Letter Cite This: Org. Lett. 2018, 20, 2028−2032
pubs.acs.org/OrgLett
Heterobicyclic Core Retained Hydroarylations through C−H Activation: Synthesis of Epibatidine Analogues Deng-Yuan Li, Zheng-Lu Huang, and Pei-Nian Liu* Shanghai Key Laboratory of Functional Materials Chemistry, Key Lab for Advanced Materials and School of Chemistry & Molecular Engineering, East China University of Science & Technology, Meilong Road 130, Shanghai 200237, China S Supporting Information *
ABSTRACT: The heterobicyclic core retained hydroarylation of oxa/azabenzonorbornadienes with quinoline N-oxides has been achieved under rhodium catalysis, giving quinoline N-oxide substituted heterobicyclic structures with excellent regioselectivity and in good yields. As the first example of the direct introduction of quinoline N-oxides onto heterobicyclic structures, the strained heterobicyclic core was well retained in the reaction. The products could be successfully transformed into a series of useful compounds, including epibatidine analogues.
H
reactions involve the strained ring-opening of oxa/azabenzonorbornadienes, while the formation of carbon−carbon bonds usually requires the prefunctionalization of coupling partner such as organoboron,8 zinc,9 and magnesium reagents,10 as well as aryl halides.11 In contrast, considerably less progress has been made toward the heterobicyclic core retained reaction of oxa/azabenzonorbornadienes,13 especially reactions using in situ generated C−H activation intermediates as reaction partners. Transition-metal-catalyzed C−H activation involving directing groups is a powerful and promising tool because it can accelerate the reaction and ensure regioselectivity. Recently, oxa/azabenzonorbornadienes were utilized as functionalized reagents for C−H activation.14,15 Notably, Li reported the rhodium- or cobalt-catalyzed ortho-naphthylated of arenes with oxa/azabenzonorbornadienes to afford heterobicyclic core opening products.14a,b Bolm reported the rhodium- or ruthenium-catalyzed hydroarylations of arenes with oxabenzonorbornadienes to give heterobicyclic core retained product,14c,d and similar results were reported by Nishimura using hydroxoiridium as a catalyst.14e,f More recently, Cheng achieved the cobalt-catalyzed oxidative annulation of aromatic/ vinylic amides with oxabenzonorbornadienes.15a Moreover, Glorius reported the cascade reaction of N-phenoxyacetamide with azabenzonorbornadiene by merging rhodium-catalyzed C−H activation and Wagner−Meerwein-type rearrangements.15b We also developed a cascade reaction of alkynols and oxabenzonorbornadienes driven by the synergistic merger
eterobicyclic structures represent increasingly important building blocks that are extensively used in agrochemicals and pharmaceuticals. For example, epibatidine and samandarine are two natural heterobicyclic core alkaloids from the neotropical poison frog (Dendrobatidae) and salamander, respectively (Figure 1).1 They are highly potent analgesics
Figure 1. Some natural products containing heterobicyclic cores.
that act on multiple neuronal nicotinic acetylcholine receptor (nAChR) subtypes. Moreover, cytisine, another important heterobicyclic core alkaloid from lupin, is a potent and a 4b2 subtype-selective partial agonist of neuronal nicotinic acetylcholine receptors (Figure 1),2a which led to the discovery of heterobicyclic core analogues, including (−)-sparteine, (+)-anagyrine, (−)-thermopsine, and varenicline.2b,c Therefore, a number of methods have been developed to construct heterobicyclic structures3,4 and their analogues.5 Most of these methods involve multistep process with low total yields. The direct heterobicyclic core retained transformation from readily available heterobicyclic structures remains a challenge due to the feasibility of strained ring-opening. Oxa/azabenzonorbornadienes are one of the most important heterobicyclic structures in modern organic synthesis6 and have been extensively used in the construction of carbon− carbon7−11 and carbon−heteroatom bonds.12 Most of these © 2018 American Chemical Society
Received: February 16, 2018 Published: March 20, 2018 2028
DOI: 10.1021/acs.orglett.8b00571 Org. Lett. 2018, 20, 2028−2032
