Gold-Catalyzed Formal [4 + 2] Cycloaddition of 5-(Ethynylamino)pent

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/OrgLett

Gold-Catalyzed Formal [4 + 2] Cycloaddition of 5‑(Ethynylamino)pent-2-yn-1-yl Esters to 1,2,3,5Tetrahydrobenzo[g]quinolines Xiaoyu Chen,† James Theodore Merrett,† and Philip Wai Hong Chan*,†,‡ †

School of Chemistry, Monash University, Clayton, Victoria 3800, Australia Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom

‡

S Supporting Information *

ABSTRACT: A synthetic method to prepare 1,2,3,5tetrahydrobenzo[g]quinolines efficiently that relies on gold(I)-catalyzed cycloisomerization of 5-(ethynylamino)pent-2yn-1-yl esters at room temperature under atmospheric conditions is described. The proposed reaction mechanism presents a unique instance of an in situ formed allenic ester and gold keteniminium species to undergo a formal [4 + 2] cycloaddition pathway.

G

starting material was then posited to give the product. A recent study also described the trapping of the organogold species of one by the CC bond of the other in reactions employing both substrates that is followed by a 1,2-acyloxy shift to give 1amino-1,3-cyclopentadienes (Scheme 1c).7 However, the analogous cascade reactions in which a gold carbenoid or allene intermediate subsequently engages with a gold keteniminium species to effect carbon−carbon bond formation has so far remained unrealized. With this in mind, we were drawn to the potential gold-catalyzed cycloisomerization chemistry of 5-(ethynylamino)pent-2-yn-1-yl esters 4 (Scheme 1d). In doing so, the putative (2-iminiovinyl)gold adduct IV furnished in this manner was discovered to undergo a formal [4 + 2] cycloaddition to give the 1,2,3,5-terahydrobenzo[g]quinoline ring system.8,9,11 Herein, we disclose the details of this Au(I)-catalyzed method for the efficient and selective synthesis of a potentially useful new member of the Nheterocyclic compound family in good to excellent yields from a ynamide-tethered propargyl ester. Accomplished at room temperature under mild conditions that did not require the exclusion of moisture or air, the transformations represent the first example of the two in situ formed cycloisomeric motifs participating in a formal [4 + 2] cycloaddition pathway. It also provides a rare instance of a gold-activated ynamide taking on the role of the 4π cycloaddition partner in gold catalysis.9 To test the feasibility of our hypothesis, the ynamidetethered propargyl acetate 4a was chosen as the model substrate to establish the optimum reaction conditions (Table 1). The ensuing study revealed that treatment of the starting material with 5 mol % of gold(I) phosphine complex A in nondistilled 1,2-dichloroethane at room temperature under atmospheric conditions for 12 h gave the best result (entry 1).

old-catalyzed cycloisomerization of propargyl esters and ynamides provides one of the most powerful and versatile strategies for carbocyclic and heterocyclic synthesis (Schemes 1a and 1b).1−10 Mechanistically, the reactions of propargyl esters have often relied on their propensity to undergo either 1,2- or 1,3-acyloxy migration to give the corresponding gold carbenoid or allene intermediates I and II.2−4 For the nitrogensubstituted alkynes, it is thought the transformations proceed via a gold keteniminium species III.5−7 In both cases, further functionalization by a remaining pendant group or a second Scheme 1. Gold-Catalyzed Reactivities of Propargyl Esters and Ynamides

Received: January 25, 2018

© XXXX American Chemical Society

A

DOI: 10.1021/acs.orglett.8b00267 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Table 1. Optimization of the Reaction Conditionsa

outcome was found in a final set of control experiments mediated by AgOTf or AgSbF6, which were observed to give an inseparable mixture of two compounds that could not be identified by 1H NMR analysis (entries 12 and 14). With the optimum reaction conditions in hand, we next turned our attention to evaluating the generality of the present procedure by surveying a variety of 5-(ethynylamino)pent-2-yn1-yl esters (Figure 1). These reactions demonstrated the

entry

catalyst

solventb

yieldc (%)

