Palladium(0)-Catalyzed Intermolecular Asymmetric Allylic

Letter pubs.acs.org/OrgLett

Cite This: Org. Lett. 2018, 20, 748−751

Palladium(0)-Catalyzed Intermolecular Asymmetric Allylic Dearomatization of Polycyclic Indoles Run-Duo Gao,† Lu Ding,‡ Chao Zheng,† Li-Xin Dai,† and Shu-Li You*,†,‡ †

State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, China ‡ School of Physical Science and Technology, ShanghaiTech University, 100 Haike Road, Shanghai 201210, China S Supporting Information *

ABSTRACT: An intermolecular Pd-catalyzed allylic dearomatization reaction of polycyclic indoles with substituted allylic carbonates was realized in the presence of a newly synthesized chiral phosphoramidite ligand. Various polycyclic indoline and indolenine derivatives were successfully synthesized in excellent yields (up to 99%) with excellent enantioselectivity (up to 98% ee). The obtained products could undergo versatile transformations, increasing the application potential of the method in organic synthesis.

P

Scheme 1. Pd-Catalyzed Asymmetric Allylic Dearomatization

olycyclic indole derivatives are structural cores for numerous natural products and pharmaceuticals with promising biological activity.1 Consequently, enormous efforts have been devoted to the development of efficient and convenient regio- and enantioselective construction and functionalization of these skeletons. Among them, catalytic asymmetric dearomatization (CADA) reactions of indole derivatives provide an efficient and rapid access to these valuable scaffolds.2−4 Recently, transition-metal-catalyzed allylic dearomatization reactions of indoles have been demonstrated to be highly efficient for the synthesis of substituted indolines not only in good yields but also with excellent diastereoselectivity and enantioselectivity.5 For the pioneering asymmetric intermolecular allylic dearomatization reaction, the Trost6 group reported an innovative enantioselective C-3 allylic allylation of 3-substituted indoles from simple allylic alcohol in the presence of a bulky borane 9-BBN-C6H13 as the promoter (Scheme 1(a)). Despite the progress in transition-metal-catalyzed asymmetric allylic dearomatization reactions of indoles, challenges remain. Highly efficient catalysts that enable good enantioselective control for polycyclic indole substrates are still limited. In addition, there are only a few reports on the allylic dearomatization reactions of sterically hindered allyl precursors, especially allylic carbonates bearing a substituent at the C2 © 2018 American Chemical Society

position. Herein, we report an asymmetric intermolecular allylic dearomatization of polycyclic indoles with sterically hindered allyl carbonates, providing polycyclic indoline derivatives in a highly enantioselective manner. A newly synthesized chiral phosphoramidite ligand plays a key role in the high enantioselective control (Scheme 1(b)). Received: December 13, 2017 Published: January 25, 2018 748

DOI: 10.1021/acs.orglett.7b03887 Org. Lett. 2018, 20, 748−751

Letter

Organic Letters

excellent enantioselectivity were obtained when acetyl was utilized (99% yield, 93% ee, entry 13, Table 1). To be noted, reducing the catalyst loading to 1 mol % led to no notable loss of the yield and enantiomeric purity (98% yield, 92% ee, entry 14, Table 1). Under the optimized reaction conditions (entry 13, Table 1), the substrate scope of the reaction was then explored. As summarized in Scheme 2, substrates bearing either an electron-

At the outset, 1,2,3,4-tetrahydrocyclopentaindole (1a) and methyl (2-methylallyl) carbonate (2a) were chosen to optimize the reaction conditions. To our delight, this reaction proceeded well and the indolenine could be formed in high yield. However, the indolenine was not stable during purification by silica gel column chromatography. Thus, a further protection of indolenine by reacting with methyl chloroformate, which has been added to the reaction mixture in one pot, was carried out to afford stable enamine 3 through isomerization. Then, various ligands were tested in the presence of Pd2dba3 in DCM 50 °C (in a sealed tube) (entries 1−4, Table 1). The

Scheme 2. Substrate Scope for Indoles with Fused Cyclopentanea

Table 1. Optimization of Reaction Conditionsa

entry

ligand

solvent

T (°C)

yieldb (%)

ee (%)

1 2 3 4 5 6 7 8 9 10 11 12 13c 14c,d

L1 L2 L3 L4 L4 L4 L4 L4 L4 L4 L4 L4 L4 L4

DCM DCM DCM DCM DCE toluene EtOAc Et2O THF dioxane DCM DCM DCM DCM

50 50 50 50 50 50 50 50 50 50 40 rt rt rt

54 87 85 75 45 47 52 47 40 54 75 72 99 98

16 38 −40 88 86 48 85 82 76 68 91 93 93 92

a

Reaction conditions: 1 (0.2 mmol), 2 (0.26 mmol), Pd2dba3 (2.5 mol %), and L4 (11 mol %) in DCM at room temperature. After the reaction was complete, pyridine (0.8 mmol) and AcCl (0.6 mmol) were added at room temperature. bThe reaction was carried out at 1.0 mmol scale.

a

Reaction conditions: 1a (0.2 mmol), 2a (0.26 mmol), Pd2dba3 (2.5 mol %), L (11 mol %) under sealed reaction. After the reaction was complete, pyridine (0.8 mmol) and ClCO2Me (0.6 mmol) were added at room temperature, R = CO2Me. bIsolated yield. cAfter the reaction was complete, pyridine (0.8 mmol) and AcCl (0.6 mmol) were added at room temperature, R = Ac. dPd2dba3 (1.0 mol %) and L4 (4.4 mol %) were utilized.

