Metal-Free Etherification of Aryl Methyl Ether Derivatives by C–OMe

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

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Metal-Free Etherification of Aryl Methyl Ether Derivatives by C−OMe Bond Cleavage Xueqiang Wang,†,* Chenchen Li,† Xia Wang,† Qingli Wang,† Xiu-Qin Dong,§ Abing Duan,‡,* and Wanxiang Zhao*,†

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†

State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan 410082, China ‡ College of Environmental Science and Engineering, Hunan University, Changsha, Hunan 410082, China § Key Laboratory of Biomedical Polymers, Ministry of Education, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, China S Supporting Information *

ABSTRACT: A general and efficient protocol was developed for the synthesis of aryl alkyl ethers through metal-free C− OMe bond cleavage under mild reaction conditions. This process displays a wide scope of methoxyarenes and alcohols, including primary, secondary, and tertiary alcohols, as well as natural products, pharmaceuticals, and biologically active alcohols. DFT calculations and experimental results simultaneously confirm that a potassium ion plays a critical role in the activation of methoxy group via binding with the nitrile and provide support for an SNAr mechanism. fragrances, and cosmetics.2c Synthesis of aryl alkyl ethers has thus gained intense interest from synthetic chemists (Figure 1b).3−6 Conventionally, aryl alkyl ethers are prepared via the Williamson ether synthesis,3 in which phenol derivative is coupled with an alkyl halide. However, tertiary alkyl halides tend to undergo E2 elimination to yield alkenes instead of aryl alkyl ethers in the presence of base.3b Alternatively, transitionmetal-catalyzed C−O bond formation reactions are one of the most reliable methods to access ethers.4 In this context, copper-mediated Ullmann ether synthesis and Chan−Evans− Lam coupling represent useful protocols to prepare aryl alkyl ethers, despite the drawbacks of using a stoichiometric amount of copper and/or harsh reaction conditions.4a−h On the other hand, palladium-catalyzed C−O bond formation has also become a powerful approach for the synthesis of aryl alkyl ethers, which have been developed intensively by Buchwald, Hartwig, as well as other groups.4g−i,5 Additionally, nucleophilic aromatic substitution (SNAr) reactions offer a convenient access to aryl alkyl ethers as well.6 However, the scope is limited to the arenes bearing strong electron-withdrawing groups and with halides as the leaving groups.6 Nevertheless, methods for halogenation generally suffer from harsh reaction conditions and poor selectivity or require multistep synthesis. On the other hand, aryl methyl ethers are ideal alternatives to aryl halides for SNAr reactions in terms of not only environmental friendliness but also synthetic accessibility and

A

ryl alkyl ethers are ubiquitous in biologically active agents and pharmaceutically important molecules,1 such as febuxostat,1c butoxycaine,1d and duloxetine1e (Figure 1a). In addition, they have emerged as synthetically valuable building blocks in the synthesis of catalysts,2a functional materials,2b

Figure 1. (a) Representative drugs containing aryl alkyl ether. (b) Aryl alkyl ether synthesis. © XXXX American Chemical Society

Received: May 30, 2018

A

DOI: 10.1021/acs.orglett.8b01696 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 1. Reaction Scopea

a

Conditions: 1 (0.5 mmol), R′OH (1.0 mmol), KOtBu (1.0 mmol), dioxane (0.5 mL). bThe substrate is piperonylonitrile. cThe reaction was conducted at ambient temperature. dWithout alcohol. eThe reaction was conducted using 2.0 equiv of KHMDS at 60 °C.

The past decade has witnessed tremendous advances in transition-metal-catalyzed (particularly nickel-catalyzed) C− OMe bond cleavage of methoxyarenes8 for borylation,9a silylation,9b amination,9c reduction9d−g and couplings with organometallic reagents,9h−s although C−OMe cleavage requires high activation energy. In contrast, metal-free C− OMe bond activation is far less common and more challenging,10 and few available examples are limited to the SNAr reactions of methoxyarenes with nitro groups.7 Not surprisingly, etherification of anisole analogues has been

reliability. However, few examples on nucleophilic aromatic substitution of aryl methyl ethers are available, likely due to the inertness of the C−O bond.7 To address this issue, at least two strong electron-withdrawing groups are required to activate the C−O bond.7a−c These drawbacks largely limit the applications of the SNAr reaction in etherification. Therefore, we questioned whether it is possible to develop an efficient and practical method to produce aryl alkyl ethers from aryl methyl ethers through the optimal combination of an electronwithdrawing group and reaction conditions. B

DOI: 10.1021/acs.orglett.8b01696 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 2. Scope of Complex Alcoholsa

a Conditions: 1 (0.5 mmol), R′OH (1.0 mmol), KOtBu (1.0 mmol), dioxane (0.5 mL). bThe reaction was performed at 80 °C. c1.0 mL of dioxane was used. dThe reaction was conducted using 2.0 equiv of KHMDS at 60 °C.