Letter
Organic Letters
ineffective for the reaction (entries 1−5), but [Cp*Rh(MeCN)3](SbF6)2 (5 mol %) afforded the heterobicyclic core retained hydroarylation product 3a in 34% yield (entry 6). Single-crystal X-ray diffraction analysis of 3a confirmed the structure of the products (see the SI). Increasing the reaction temperature to 50 °C provided 40% yield of 3a (entries 7 and 8). The use of various additives revealed that AcOH (2 equiv) facilitated the heterobicyclic core retained hydroarylation, giving 3a in 91% yield (entries 9−13), suggesting that it is essential for the formation of 3a to remain suitably acidic conditions. Changing the reaction solvents revealed that dioxane provided the best yield (entries 14−19). Increasing or decreasing [Cp*Rh(MeCN)3](SbF6)2 loading did not improve yield (entries 20 and 21). Changing the ratio of 1a:2a from 1:1.5 to 1:1.2 gave the 3a in 95% yield (entry 22) while continuously decreasing the ratio of 1a:2a from 1:1.2 to 1:1 led to a lower yield of 91% (entry 23). Omitting [Cp*Rh(MeCN)3](SbF6)2 hindered the formation of 3a, confirming its essential role in the reaction (entry 24). In addition, we proved that the reaction could be conducted on a gram scale without a significant decrease in yield (1.66 g of 3a, 82% yield) with a catalyst loading of 2.5 mol % (entry 25). After the optimization of the reaction conditions, we explored the substrate scope of the hydroarylation (Scheme 1). Quinoline N-oxides with functional groups such as Me, MeO, F, Cl, Br, or I at the R1 position of the benzene ring of
of rhodium and Sc(OTf)3 catalysts to afford spirocyclic dihydrobenzo[a]fluorenefurans.15c Taking advantages of the direct functionalization of quinoline N-oxides by C−H activation16 and our ongoing research on the construction of heterocycles using rhodium-catalyzed C−H activation,15c,17 we describe here a method to directly access substituted heterobicyclic structures via hydroarylation of oxa/ azabenzonorbornadienes with quinoline N-oxides. The reaction was promoted by a rhodium catalyst and demonstrated excellent regioselectivity and good yields. The mild reaction conditions completely inhibited the ring-opening of oxa/ azabenzonorbornadienes, and the heterobicyclic core was well retained. Moreover, the synthesis of epibatidine analogues was achieved by a one-step transformation of the product. To the best of our knowledge, this protocol is the first example of the direct introduction of quinoline N-oxides onto heterobicyclic structures. We began our investigations of the hydroarylation using 1a and 2a as model substrates in DCE at 25 °C. As shown in Table 1, common catalysts such as Pd(OAc)2, [RuCl2(p-cymene)]2, [Cp*IrCl2]2, [Cp*RhCl2]2, and [Cp*RhCl2]2/AgSbF6 were Table 1. Optimization of Reaction Conditionsa
Scheme 1. Scope of Substratesa entry
catalyst
additive
solvent
yieldb (3a, %)
1c 2c 3c 4c 5c,d 6c 7 8e 9 10 11 12 13 14 15 16 17 18 19 20f 21g 22h 23i 24 25j
Pd(OAc)2 [RuCl2(p-cymene)]2 [Cp*IrCl2]2 [Cp*RhCl2]2 [Cp*RhCl2]2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 [Cp*Rh(MeCN)3](SbF6)2 − [Cp*Rh(MeCN)3](SbF6)2
− − − − − − − − HOAc HOPiv HO2CPh HO2CCF3 HOTf HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc
DCE DCE DCE DCE DCE DCE DCE DCE DCE DCE DCE DCE DCE MeCN DMF toluene MeOH THF dioxane dioxane dioxane dioxane dioxane dioxane dioxane
ND ND ND ND trace 34 40 38 91 88 62 ND ND 83 90 trace 78 85 95 84 81 95 91 ND 82
a Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), catalyst (5 mol %), additive (2.0 equiv), solvent (2 mL), at 50 °C, 5 h, under Ar. b Yields of isolated products are given. c25 °C. dAgSbF6 (0.02 mmol) was added. e80 °C. f2.5 mol % instead of 5 mol %. g10 mol % instead of 5 mol %. h1a (0.2 mmol) and 2a (0.24 mmol) were used. i1a (0.2 mmol) and 2a (0.2 mmol) were used. jThe reaction was scaled up to 1.0 g (7 mmol) of 1a with a catalyst loading of 2.5 mol %.
a Reaction conditions: 1a (0.2 mmol), 2a (0.24 mmol), [Cp*Rh(MeCN)3](SbF6)2 (0.01 mmol), AcOH (2.0 equiv), dioxane (2 mL), at 50 °C, 5 h, under Ar. bIsolated yields of products are given. cThe reaction was scaled up to 1.7 g (7 mmol) of 1z with a catalyst loading of 2.5 mol %.