donating group (5-Me, 5-MeO, 3ba, 3ca, Scheme 2) or an electron-withdrawing group (5-F, 5-Cl, 5-Br, 6-Cl, 6-Br, 3fa− 3ja, Scheme 2) on the indole moiety could all lead to their corresponding dearomatized products in excellent yields and enantioselectivity (89−99% yields, 87−94% ee). Notably, almost the same results (98% yield, 94% ee) were obtained when the synthesis of 3ca was performed at 1.0 mmol scale. For disubstituted polycyclic indole, a quantitative yield was achieved, despite a minor decrease in enantioselectivity (3da, 99% yield, 87% ee). The absolute configuration of the dearomatized products was determined as (R) by an X-ray crystallographic analysis of a single crystal of enantiopure 3ja.10 Next, the generality of the reaction with respect to allylic carbonates was tested using 1a as the nucleophile. For the bulkier 2-phenyl substituted allylic carbonate, the dearomatized product 3ab was obtained in 99% yield and 82% ee. To our delight, for the substituents with different electronic properties on the phenyl moiety (4-MeC6H4, 4-MeOC6H4, 4-ClC6H4, 3ClC6H4), their corresponding dearomatized indolines were

Feringa ligand L1 could give a moderate yield but low enantioselectivity (16% ee).7 The reaction with the Carreira P/ olefin ligand L2 gave an increasing yield with 38% ee.8 The utilization of BHPphos (L3),9 developed by our group, afforded a slight improvement of enantioselectivity. Inspired by these results, we decided to modify this ligand and then synthesized 3,3′-(Me)2-BHPphos (L4), which has a similar skeleton to L3. To our delight, good yield and excellent enantioselectivity could be obtained by using L4 as the ligand (75% yield, 88% ee, entry 4, Table 1). After screening several other solvents, including DCE, toluene, EtOAc, Et2O, THF, and dioxane (entries 5−10, Table 1), DCM was still found to be the best choice. By running the reaction at room temperature, the enantioselectivity could be increased to 93% ee with almost no effect on the reactivity (72% yield, 93% ee, entry 12, Table 1). Different protecting groups were tested, and a quantitative yield and 749

DOI: 10.1021/acs.orglett.7b03887 Org. Lett. 2018, 20, 748−751

Letter

Organic Letters

NaBH3CN, affording indoline 5 and converted to enamine 6 after acyl protection, respectively (Scheme 4). In summary, we have developed an intermolecular Pdcatalyzed asymmetric allylic dearomatization reaction of polycyclic indoles with substituted allylic carbonates. With a newly synthesized BINOL-derived phosphoramidite ligand, the reactions proceeded efficiently in excellent yields and enantioselectivity. The reaction conditions tolerated polycyclic indoles with the fused rings ranging from cyclopentane to cyclooctane. The products have been shown to undergo useful transformations.

obtained all in excellent yields and good enantioselectivity (95− 99% yields, 74−85% ee). Scheme 3. Substrate Scope for the Synthesis of Indoleninesa

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03887. Experimental procedures and analysis data for all new compounds (PDF) Accession Codes

a Reaction conditions: 1 (0.2 mmol), 2a (0.26 mmol), Pd2dba3 (2.5 mol %), and L4 (11 mol %) in DCM at room temperature.

CCDC 1590190 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.

For indole substrates with a fused cyclohexane ring, their corresponding indolenines are stable for isolation. The better stability compared with the indolenines derived from cyclopentane-fused indoles is likely due to the release of the ring strain brought by the larger ring size. Under the above optimized conditions, the reaction of cyclohexane fused indole with methyl (2-methylallyl) carbonate (2a) proceeded smoothly to give 4a in 87% yield and 92% ee. Pleasingly, the dearomatization reactions proceeded well with excellent yields and enantioselectivity regardless of the electronic property and position of the substituent on the indole core (5-Me, 5-OMe, 5F, 5-Cl, 5-Br, 6-Cl, 69−94% yields, 87−92% ee, 4b−4g, Scheme 3). Of particular note, for the substrates bearing substituents on the cyclohexane part, dearomatized indolenines 4h−4j were afforded with excellent yields and much improved enantioselectivity (90−98% yields, 95−98% ee). The good compatibility with functional groups such as double bond (4i) and cyclic acetal (4j) will certainly allow more diverse transformations of the products. For substrates bearing a larger ring, such as cycloheptane and cyclooctane, the dearomatization reactions also proceeded well in excellent yields albeit with slightly decreased enantioselectivity for a cyclooctane fused indole substrate (84−93% yields, 78−92% ee). For the indolenine product derived from the dearomatization of cyclohexane fused indole, it could be successfully reduced by

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chao Zheng: 0000-0002-7349-262X Shu-Li You: 0000-0003-4586-8359 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Key Research and Development Program of China (2016YFA0202900), National Basic Research Program of China (973 Program 2015CB856600), the NSFC (21332009, 21421091, 21572252), Science and Technology Commission of Shanghai Municipality (16XD1404300), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000, QYZDYSSW-SLH012) for generous financial support.

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Scheme 4. Transformation of Products

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DOI: 10.1021/acs.orglett.7b03887 Org. Lett. 2018, 20, 748−751