XZ215429), and the Foundation for Transformation of Sci-tech Achievements (Grant No. 2016XZH02) of Third Military Medical University.

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knowledge, because of the relative electron deficiency of the metal center, intermediate V is more reactive to decompose azide 11a for nitrene formation than intermediate II.21 Based on these facts, we preferred a stepwise catalytic pathway to illuminate the generation of intermediate VII (Scheme 1I, II → V → VI → VII). In addition, the confirmed structure of 12e and the excellent regioselectivity of the reaction might be rationalized by the [1,3]-migration22 and the follow-up insertion from intermediate VII to intermediate IX (Scheme 1I). In conclusion, we have disclosed an annulation reaction between benzocyclobutenols and isocyanates as well as a threecomponent reaction of benzocyclobutenols, isonitriles, and sulfonyl azides for the expeditious synthesis of isoquinolone derivatives. These reactions show excellent regioselectivity and functional-group tolerance and provide an efficient approach to isoquinolone[c]-fused cyclic compounds and isoquinolin1(2H)-imines, which are otherwise difficult to prepare. In these transformations, a variety of substituent groups can be easily introduced to the nitrogen atoms of the products, while isoquinolone scaffolds are constructed. Moreover, the reactions afford the first examples of formal amide and amidino fragment insertion into a C−C bond, which may open new possibilities in the development of other “cut and sew” transformations for the construction of heterocyclic compounds.

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REFERENCES

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Letter

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DOI: 10.1021/acs.orglett.8b03653 Org. Lett. 2019, 21, 185−189