Annulation Cascades of N-Allyl-N-((2-bromoaryl)ethynyl)amides

Apr 2, 2019 - An annulation cascades of N-allyl-N-((2-bromoaryl)ethynyl)amides with terminal alkynes or 1,3-dicarbonyls involving C–H functionalizat...
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Annulation Cascades of N‑Allyl‑N‑((2-bromoaryl)ethynyl)amides Involving C−H Functionalization Rongkui Su,†,‡ Xu-Heng Yang,*,§ Ming Hu,† Qiu-An Wang,† and Jin-Heng Li*,† †

Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, China ‡ Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, School of Metallurgy and Environment, Central South University, Changsha 410083, China § College of Arts and Sciences, National University of Defense Technology, Changsha 410073, China

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S Supporting Information *

ABSTRACT: An annulation cascades of N-allyl-N-((2-bromoaryl)ethynyl)amides with terminal alkynes or 1,3-dicarbonyls involving C−H functionalization for producing 2,3-functionalized indoles has been first developed by means of Cu catalysis. The method is enabled by the formation of the ketenimine intermediates to deliver 2,3-disubstituted indoles through a sequence of aza-Claisen rearrangement, C−H functionalization, Ullmann C−N coupling, and cyclization.

I

continuous efforts toward the development of general and efficient methods to build 2,3-difunctionalized indole architectures.1,3−5 Attractive methods include transition-metalcatalyzed annulation reactions,4,5 which represent an important and uniquely practical alternative to the classical Fischer indole synthesis.3 However, such transition-metal-catalyzed transformations still remain a great challenge of one-step 2,3difunctionalizing indoles which simultaneously incorporate two functional groups with two new chemical bonds. In recent years, the ynamide chemistry has become a powerful entry to increase molecular complexity and build diverse cyclic compounds.6,7 Typical methods include the rearrangement reactions of N-allyl ynamides, in which the ynamide moiety is generally converted into the highly reactive ketenimine intermediate by N-to-C allyl transfer.7 However, such transformations are much less abundant and are restricted to functionalization across the C−C double bond of the ketenimine moiety with limited scope of nucleophiles (e.g., alcohols, amines, alkenes, and imines). Herein, we report a new copper-catalyzed aza-Claisen rearrangement,8 C−H functionalization, Ullmann C−N coupling,9 and cyclization cascades of N-allyl-N-((2-bromoaryl)ethynyl)amides with terminal alkynes or 1,3-dicarbonyls for accessing 2,3-difunctionalized indoles (Scheme 2); this reaction represents a novel strategy for the utilization of the ketenimine intermediates A by functionalizing across its C−N double bonds, thus providing a highly step-

ndoles are a well-known class of N-heterocycles due to their prevalence in natural products, drugs, and functional materials as well as their wide utilization as versatile building blocks for chemical synthesis.1,2 In particular, 2,3-difunctionalized indoles have been of growing interest from synthetic and biological value perspectives (Scheme 1).2 Among them, Scheme 1. Examples of Important 2,3-Difunctionalized Indoles

azepinoindole,2a MDL 203371,2f and Arbidol2g exhibit antifungal properties and broad-spectrum biological function as H1-receptor antagonists, glycine receptor antagonists, and viral fusion inhibitors. Tryprostatins A and B, isolated from the fermentation broth of Aspergillus f umigatus BM939, can selectively arrest the cell cycle at the mitotic phase in tsFT210 cells.2b−d Grossularine-1 is the first example of an important naturally occurring α-carboline with antitumor properties.2h For these reasons, chemists are urged to devote © XXXX American Chemical Society

Received: February 27, 2019

A

DOI: 10.1021/acs.orglett.9b00740 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

deleterious effect on the reaction (entries 14 and 15). Notably, this reaction could be applicable to a 1 mmol scale of 1a, giving 3 in good yield (entry 16). Having established the optimal reaction conditions, the scope of the Cu-catalyzed cascade reaction was exploited with regard to N-allyl-N-((2-bromoaryl)ethynyl)amides (1) and alkynes (2) (Scheme 3). The optimal conditions were

