Metal-free electrochemical fluorodecarboxylation of aryloxyacetic acids to fluoromethyl aryl ethers
Electrochemical decarboxylation of aryloxyacetic acids followed by fluorination provides easy access to fluoromethyl aryl ethers. This electrochemical fluorodecarboxylation offers a sustainable approach with electric current as traceless oxidant. Using Et 3 N$5HF as fluoride source and as supporting electrolyte, this simple electrosynthesis affords various fluoromethoxyarenes in yields up to 85%.
Introduction<!>Results and discussion<!>Conclusions<!>Conflicts of interest
<p>Fluoromethyl aryl ethers have intriguing properties 1 and show emerging importance in agrochemical and pharmaceutical applications. 2 Replacing a hydrogen atom by uorine as a bioisostere for example in methyl groups has advanced to a common method in molecular editing of drugs. The incorporation of uorine can improve the metabolic stability of a drug and increase its potency. 3 Furthermore, uoromethyl aryl ethers feature a striking shi in volatility and scent compared to their non-uorinated analogues, making them possible candidates for pheromones or fragrances. 4 Therefore, widely applicable methods installing uorine selectively into a molecule are of high interest.</p><p>The rst synthetic method for the formation of uoromethyl aryl ethers involves an electrophilic monouoromethylation of a phenol under basic conditions using FCH 2 Cl with chloride as leaving group. 5 Furthermore, direct nucleophilic monouoromethylation of phenols and thiophenes with monouoromethyl-substituted sulfonium ylides has been reported. 6 Although their scope shows wide applicability, rstly, the monouoromethylating agent is considered as an impactful green-house gas (CH 2 FCl) and secondly the reagent has to be prepared by a tedious synthesis including several steps. 7 In a much less troublesome approach, phenoxyacetic acids are used in decarboxylation reactions followed by the introduction of uorine. One of the earliest methods for this reaction type involves the use of XeF 2 for a radical uorodecarboxylation, 8,9 but this reagent is expensive and difficult to handle due to its enormous reactivity. In the recent years, several methods for decarboxylative uorinations have been reported. For example, MacMillan showed the decarboxylative uorination of aliphatic carboxylic acids using photoredox catalysis in combination with Selectuor™. 10 Furthermore, Sammis and Hartwig reported on Hunsdiecker-type uorodecarboxylation reactions 11,12 using AgF 2 starting either from phenoxyacetic acids, a-uoroor a,a-diuorocarboxylates to obtain the corresponding mono-, di-and triuoromethyl aryl ethers. Additionally, Tang showed the decarboxylative uorination of electron-rich heteroaromatic carboxylic acids using Selectuor™ in combination with KF in dichloroethane/H 2 O mixtures. 13 However, these methods require either a photocatalyst or an excess of oxidizing and uorinating agents like Selectuor/NFSI, 11,14 XeF 2 or AgF 2 which can be very powerful, 8,12 but have many drawbacks like their hazardousness and high costs for the reagents and in particular the ruthenium based catalyst.</p><p>Thus, an electrochemical approach for uorodecarboxylation reactions could tweak these ndings, since electric current can be used as a green oxidant to generate reactive intermediates in situ, like the rst electrosynthetic decarboxylation reaction shown by Kolbe in 1849. 15 The well-known Kolbeelectrolysis gives the dimer of two aliphatic carboxylic acids by decarboxylation. 15,16 Organic electrochemistry offers many advantages over traditional, reagent-based reactions, because usual reagents are oen toxic, costly and generate a lot of reagent waste. It solely depends on electric current as a renewable, inexpensive and inherently safe reagent. 17 Therefore, organic electrochemistry attracted a lot of attention in the recent years, 18 and especially electrochemical uorination reactions proved to be powerful tools for organouorine synthesis. 19 Recently, we worked on the electrochemical synthesis of aryl methoxymethyl ethers by electrochemical decarboxylation of phenoxyacetic acids (Scheme 1). 