Letter pubs.acs.org/OrgLett
Benzene C−H Etherification via Photocatalytic Hydrogen-Evolution Cross-Coupling Reaction Yi-Wen Zheng,†,‡ Pan Ye,† Bin Chen,*,‡ Qing-Yuan Meng,‡ Ke Feng,‡ Wenguang Wang,† Li-Zhu Wu,‡ and Chen-Ho Tung*,†,‡ †
Key Laboratory of Colloid and Interface Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China ‡ Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry & University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, PR China S Supporting Information *
ABSTRACT: Aryl ethers can be constructed from the direct coupling between the benzene C−H bond and the alcohol O−H bond with the evolution of hydrogen via the synergistic merger of photocatalysis and cobalt catalysis. Utilizing the dual catalyst system consisting of 3-cyano-1-methylquinolinum photocatalyst and cobaloxime, intermolecular etherification of arenes with various alcohols and intramolecular alkoxylation of 3-phenylpropanols with formation of chromanes are accomplished. These reactions proceed at remarkably mild conditions, and the sole byproduct is equivalent hydrogen gas.
A
Scheme 1. Aryl Ether Synthesis
ryl ethers are structural constituents of pharmaceuticals and bioactive natural products,1 and these ethers represent one of the most ideal substrates that can be used to replace aryl halides for catalytic cross-coupling reactions in terms of availability, cost, safety, and atom efficiency.2 Over the years, typical methodologies for aryl etherification have been well established, such as the Ullmann ethers synthesis3 and the Chan−Evans−Lam reaction.4 The scope of aryl ethers was largely extended by the Buchwald−Hartwig cross-coupling approach. This protocol was demonstrated to be efficient for the coupling of aryl halides with phenols or alcohols (Buchwald’s group)5 and aryl halides with alkoxides (Hartwig’s group)6 in the presence of palladium catalyst and base (Scheme 1a). Recently, MacMillan and co-workers developed a highly efficient and general carbon−oxygen formation reaction employing simple alcohols and aryl halides via the synergistic merger of photoredox catalysis and nickel catalysis (Scheme 1b).7 However, in the above methods, the leaving groups in the desired substituted position of the starting materials have to be installed in preceding synthetic steps. In addition, by using any kind of leaving group stoichiometric amounts of unwanted coproducts or chemical waste will be inevitably produced. Therefore, cross coupling between aromatic C−H bond and alcoholic O−H bond will be an ecological and economical methodology for aryl ether synthesis. Cross-dehydrogenative coupling (CDC) builds C−C bonds or C−heteroatom bonds through a direct coupling either from two different C−H bonds or from one C−H bond and one heteroatom−H bond.8 It eliminates the prefunctionalization of the starting material, and the overall skeletal structure of the product resembles that of the starting materials. In the past few © 2017 American Chemical Society
years, a few examples of aryl ether synthesis from the coupling between simple arenes and alcohols were successfully demonstrated by the utilization of CDC strategy in the presence of Pd, Cu/Ag or Co catalysis.9 However, in these reactions, stoichiometric oxidants were used, and large amounts of waste were produced. To date, to the best of our knowledge, Received: February 15, 2017 Published: April 14, 2017 2206
DOI: 10.1021/acs.orglett.7b00463 Org. Lett. 2017, 19, 2206−2209
Letter
Organic Letters only one example of direct etherification of benzene was reported by Fukuzumi and co-workers, who used oxygen as the oxidant.10 However, in the oxidative conditions, the reaction yield and selectivity were low, and the substrate scope was still limited (Scheme 1c). Herein, we describe an oxidant-free hydrogen-evolution cross-coupling approach for photocatalytic one-step benzene C−H etherification by alcohols with high selectivity, and an equivalent amount of hydrogen was obtained as the sole byproduct (Scheme 1d). Photocatalytic hydrogen-evolution cross-coupling, which consists of a photoredox cycle and hydrogen-evolution system, was developed by Wu, Tung, and co-workers.11 Utilizing this strategy, a quinolinum ion photocatalyst (QuH+ or QuCN+) with a cobalt catalyst [Co(dmgBF 2 ) 2 (CH3 CN) 2 ] were combined for the successful synthesis of aniline directly from benzene and ammonia and phenol from benzene and water with evolution of hydrogen gas under remarkably mild conditions in excellent yields in our preceding work.12 We anticipate that it should be possible to utilize the dual catalyst system to synthesize aryl ethers directly from coupling of the benzene C−H bond and the alcohol O−H bond with evolution of hydrogen. An outline of a possible mechanism for photocatalytic hydrogen-evolution cross-coupling of benzene with alcohol is illustrated in Scheme 2. On the basis of the established one-
(5 mol %), and Co(dmgBF2)2(CH3CN)2 (3 mol %) in dry and degassed acetonitrile by mercury lamp (λ > 300 nm) at room temperature provided the anisole in 95% yield based on the consumption of benzene. Generally, after 5 h of irradiation, the conversion of benzene was 80%. A quantitative yield of H2 was also obtained (Scheme 3 and Table S1). Control experiments Scheme 3. Photocatalytic Etherification of Benzene with Alcoholsa
a
Reaction conditions: benzene (0.2 mmol), ROH (10 equiv). Conversions (based on benzene) were determined by GC and yields were determined by GC (3a and 3b) or NMR (3c−l) using ntetradecane as an internal standard. Isolated yields were given in parentheses. Selectivity = GC yield or NMR yield/conversion.