1 2 3 4 5 6 7 8 9 10 11 12 13h 14

A A A A A A B Ph3PAuNTf2 Ph3PAuNTf2 C C AgOTf AgOTf AgSbF6

(CH2Cl)2 CH2Cl2 MeCN THF toluene (CH2Cl)2e (CH2Cl)2 (CH2Cl)2 (CH2Cl)2e (CH2Cl)2 (CH2Cl)2e (CH2Cl)2 toluene (CH2Cl)2

90 67 36 d 46 f 75 50 f 54 f g d g

a

All reactions were performed with 0.1 mmol of 4a and 5 mol % of gold(I) catalyst in 1 mL of solvent at room temperature under atmospheric conditions for 12 h. bReaction performed in nondistilled solvent. cIsolated yield. dMixture of unidentifiable decomposition products were obtained based on TLC analysis and 1H NMR measurements. eReaction performed in distilled (CH2Cl)2 and with 4 Å MS (50 mg) under a nitrogen atmosphere at room temperature for 3 h and then at reflux temperature for 4 h. fSubstrate recovered in 70− 96% yield. gMixture of two compounds were obtained that could not be identified by 1H NMR analysis. hReaction conducted with 10 mol % of AgOTf at reflux temperature for 5 h.

Under these reaction conditions, the anticipated formal [4 + 2] cycloaddition product 5a was obtained in 90% yield. The structure of the nitrogen-containing cyclic adduct was determined by comparison with the NMR measurements and X-ray crystallography of a closely related analogue vide infra. Lower product yields of 36−67% were furnished on repeating the reaction with dichloromethane, acetonitrile, or toluene in place of 1,2-dichloroethane as the solvent (entries 2, 3, and 5). Likewise, the analogous control reactions mediated by the gold(I) phosphine complex B, Ph3PAuNTf2 or NHC−gold(I) (NHC = N-heterocyclic carbene) complex C were found to give product yields of 50−75% (entries 7, 8, and 10). In contrast, the introduction of 4 Å molecular sieves (MS) to the gold(I) phosphine complex A- or C- or Ph3PAuNTf2-catalyzed experiments in distilled 1,2-dichloroethane was found to result in no reaction being detected by TLC analysis after 3 h (entries 6, 9, and 11). Continuing the experiments at reflux temperature for 4 h was also observed to have no influence on the course of the reaction and led to the recovery of only the substrate in 70−96% yield. The analogous reactions catalyzed by gold(I) phosphine complex A with THF as the solvent or AgOTf (10 mol %) in toluene at reflux temperature for 5 h, on the other hand, were found to give a mixture of unidentifiable decomposition products (entries 4 and 13).12 A similar

Figure 1. Formal [4 + 2] cycloaddition of 4b−v catalyzed by Au(I) complex A. All reactions were performed at the 0.1−1 mmol scale with 5 mol % of Au(I) complex A in 1,2-dichloroethane (0.1 M) at room temperature under atmospheric conditions for 1−18 h. Values in parentheses denote isolated product yields. (a) Reaction conducted at 60 °C.

gold(I) complex A-catalyzed reaction conditions to be broad, providing a series of 1,2,3,5-tetrahydrobenzo[g]quinolines in 32−89% yield from the corresponding substrates 4b−v. The rearrangement of substrates with a OPNB (4b) or OPMB (4c) instead of OAc migrating group were found to proceed to afford the corresponding N-heterocycles 5b and 5c in yields of 64 and 62%, respectively. The structure of the former partially hydrogenated quinoline was also confirmed by X-ray singlecrystal analysis. Reactions with starting materials containing other aryl (4d−i), 3-furanyl (4j), alkyl (4k,l), or cycloalkyl (4m,n) groups at the ester carbon center were also found to be well tolerated and gave the anticipated nitrogen-containing heterocycles 5d−n in 32−89% yield. Similarly, the presence of an electron-withdrawing (4p−r,v) or -donating (4s−u) substituent at the ortho- or para-position of the phenyl group B