20 Furthermore, Baran could demonstrate trapping carbocations generated by electrochemical decarboxylation of tertiary carboxylic acids. This way, uorine has been introduced in few examples using KF/18crown-6 and AgClO 4 as sacricial oxidant. 21 That uorodecarboxylation relies on silver(I) salts, which are known for Hunsdiecker-type decarboxylations (see Scheme 1). 12 In contrast, we present a novel metal-free electrochemical uorodecarboxylation of simply accessible aryloxyacetic acids 22 to uoromethyl aryl ethers by a pseudo-Kolbe pathway.</p><!><p>The screening experiments were conducted in undivided cells equipped with a simple two-electrode arrangement using constant current conditions. 23 Electrolyses were performed at isostatic graphite electrodes (C gr ), a common anode material for pseudo-Kolbe electrolyses. The conversion of 4-tert-butylphenoxyacetic acid (1a) in dichloromethane with a substrate concentration of 0.1 mol L À1 served as benchmark reaction for optimisation studies. Various parameters and their inuence on the reaction outcome like current density, applied charge, different uoride sources and electrode materials were investigated. Since the electrolyte system turned out to be the most critical parameter, a selection of screening experiments is shown in Table 1, additional screening experiments are summarized in the ESI. † The yield of the optimisation experiments was determined by 1 H NMR using 1,3,5-trimethoxybenzene as internal standard (Table 1). With an applied charge of 3 F, a current density of 5.5 mA cm À2 , potassium uoride (3.0 equiv.), 18-crown-6 (3.0 equiv.), 2,4,6-collidine (3.0 equiv.) in a 0.1 M NBu 4 PF 6 in dichloromethane (5 mL) it was possible to generate the desired uorinated product 2a in 30% yield (Table 1, entry 1). The control experiment without applied current (no conversion of starting material) indicated, that the anodic oxidation of the carboxylic acid is crucial for the conversion of 1a. Furthermore, a control experiment without adding a base demonstrated its signicance (no product formation observed) for the reaction. Higher quantities of KF did not increase the yield (Table 1, entry 2). Replacing 2,4,6collidine with DBU as base resulted in no product formation, while the starting material was consumed during electrolysis. However, the formation of Kolbe-type products was observed as side reaction when KF was used as uoride source. Therefore, other uoride sources such as amine uorides were tested. Using Py$9HF (Table 1, entry 3) as uoride source, the Kolbe reaction was completely suppressed, but the yield of 2a dropped as well. With the use of triethylamine trihydrouoride (Et 3 -N$3HF) also no product formation was observed. Yet, with triethylamine pentahydrouoride (Et 3 N$5HF) as uoride source, uoromethyl aryl ether 2a was obtained in 58% yield with the starting material being completely consumed (Table 1, entry 5). Therefore, the following experiments were conducted with Et 3 N$5HF as uoride source. Neither higher Et 3 N$5HF (CH 2 Cl 2 /Et 3 N$5HF, 3 : 2) nor lower amounts (9 : 1) increased the yield of 2a. When the amount of amine-HF was raised even higher (CH 2 Cl 2 /Et 3 N$5HF, 2 : 3), the graphite anode degraded during electrolysis. Raising the uoride concentration by adding KF also did not improve the yield, indicating that larger quantities of uoride do not benet the reaction outcome (Table 1, entry 8). In further optimisation studies, different current densities, amounts of charge and anode materials were tested. Both, with lower (2.8 mA cm À2 ) and higher current densities (11 mA cm À2 ) the yield of the uorinated product 2a (47% and 0%) decreased. For the oxidation of the carboxylic   acid 1a a theoretical charge of 2.0 F is needed, but lowering the applied charge from 3.0 F to 2.5 F led to lower yields (Table 1, entry 9). Lower quantities of 2,4,6-collidine decreased the yield substantially (Table 1, entry 10). The use of platinum or boron-doped diamond (BDD) as anode material lead to almost no formation of product 2a (Table 1, entries 11 and 12). Also using glassy carbon, another carbon allotrope close to graphite, as anode did not give the