Scheme 2. Proposed Pathway for Benzene Etherification via Photocatalytic Hydrogen-Evolution Cross-Coupling
demonstrated that photocatalyst, metal cocatalyst, and light were necessary to achieve C−O bond formation (Table S1). It should be noted that the produced anisole cannot undergo further photoreactions to yield dimethoxybenzene even at high benzene conversion. To verify this observation, anisole instead benzene was employed as starting material. Though anisole (Ep/2 = 1.81 V vs SCE)16 possesses a lower oxidation potential than benzene, no consumption of anisole occurred under the same conditions. According to the previous research,13a,17 the resistance of anisole during photocatalytic alkoxylation may result from the fast back-electron transfer of the generated charge pair of the photocatalyst anion radical and the anisole radical cation. Using the optimal conditions, we examined the scope of this hydrogen-evolution crossing-coupling method for benzene etherification by various alcohols (Scheme 3). For coupling of benzene with primary alcohols, good to excellent selectivities were obtained with a stoichiometric amount of hydrogen gas. For example, a series of simple alkyl alcohols were effective coupling partners (3a−d). Moreover, halohydrins were also found to participate efficiently (3e and 3f) in the reactions. Importantly the use of glycol ethers as starting materials did not compromise the efficiency of the cross-coupling (3g−i). Notably, as an alkyl alcohol bearing two hydroxyl groups (ethylene glycol) was used, only one of them could be coupled to benzene with a high level of efficiency to form 2phenoxyethanol (3j), which has antimicrobial activity18 and acts as a synthetic intermediate in the production of plasticizers, pharmaceuticals, and fragrances.19 Carbonyl and ester groups were also tolerated. Corresponding products 3k and 3l were obtained in 91% and 93% yield, respectively, based on the
electron redox potentials of QuCN+* (Ered = 2.72 V vs SCE)13 and benzene (Eox = 2.48 V vs SCE),14 we assumed that the reaction could be initiated via electron transfer from benzene to the singlet excited state of QuCN+ upon irradiation. This crucial electron-transfer step could lead to a benzene radical cation and a photocatalyst radical anion that would then transfer an electron to the Co II (dmgBF 2 ) 2 (CH 3 CN) 2 [Eox(QuCN• ) = E red(QuCN+) = −0.60 V vs SCE,13b E0(CoII/I) = −0.55 V vs SCE15] to produce CoI complex and ground-state QuCN+, accordingly completing the photocatalysis cycle. The resulting benzene radical cation could react with a nucleophile (ROH) to give a dienyl radical and eliminate a proton, which would be captured by the CoI complex to form CoIII hydride. The intermediate metal hydride accepts an electron from the dienyl radical and decomposes by proton attack to return to CoII, concluding the cobalt catalysis cycle and producing H2. The generated aryl cation loses a proton to yield aryl ether. We began our direct benzene etherification studies by using methanol as alkoxylation reagent with the photocatalyst QuCN+ and metal cocatalyst Co(dmgBF2)2(CH3CN)2. Upon investigating a range of reaction parameters, irradiation of benzene (1, 40 mM), methanol (2a, 10 equiv), QuCN+ClO4− 2207
DOI: 10.1021/acs.orglett.7b00463 Org. Lett. 2017, 19, 2206−2209
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Organic Letters
Montmorillonite K10.22 Finally, butoxycaine 6 was prepared as the previous literature23 in 91% yield by coupling 5 with CO and (diethylamino)ethanol. The buloxycaine was successfully de novo synthesized from benzene by the application of the photocatalytic hydrogen-evolution cross-coupling method in three steps. Analogous to the intermolecular direct benzene C−H etherification, intramolecular C−H alkoxylation of substituted arenes also performed well using this synergistic protocol (Scheme 6). 3-Phenyl-1-propanol was preliminarily investigated
consumption of the starting material. However, secondary alcohols that were readily oxidized into the corresponding ketones20 were not suitable for this method. We next turned our attention to explore the scope of arene components (Scheme 4). A diverse array of electron-deficient Scheme 4. Photocatalytic Etherification of Arenes with Methanola
Scheme 6. Photocatalytic Intramolecular Etherification of Arenesa
a
Reaction conditions: arene (0.2 mmol), CH3OH (10 equiv). Conversions (based on arene) and yields were determined by NMR using n-tetradecane as an internal standard. Isolated yields were given in parentheses. Selectivity = NMR yield/conversion. The ratio of the isomer was determined by NMR. a
Reaction conditions: arene (0.2 mmol). Conversions (based on arene) and yields were determined by NMR using n-tetradecane as an internal standard. Isolated yields were given in parentheses. Selectivity = NMR yield/conversion. The ratio of the isomer was determined by NMR.