Letter

Organic Letters at the distal ynamide carbon center of the substrate was found to have little influence on the course of the reaction. In these reactions, the corresponding tricyclic adducts 5p−v were furnished in 46−82% yield. The cycloisomerization of the tertiary ester 4o was the only instance that required a slight modification of the reaction conditions, with a reaction temperature of 60 °C needed to furnish the desired Nheterocyclic product 5o in 65% yield. Under the gold(I)catalyzed standard conditions at room temperature, the allene 6o was obtained in 71% yield. A tentative mechanism for the present Au(I)-catalyzed formal [4 + 2] cycloaddition is outlined in Scheme 2. With

Scheme 3. Control Experiment with 6o

of substrate in control reactions with 4 Å MS and a nitrogen atmosphere detailed in Table 1, entries 6, 9, and 11.10 In summary, we have elucidated an efficient gold(I)-catalyzed formal [4 + 2] cycloaddition process to assemble 1,2,3,5tetrahydrobenzo[g]quinolines from 5-(ethynylamino)-pent-2yn-1-yl esters under mild conditions at room temperature. The cycloisomerization was shown to be robust as it did not require the exclusion of air or moisture and demonstrated a new mode of reactivity between in situ formed allenic esters and gold keteniminium species. Efforts to realize an asymmetric version and explore the potential synthetic applications of the present reaction are currently in progress.

Scheme 2. Proposed Mechanism for the Au(I)-Catalyzed Formal [4 + 2] Cycloaddition of 5-(Ethynylamino)pent-2yn-1-yl Esters Represented by 4a

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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.8b00267. Detailed experimental procedures, characterization data and 1H and 13C NMR spectra for all starting materials and products (PDF) Accession Codes

CCDC 1817966 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.

4a as representative example, this could involve the activation of the propargyl ester moiety in the substrate by the metal complex to give the gold-coordinated species Va. As a result, this may trigger [3,3]-sigmatropic rearrangement of the acetate group and formation of the allenic ester VIIa via the 1,3-dioxin1-ium species VIa. This is followed by gold-activated ynamide− gold keteniminium tautomerization to provide the putative organogold intermediate IVa. This is the putative active species that undergoes the formal [4 + 2] cycloaddition process involving nucleophilic attack of the allenic ester group to the gold keteniminium motif in IVa.8,9 Subsequent Friedel−Craftstype alkylation of the phenyl group by the vinyl oxonium unit in the ensuing vinyl gold species VIIIa would deliver the Wheland-type intermediate IXa. Deprotonation and protodeauration of this newly formed tricyclic carbocation species might then produce the product 5a.10 The proposed sequential 1,3-acyloxy migration followed by a gold-activated ynamide− gold keteniminium tautomerization pathway would be consistent with our findings showing 5o and 6o being afforded in the respective reactions at 60 °C and room temperature as described in Figure 1. It also supports the posited involvement of the putative (2-iminiovinyl)gold adduct IV put forward in Scheme 1. This argument was further corroborated by the observation that when 6o was resubjected to the Au(I)catalyzed standard conditions at 60 °C, the expected benzofused quinoline product 5o was obtained in 71% yield (Scheme 3). The role of an advantageous trace amount of water in facilitating the rearomatization and protodeauration of the vinyl gold species IXa would be in agreement with the recovery

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AUTHOR INFORMATION

Corresponding Author

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

Philip Wai Hong Chan: 0000-0002-8786-6143 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Discovery Project Grant (DP160101682) from the Australian Research Council. REFERENCES

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Gold-Catalyzed Formal [4 + 2] Cycloaddition of 5-(Ethynylamino)pent

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