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Photoredox-Catalyzed Cascade Radical Cyclization of Ester Arylpropiolates with CF3SO2Cl to Construct 3-Trifluoromethyl-Coumarin Derivatives Long Chen, Lianlian Wu, Weijie Duan, Tao Wang, Letian Li, Keke Zhang, Jingmei Zhu, Zhi Peng, and Fei Xiong J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00581 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018
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The Journal of Organic Chemistry
Photoredox-Catalyzed Cascade Radical Cyclization of Ester Arylpropiolates with CF3SO2Cl to Construct 3-TrifluoromethylCoumarin Derivatives Long Chen, †, ‡ Lianlian Wu, ‡ Weijie Duan, ‡ Tao Wang, *,†, ‡ Letian Li, ‡ Keke Zhang, ‡ Jingmei Zhu, ‡ Zhi Peng, ‡ Fei Xiong *,†, ‡ †
National Research Center for Carbohydrate Synthesis and Key Laboratory of Chemical Biology, Jiangxi Normal University, Nanchang, Jiangxi 330022, P. R. China ‡ College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, Jiangxi 330022, P. R. China
We reported a highly efficient method to construct 3-trifluoromethy coumarins using CF3SO2Cl as the trifluoromethyl radical source with ester 3-arylpropiolates. The reaction incorporated a cascade cyclization/dearomatization/ester migration/ oxidization/ rearomatization process to afford various 3-trifluoromethyl coumarins under visible light irradiation in good to excellent yields. ABSTRACT:
KEYWORDS: Photoredox-catalyzed; Trifluoromethylation; Coumarin; Radical; Cascade reaction
Naturally occurring coumarins are found extensively in plants, fungi and bacteria.1 This class of compounds is well known for its significant biological and pharmacological properties.2 Some coumarin analogues are used as P450s inhibitor.2a Some show anti-inflammatory,2b antitumor,2c antineurodegenerative,2d anticoagulant,2e antimicrobial,2e anti-HIV,2e and antioxidant activities.2e Coumarins have also found widespread applications in several fields, including cosmetics, pesticides, fluorescent dyes, chemical and biological probes.3 Over the past several decades, various synthetic strategies have been developed for preparing coumarins. In 2000, Fujiwara’s group discovered Pd-catalytic intramolecular hydroarylation of C-C triple bond as an efficient methodology to construct coumarin moieties.4 Other Lewis acids, such as Au, Fe, etc., also smoothly promoted the transformation.5 Recently, direct functionalization of activated alkynoates has been utilized to synthesize 3-substitued coumarins,6 including, 3-trifluoromethylation with Togni’s reagent,6b 3 ‑ phosphorylation with dialkyl H‑phosphonate,6c, 6d 3‑difluoroacetylation with ethyl bromodifluoroacetate,6e 3trifluoromethylthiolation and thiocyanation with AgSCF3 and AgSCN, respectively,6f and 3-sulfonation with aryldiazonium,6g among many others. These transformations proceeded in a similar reaction pathway. First, a radical is generated by external oxidants, reductants or photocatalysts, and is added to the α-position of C=O in alkynoates to produce the vinyl radical intermediate. Second, an intramolecular radical cyclization forms a ring-closed radical intermediate via 6-endo or 5-exo annulation with ester migration. Third, the radical intermediate is oxidized to the corresponding cation intermediate. Finally, deprotonation and aromatization afford the corresponding 3-subsititued coumarin derivatives.