arenes with functional groups such as ketones, acid, and ether were viable substrates, affording a mixture of ortho, meta, and para isomers (3m−p). Among them, ortho methoxylation products were substantial for phenyl ketones and benzoic acid, while similar amounts of ortho and para isomers for benzoate (3p) were produced. Additionally, a variety of chlorobenzenes were found to be competent substrates in this transformation (3q−s, 69−87% conversion, 89−97% selectivity). We sought to demonstrate that this methodology is applicable for the synthesis of practical and high-value building blocks such as pharmaceuticals. More specifically, we envisaged butoxycaine that could be de novo synthesized from benzene in utility of the photocatalytic hydrogen-evolution cross-coupling C−O bond formation strategy (Scheme 5). In contrast to the
without further optimization, and chromane (8a) was obtained in 21% yield with >99% selectivity. The unreacted 3-phenyl-1propanol was quantitatively recovered. Various functional groups could be well compatible to produce chromanes in modest yields with nearly 100% selectivity. Transformation proceeds smoothly in the presence of electron-withdrawing substituents (8b, 52% yield). The synthetically useful halogens were well tolerated, and the influence of regioisomeric substitution was also explored. The chloro- or bromosubstituent in the ortho position leads to the single regioisomer 5-chloro- or bromochromane (8c and 8d). However, two regiosomers, 6- and 8- halogenated chromanes (8e−f′), were found for meta-halogen-substituted substrates with regioselectivity ratio greater than 4:1. In summary, the synthesis of aryl ethers through direct etherification of benzene with alcohols via an external oxidantfree dual catalytic strategy has been developed. The only byproduct for this reaction is equivalent hydrogen gas. This catalytic system also permits the direct intramolecular alkoxylation of 3-phenyl-1-propanols for chromane construction. These results demonstrate that hydrogen-evolution crosscoupling can be a useful strategy to achieve a variety of challenging bond constructions as long as the redox potentials of substrate, photocatalyst, and cocatalyst are matched.
Scheme 5. De novo Synthesis of Buloxycaine from Benzene
previous report,21 benzene was used as a starting material for the synthesized butoxycaine. First, butoxybenzene was synthesized by hydrogen-evolution cross-coupling of benzene (0.2 mmol) with 1-butanol under the optimal conditions as described above in a yield of 56% with 99% selectivity (33% yield of 4 was obtained when benzene was scaled up to 4 mmol). Subsequently, regioselective bromination of 4 with anhydrous sodium bromide gave 90% yield of 5 in the presence of sodium chlorite and Mn(acac)3 with the aid of moist 2208
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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.7b00463. Experimental procedures and characterization data of products (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Bin Chen: 0000-0003-0437-1442 Wenguang Wang: 0000-0002-4108-7865 Li-Zhu Wu: 0000-0002-5561-9922 Chen-Ho Tung: 0000-0001-9999-9755 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology of China (2013CB834804, 2013CB834505, and 2014CB239402), the National Natural Science Foundation of China (21390404, 91427303, and 21402217), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB17030400), and the Technical Institute of Physics and Chemistry, CAS.
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DOI: 10.1021/acs.orglett.7b00463 Org. Lett. 2017, 19, 2206−2209