virtually unexplored via transition-metal-catalyzed or metal-free C−OMe bond scission. Traditionally, etherification of aryl methyl ethers requires two-step routes, namely, demethylation and alkylation.11 Taking into consideration the significance of aryl alkyl ethers and enlightened by Ogata and Okano’s work on the SNAr reactions of 2,4-dinitroanisoles with alcohols,12 herein we describe an efficient method to prepare aryl alkyl ethers from methoxyarenes with a cyano group via C−OMe bond cleavage. We began our studies of alcohol exchange reactions with 2methoxybenzonitrile 1a as the model substrate and 2-propanol as the nucleophile. After efforts toward the evaluation of

reaction parameters, we found the best yield was obtained when KOtBu was used as the base and 1,4-dioxane as the solvent (see details in the Supporting Information). With the optimal conditions determined, we then looked into the substrate scope of this reaction. A series of o-methoxy aromatic nitriles with different substituents at different positions reacted smoothly with 2-propanol to afford the corresponding aryl alkyl ethers in good to excellent yields (Scheme 1). Various functional groups, including bromide (2ab), chloride (2ac), trifluoromethyl (2ae), and nitrile (2af), are well tolerated. In addition, the mild reaction conditions are compatible with a range of heterocycles, such as dibenzofuran (2ah), furan (2ai), C

DOI: 10.1021/acs.orglett.8b01696 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

proposed that this alcohol-exchange reaction underwent an SNAr mechanism. To further confirm this, we performed a series of density functional theory (DFT) calculations using the methoxybenzonitriles (include o-, m- and p-methoxybenzonitrile) as the aromatic substrates and alkali isoproxides (MOiPr, including KOiPr, NaOiPr and LiOiPr) as the nucleophiles (see the Supporting Information). We also studied the position effect of the methoxy using the methoxybenzonitriles and KOiPr. On the basis of the calculation results, we conclude that the nature of the alkalimetal ion of MOiPr and the substrates themselves have profound effects in terms of reactivity. That is to say, the o-methoxybenzonitrile is the more reactive substrate than p-methoxybenzonitrile, while the mmethoxybenzonitrile turns out not to be suitable for the reaction with KOiPr. Combing the experimental and computational studies results, we believe that our metal-free etherification of aryl methyl ethers by C−OMe bond cleavage involves an SNAr-type (addition−elimination) mechanism. The utility of this methodology is demonstrated through a gram-scale reaction of 2-methoxy-1-naphthonitrile with (+)-fenchol, which furnished the etheric product 2bs in nearly quantitative yield (Figure 2a). We also carried out a one-pot

thiophene (2aj), and pyridine (2ak). This is particularly noteworthy as not only o-methoxy aromatic nitriles but also pmethoxy aromatic nitriles successfully participated in this etherificaton reaction, although the m-OMe is unreactive under the standard conditions. The p-methoxy substrates bearing trifluoromethyl and methoxy all worked well, yielding the products 2am and 2an in 89% and 98% yields, respectively. The hindered C−OMe bond could also efficiently undergo a substitution reaction to form the product 2ao. Notably, the isopropoxyl group can be incorporated into both the ortho- and para-positions at the same time (2ap). Piperonylonitrile reacted smoothly with 2-propanol to furnish the ring-opening product 2aq in 85% yield. Naphthalenes are also suitable substrates for this C−O bond cleavage and formation process and proceeded smoothly at room temperature to give the corresponding products 2ar and 2as in excellent yields. Furthermore, we were also interested in evaluating the scope of the alcohols for our protocol. As shown in Scheme 1, a variety of alcohols were applicable to provide the aryl alkyl ethers with high efficiency, including primary, secondary, and tertiary alcohols. The mild conditions can tolerate a diverse set of functional groups, including chloride (2av), alkyne (2ba), and alkene (2bb). Pyrazole could also be well accommodated in this reaction (2bc). It is worth noting that the chiral alcohol (+)-2-butanol is a competent coupling partner for this transformation (2av). Surprisingly, the bulky tert-butoxy can act as the nucleophile to yield the tert-butyl ethers 2bh and 2bi in the absence of the alcohol. Consequently, 2.0 equiv of KHMDS was utilized as the base instead of the KOtBu to avoid the incorporation of the tert-butoxy when tertiary alcohols were employed (2bj−bl). To further demonstrate the generality of our method, a range of complex alcohols were subjected to the standard reaction conditions, including natural products, pharmaceuticals, and biologically active molecules, and were all successfully transformed to the corresponding aryl alkyl ethers in good yields (Scheme 2). The terpenoids such as (−)-perillyl alcohol and nerol were found to be compatible nucleophiles (2bm and 2bn). Perphenazine used as antipsychotic drug also proceeded smoothly to give the etherification product 2bo, albeit in 32% yield. Additionally, a collection of secondarynatural alcohols, such as (+)-menthol, (+)-fenchol, and (−)-borneol, were excellent substrates for this transformation (2bp−bt). We were pleased to find that nitrogen-containing alcohols (N-Bn-pyrrolidinol and 3-quinuclidinol) provided the etheric products in 55% (2bu) and 99% yields(2bv), respectively. Upon applying our reaction conditions to estradiol, abiraterone, cholesterol, oleanolic acid (methyl ester), and vitamins D2 and D3, the desired products (2bw−cb) were all successfully obtained. With regard to bulky tertiary alcohols, (+)-cedrol and the alcohol derived from estrone were also converted efficiently to the ethers 2cc and 2cd. All of these results demonstrate the extraordinarily broad applicability of our method and immense utilization potentiality for late-stage modifications of complex molecules. To gain insightful mechanistic information from this transformation, a series of complementary experiments were carried out. We first investigated the possibility of a radical mechanism. In the presence of a radical scavenger such as TEMPO and BHT, no significant change was observed in terms of yield and reaction rate (see the Supporting Information). These results might allow us to exclude the possibility of a radical pathway. On the other hand, we