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DOI: 10.1021/acs.orglett.8b00571 Org. Lett. 2018, 20, 2028−2032
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Organic Letters
this methodology in the synthesis of epibatidine analogues are shown in Scheme 2b. A one-pot removal of the oxygen atom and protecting group (Boc) from 3z afforded epibatidine analogue 13 in 72% yield. Moreover, 3z was converted the epibatidine analogue 14 (77%) by a reaction with thionyl chloride To investigate the mechanism of this reaction, we examined the behavior of the present catalytic system using deuterated acetic acid (Scheme 3). In the reaction of 2a without 1a, a
the quinoline were tolerated, giving the desired products 3b−g in 64−95% yields. However, quinoline N-oxides bearing strongly electron-withdrawing groups (CF3, CN, or NO2) were unreactive. Quinoline N-oxides substituted with Me, MeO, F, Cl, Br, or p-tolyl at the R2 position of the benzene ring of the quinoline were also tolerated, giving products 3h−l in 61−93% yields. Quinoline N-oxides substituted with Me or Cl at the para position of the pyridine ring of quinoline reacted well with 2a, producing 3m and 3n in 82% and 71% yields, respectively. Quinoline N-oxides substituted with Me at the ortho or meta positions of the pyridine ring of quinoline gave the desired product 3o and 3p in respective 89% and 75% yields. Moreover, acridine 10-oxide reacted with 1a, giving a single product 3q in 61% yield. Bicyclic olefin 1a and substituted 7-oxabenzonorbornadienes reacted efficiently with 2a, giving products 3r−w in 77−94% yields. Notably, other alkyl or aryl alkenes did not react with 2a to give the corresponding bicyclic products. Interestingly, azabenzonorbornadienes reacted under the same reaction condition, giving the N-heterobicyclic core retained products 3x−z, 3aa, and 3ab in 58−88% yields. To prepare the Nheterobicyclic core retained products on a gram scale, we conducted the scaled-up experiment using a catalyst loading of 2.5 mol % and successfully produced 3z (1.90 g of 3z, 70% yield) without a significant decrease in yield. To demonstrate the synthetic utility of the developed approach, we performed various downstream transformations of the heterobicyclic core retained products (Scheme 2a). For
Scheme 3. Mechanistic Study
reversible H/D exchange at the C-8 position was observed in CD3CO2D (10 equiv) at 50 °C after 5 h in the presence of [Cp*Rh(MeCN)3](SbF6)2 (eq 1), although no H/D exchange occurred in the absence of [Cp*Rh(MeCN)3](SbF6)2. This result showed that the C−H activation was a reversible process. For the reaction of 1a and 2a under standard conditions in the presence of CD3CO2D (10 equiv), [D]3a (60% deuteration ratio in O-heterobicyclic core) was isolated in 94% yield (eq 2). This result further demonstrated that the reaction involved intermolecular protonolysis. We also attempted to detect the reaction intermediates in this reaction. When 2a was treated with the catalyst under typical conditions for 15 min with or without 1a, rhodacyclic intermediate A was generated and could be detected by ESI-HRMS (calcd for C19H21NORh+ [M]+ 382.0673, found 382.0681; the experimental isotopic distribution matched the theoretical isotopic distribution, SI). On the basis of the aforementioned experimental investigations and the available literature,16 we propose a plausible mechanism for the hydroarylation (Scheme 4). The reaction likely begins with the cyclorhodiation of 2a to afford fivemembered cyclic intermediate A, which coordinates with 1a to generate RhIII−alkene species B. Then a migration insertion of the alkene into the Rh−C bond gives seven-membered rhodacyclic intermediate C. Finally, intermolecular protonolysis
Scheme 2. Transformations of Products and Synthesis of Epibatidine Analogues
Scheme 4. Proposed Mechanism
example, 3a could be readily converted into a heterobicyclic core retained quinoline (4, 71%) by the selective removal of the oxygen atom of the N-oxide moiety. Moreover, the N-oxide moiety in 3a could also be readily substituted at the C-2 position by a number of groups such as alkoxy (5, 60%), amido (6, 62%), amide (7, 60%), sulfamide (8, 48%), tosyl (9, 68%), cyano (10, 71%), alkyl (11, 40%), and aryl (12, 70%) groups via various carbon−carbon and carbon−heteroatom bondforming reactions. In addition, the important applications of 2030
DOI: 10.1021/acs.orglett.8b00571 Org. Lett. 2018, 20, 2028−2032
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of C gives desired product 3a with concomitant regeneration of the active Cp*Rh(III) species. In summary, we have developed the heterocyclic core retained hydroarylation of oxa/azabenzonorbornadienes with quinoline N-oxides using rhodium as a catalyst under mild conditions. The quinoline N-oxide substituted heterobicyclic structures were obtained directly in excellent regioselectivity and in good yields. Mechanistic investigations demonstrated that the reaction proceeds through an N-oxide group directed C−H activation, coordination, migration insertion of an alkene, and intermolecular protonolysis. The downstream transformations of the products, especially one-step access to epibatidine analogues, confirmed the usefulness of this protocol in the synthesis of substituted heterobicyclic structures.
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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.8b00571. Detailed experimental procedures, characterization data of products (NMR, HRMS, etc.), spectra of the products (PDF) Accession Codes
CCDC 1824334 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.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Pei-Nian Liu: 0000-0003-2014-2244 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Project Nos. 21602059, 21561162003, and 21672059), the Eastern Scholar Distinguished Professor Program, the China Postdoctoral Science Foundation (2016T90341), and the Fundamental Research Funds for the Central Universities.
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REFERENCES
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