desired product with almost half of the starting material remaining non-converted upon electrolysis (Table 1, entry 13). In contrast to that, graphite foil as anode afforded the uorinated product 2a in 54% yield but did not exceed previous results (Table 1, entry 14). Finally, with an optimised electrolysis protocol, we explored the scope of this reaction with various aryl moieties (Scheme 2). The electrochemical uorodecarboxylation of phenoxyacetic acid 1a at optimised conditions gave product 2a in 50% isolated yield. An experiment with the a,a-dimethylated homologue of 2a did not provide the tertiary uoride, though it was anticipated that the resulting tertiary carbocation would stabilize the oxocarbenium ion. Tertiary uorides might not be electrochemically stable (too ionizable) or undergo elimination reactions. An additional methyl substituent in ortho-position afforded the uorinated product 2b in 61% yield. Starting from the para-nitro-substituted substrate gave uoromethoxybenzene 2c in 35% yield although nitro groups oen tend to cathodic side reactions. With another methyl group in orthoposition to the nitro group product 2d was obtained in 31% yield.</p><p>Furthermore, the 4-bromo-substituted phenoxyacetic acid gave the uorinated product 2e in 42%. The corresponding thiophenoxyacetic acid gave an even higher yield with 59% of uoromethyl aryl thioether 2f. With 4-cyano-phenoxyacetic acid only 24% of the uorinated product 2g was obtained under standard conditions. However, using graphite foil the yield of uoromethyl aryl ether 2g could be increased to 32%. In contrast, 4-methoxyphenoxyacetic acid did not show any product formation either with isostatic graphite (C gr ) or with graphite foil as electrodes. For the mechanism of this uorodecarboxylation process we postulate a pseudo-Kolbe pathway 24 (see ESI † for more information). Electron rich derivatives are therefore prone to possible side reactions at accessible ortho-positions. Blocking these positions should therefore prevent side reactions. Accordingly, with an additional sterically demanding tert-butyl-substituent in ortho-position, the uorodecarboxylation gave 59% yield of derivative 2h. Further, a phenoxyacetic acid bearing two tert-butyl substituents in metaposition was tested giving 72% yield of uoromethyl aryl ether 2i. To demonstrate the scalability of our method, that electrosynthesis was also performed on a 2.5 mmol scale and gave the uorinated product 2i in 85% yield. Moreover, derivatives based on natural products like chloro-substituted thymol and dtocopherol (vitamin E) were tested. In both cases, the electrolysis with graphite foil was superior to isostatic graphite, giving the uorinated thymol derivative 2j in 48% and the uorinated d-tocopherol 2n in 21% yield. Even with nitrogen heterocycles the uorodecarboxylation was successful. The uoromethoxy pyrimidine 2l was afforded in 44% and pyrimidine 2m with a 2-uoromethoxy group in 34% yield, respectively. It was also possible to generate uoro-ethoxypyridine 2n from the corresponding propionic acid in 53% yield.</p><p>In addition to that, for uoromethoxyarenes 2o-2t NMR yields up to 70% were achieved. However, due to their enhanced volatility, these products are difficult to isolate, something that has been reported previously 11 (see ESI † for more detailed information).</p><!><p>In summary, we established the rst synthesis of uoromethoxy aryl ethers by electrochemical uorodecarboxylation of aryloxyacetic acids using electric current as traceless oxidant. It enabled access towards a variety of uoromethoxyarenes. This electrochemical protocol is very easy to conduct with a simple setup in undivided cells at constant current conditions with different aryl and heterocyclic moieties and substitution patterns in yields up to 85%. The successful uorodecarboxylation of natural products and heterocycles demonstrated the broad applicability of this method even to demanding substrates. Besides, this electro-conversion is readily scalable with an increase in yield expected in the distillative solvent removal from the volatile uoromethyl ethers.</p><!><p>There are no conicts to declare.</p>