Much attention has focused on synthesis of organic fluorine compounds due to their unique properties, such as improving lighting fastness for dyes.7 Organic fluorine compounds also provide better membrane permeability and bioavailability than their non-fluorinated analogues.8 Trifluoromethylation has gained increasing interest in recent years. The trifluoromethyl group(CF3-) is a versatile component and essential structural motif in pharmaceuticals, agrochemicals, dyes, and organic materials.9 Addition of the CF3 moiety is typically accomplished by means of transition metals catalyzed crosscoupling technology. However, these strategies generally required the stoichiometric metal reagents. Recently, the introduction of CF3 was achieved in a catalytic fashion by copper, or palladium-catalyzed reaction using either a nucleophilic or electrophilic source of CF3 (TMSCF3, CF3SO2Na or Togni’s reagent, Umemoto’s reagent, CF3SO2Cl, etc.).10 Among these reagents, CF3SO2Cl is also an efficient reagent for introducing CF3 moiety onto a wide range of substrates by transition metal or photoredox-catalyzed reaction.11 For example, Kamigata and co-workers discovered the RuCl2(PPh3)3-catalyzed trifluoromethylation of arenes and heteroarenes in 1990.12 MacMillan’s group reported a similar transformation in 2011 using a photocatalyst instead of RuCl2(PPh3)3. This strategy could considerably extend the scope of substrates (Scheme 1, a).9 Trifluoromethylation of silyl enol ethers and olefins generally constructed CF3-Csp3 bond in 2014 (Scheme 1, b).10c, 13 Gu’s group presented a direct annulation approach for preparation of a variety of indole derivatives using arylsulfonyl chlorides with oazidoarylalkynes via visible-light induced cyclization process in 2017 (Scheme 1, c).14 Furthermore, visible light-promoted chlorotrifluoromethylation of alkynes was described by Han’s group in 2017.15 In addition, photoredox catalysis, a powerful
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synthetic strategy, has been widely applied in catalytic trifluoromethylation over the past several years.16 Herein we report a highly efficient method to construct 3trifluoromethyl coumarins using CF3SO2Cl via visible lightpromoted cascade radical cyclization with ester 3arylpropiolates. Ding’s group reported a similar transformation using Togni’s reagent in 2014.6b Their strategy delivered a moderate yield but limited scope of substrates. Use of electron-deficient 3-arylpropiolates gave low yields. In addition, the characterization of structures seemed incorrect in comparation 1HNMR data of 3a in their report (the structure of 3a, see Scheme 1, e) and those of 3b in this work. 1HNMR spectra of two compounds were identical, however, the structure of 3b in this work was unambiguously confirmed by X-ray crystal analysis (Figure 1, Figure S1). Scheme 1: Trifluoromethylation of sp2 and sp3-C using CF3SO2Cl as trifluoromethyl source Previous works: H
CF 3
Photocatalyst(1-2 mol%)
R
+
CF3SO 2Cl
R
K 2HPO 4,MeCN, rt 26w light source
X
(a) X
MacMillan, D.W.C Nature 2011, 224 R2
R
N R1
O
R2
CF3SO 2Cl(2 equiv) Ru(Phen)3Cl2(1 mol%)
+ CF SO Cl 3 2
CF3
R
NaOAc(3.5 equvi)/AcOH Visible Light, rt, 12h
O N R 1 Yield: 50-95%
(b)
Dolbier W. R., jr; Org. Lett. 2014, 4594 Eosin Y (3 mol%) Na2HPO 4(1 equiv)
Ph
CF 3
+ CF3SO 2Cl
Ph 1,4-CHD/CH3CN rt,14h,blue LEDs
N3
N H
(c) Yield: 31%
Gu, L-J; Chem. Commun. 2017, 4203 Ir(ppy) 3 (2 mol%) Li2CO 3 (0.5 equiv)
R + CF3SO 2Cl
Ar
acetone, 25 oC, blue LEDs, 12h
R=alkyl, amide, ester R2 R1
O
+ I O
O
CF3
obtained in 45% yield (Table 1, entry 1). Encouraged by this result, we optimized the reaction conditions with respect to photocatalysts, solvents, trifluoromethyl sources, etc... The results are outlined in Table 1 and Table S4. A survey of photocatalysts demonstrated that Ru(bpy)3Cl2 exhibited the best catalytic effect. Other photocatalysts, such as Eosin Y, Rose Bengal and fac-Ir(ppy)3, showed lower catalytic efficiency (Table 1, entries 1-4). The screening of solvents showed that use of single solvent, such as DCM, DMF, DMSO, H2O, DCM, THF, MeOH, NMP, etc., did not favor the reaction (isolated yield