Scheme 2. Synthesis of 2,3-Disubstituted Indoles

Scheme 3. Annulation Cascades of N-Allyl Ynamides (1)a economical route to 2,3-difuctionalized indoles with three new chemical bonds formation in a single reaction. Our initial study focused on the cascade reaction of N-allylN-((2-bromophenyl)ethynyl)-4-methylbenzenesulfonamide (1a) with phenylacetylene (2a) (Table 1). The results Table 1. Optimization of the Reaction Conditionsa

entry

variation from the standard conditions

yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16b

none without Phen without Cu(OAc)2 Cu(OAc)2 (5 mol %) and Phen (5 mol %) Cu(OAc)2 (20 mol %) and Phen (20 mol %) CuO instead of Cu(OAc)2 CuCl2 instead of Cu(OAc)2 CuOAc instead of Cu(OAc)2 Cs2CO3 instead of K2CO3 without K2CO3 n-PrCN instead of MeCN DMF instead of MeCN toluene instead of MeCN at 100 °C at 130 °C none

68 13 0 45 70 42 61 45 43 0 63 28 32 26 57 70

a

Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol), Cu(OAc)2 (10 mol %), Phen (10 mol %), K2CO3 (2 equiv), MeCN (2 mL), argon, 120 °C, and 12 h. bUsing N-((2-bromophenyl)ethynyl)-N- (but-2-en1-yl)-4-methylbenzenesulfonamide (1h) as substrate.

a

Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), Cu(OAc)2 (10 mol %), Phen (10 mol %), K2CO3 (2 equiv), MeCN (2 mL), argon, 120 °C and 12 h. b1a (1 mmol) for 36 h.

compatible with a wide array of arylalkynes, and several aryl groups, namely naphthalen-1-yl, 4-MeC6H4, 4-MeOC6H4, 4ClC6H4, 4-BrC6H4, 4-CNC6H4, 3-MeC6H4, 2-MeC6H4, and thiophene-2-yl, at the terminal alkyne were well tolerated (products 4−12). Moreover, the position and electron effect had an influence on their reactivity. Alkyne bearing an electron-donating 4-MeOC6H4 group was converted to indole 6 in good yield, whereas alkyne with an electron-withdrawing 4-CNC6H4 group delivered indole 9 in a lower yield. Using 4MeC6H4- or 3-MeC6H4-substituted alkynes afforded indoles 5 and 10, respectively, in high yields, but bulky 2-MeC6H4substituted alkyne generated indole 11 with a diminished yield. Gratifyingly, the optimal conditions were applicable to an aliphatic alkyne, thus producing 13 in 51% yield. Subsequently, the generality of the N-allyl-N-((2-bromoaryl)ethynyl)amides (1) was examined under the optimal conditions (products 14− 20). We found that another substituent, SO2Ph or Ms, on the nitrogen atom instead of the Ts group was viable for separately assembling indoles 14 and 15, albeit with diminished yields. Unfortunately, replacement of Ts by Ac led to no reaction (product 16). Substrates 1e−f, bearing a Me group or a Cl group on the aryl ring of the 2-bromoaryl moiety, successfully delivered the indole systems (products 17 and 18). Notably, substrates 1g−h, containing an N-(2-methylallyl) group or an