Figure 2. (a) Gram-scale reaction. (b) Iterative C−O bond formation. (c) Synthesis of butoxycaine.

iterative C−O bond formation process from 2-fluoro-4methoxybenzonitrile. Treated with KOtBu, the C−F bond underwent an SNAr reaction with (+)-fenchol. Without purification, the reaction mixture was then subjected to 2.0 equiv of 2-propanol to form the product 2ce in 48% total yield via C−OMe bond cleavage (Figure 2b). Thus, our protocol is in principle capable of assembling any two different alcohols through this sequential process. Finally, this method was also applied in the synthesis of butoxycaine with high efficiency D

DOI: 10.1021/acs.orglett.8b01696 Org. Lett. XXXX, XXX, XXX−XXX

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from readily available 4-methoxybenzonitrile in three steps (Figure 2c). Diversification of the cyano group in aryl alkyl ether 2bp is shown in Figure 3. The C−CN bond could be easily reduced

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01696. Experimental procedures, NMR spectra, and computational data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Xueqiang Wang: 0000-0003-1631-9158 Chenchen Li: 0000-0002-2956-7639 Wanxiang Zhao: 0000-0002-6313-399X Notes

The authors declare no competing financial interest.

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Figure 3. Diversification of product. Reaction conditions: (a) [RhCl(cod)]2 (5 mol %), P(OnBu)3 (10 mol %), iPr3SiH (2 equiv), ethylcyclohexane, 130 °C, 15 h. (b) B2(nep)2 (2 equiv), [RhCl(cod)]2 (5 mol %), Xantphos (20 mol %), DABCO (1 equiv), toluene, 100 °C, 15 h. (c) DIBAL (1.1 equiv), toluene, 0 °C to rt. (d) MeMgBr (1.2 equiv), Et2O, 0−60 °C, overnight. (e) KOH (aq), EtOH, 80 °C. (f) NaOH (6 M), MeOH, 70 °C, 7 h.

ACKNOWLEDGMENTS Financial support was provided by the National Natural Science Foundation of China (Grant No. 21702056, 21605070), the National Program for Thousand Young Talents of China, the Fundamental Research Funds for the Central Universities, and the State Key Laboratory of Analytical Chemistry for Life Science (Grant No. SKLACLS1505).

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to C−H bond by Chatani’s method in the presence of rhodium catalyst and triisopropylsilane (Figure 3a).13 The nitrile was also successfully converted to boronic ester through rhodiumcatalyzed borylation reaction (Figure 3b).14 Moreover, the nitrile underwent reduction, addition with Grignard reagent and hydrolysis to provide the corresponding aldehyde, ketone, carboxylic acid, and amide in moderate to excellent yields (Figure 3c−f). In summary, we have developed a highly efficient and general method for the synthesis of aryl alkyl ethers from methoxy arenes and alcohols. This reaction underwent a metalfree C−OMe bond cleavage and C−O bond formation process and represents one of the few metal-free C−O bond activation reactions. More importantly, the mild reaction conditions are compatible with a wide range of methoxyarenes and alcohols, including primary, secondary, and even bulky tertiary alcohols. The protocol is also applicable to a series of natural products, drugs, and biologically active molecules. The great utility of our methodology has been demonstrated by the gram-scale reaction, the iterative C−O bond formation, and diversified product derivatization as well as its application in the synthesis of butoxycaine. Perfect consistency between the computational and experimental results was observed and suggested an SNArtype pathway. This noteworthy transformation, employing readily available reagents and possessing broad substrate scope and functional group compatibility, is anticipated to find applications in late-stage modifications of complex molecules, especially in pharmaceuticals.

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DOI: 10.1021/acs.orglett.8b01696 Org. Lett. XXXX, XXX, XXX−XXX