demonstrated that a combination of Cu(OAc)2 with 1,10phenanthroline (Phen) ligand play an important role in the reaction (entries 1−3 and Table S1). While substrate 1a reacted with alkyne 2a, Cu(OAc)2, 1,10-phenanthroline (Phen), and K2CO3 efficiently furnished the desired indole 3 in 68% yield (entry 1), the yield of 3 sharply decreased from 68% to 13% in the absence of Phen (entry 2), and the reaction could not take place without Cu(OAc)2 (entry 3). Notably, the amount of Cu(OAc)2 and Phen affected the reaction, and a combination of 10 mol % of Cu(OAc)2 with 10 mol % of Phen was preferred (entries 1, 4, and 5). Subsequently, three other Cu salts, including CuO, CuCl2, and CuOAc, were evaluated: they had lower catalytic activity than Cu(OAc)2 (entry 1 versus entries 6−8). We found that the reaction afforded product 3 in a lower yield when Cs2CO3 was used instead of K2CO3 (entry 9). However, the reaction failed to execute without bases (entry 10). Among the effect of solvents examined, both MeCN and nPrCN turned out to be more reactive than DMF and toluene (entries 1 and 11−13). A screen of the reaction temperature effect revealed that either a lower (100 °C) or a higher (130 °C) temperature had a B

DOI: 10.1021/acs.orglett.9b00740 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters N-(but-2-en-1-yl) group, underwent the reaction smoothly, giving products 19 and 20 in moderate yields. We were pleased to find that this current annulation could be expanded to 1,3-dicarbonyl compounds (Scheme 3). In the presence of substrate 1a, Cu(OAc)2, Phen, and K2CO3, a variety of 1,3-dicarbonyl compounds, namely 1,3-diketones (products 21−25) and 1,3-keto esters (products 26−28), were found to be suitable substrates for the reaction, giving the corresponding indoles 21−28 in moderate to good yields. 2,3-Difunctionalized indoles are known versatile building blocks in synthesis. As shown in Scheme 4, 2-ethynyl-3allylindole 3 could be readily transformed to nitrile 29,10a polycyclic compound 30,10b,2i N−H free indole 31,10c nitrile 32,10d and aldehyde 33.10e

to expand the applications of this cascade strategy in cycle synthesis are currently underway in our laboratory.

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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.9b00740. Descriptions of experimental procedures for compounds, and analytical characterization (PDF) Accession Codes

CCDC 1471749 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.

Scheme 4. Utilization of Product 3

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

To understand the current annulation cascade reaction, the possible mechanism was proposed on the basis of the present results and the literature reports (Scheme 5).6−9 Aza-Claisen

Jin-Heng Li: 0000-0001-7215-7152

Scheme 5. Possible Mechanism

ACKNOWLEDGMENTS We thank the Natural Science Foundation of China (Nos. 21625203 and 21871126) and the Jiangxi Province Science and Technology Project (Nos. 20171ACB20015 and 20165BCB18007) for financial support.

Notes

The authors declare no competing financial interest.

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REFERENCES

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rearrangement of N-allyl-N-((2-bromoaryl)ethynyl)amide8 readily takes place under heating to produce the ketenimine intermediate A, which sequentially undergoes nucleophilic addition of alkyne 2a to the ketenimine intermediate A to form the intermediate B.6,7 Oxidative insertion of the active CuILn species into the C−Br bond of the intermediate B resulted in the formation of the intermediate C, followed by ligandexchange, to produce the metallocycle intermediate D.9 Finally, reductive elimination of the intermediate D occurs to assemble indole 3 and regenerate the active CuIILn species. In summary, we have established a new copper-catalyzed annulation cascade of N-allyl-N-((2-bromoaryl)ethynyl)amides with a variety of C−H bonds, including C(sp)−H bonds of terminal alkynes and α-C(sp3)−H bonds of 1,3-dicarbonyls. The method proceeds via aza-Claisen rearrangement, C−H functionalization, Ullmann C−N coupling, and cyclization cascades and achieves the selective generation of 2,3difunctionalized indoles in one step by incorporating two functional groups with formation of three new chemical bonds. Importantly, the method features good compatibility of substrates and excellent tolerance of functional groups. Efforts C

DOI: 10.1021/acs.orglett.9b00740 Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.9b00740 Org. Lett. XXXX, XXX, XXX−XXX