Fluorination

Living Edition
| Editors: Jinbo Hu, Teruo Umemoto

Fluoroolefin-Amine Adduct Deoxofluorination

  • Viacheslav PetrovEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-981-10-1855-8_24-1

Fluoroolefin-amine adducts (FAAs) belong to a broader group of α,α-fluoroalkyl amino reagents (FAR). FAAs are widely used for the conversion of alcohols and acids into the corresponding alkyl or acyl fluorides. Fluoroolefin-amine adducts (FAAs) are one of the oldest group of organic fluorinating agents, which became very popular due to their availability, simple synthesis, and ability to convert alcohols and acids into alkyl and acyl fluorides. The application of the first representative of this group – adduct of chlorotrifluoroethylene and diethylamine (Yarovenko reagent) – as a reagent for conversion of alcohols into alkyl fluorides was reported in 1959 [1].

The chemistry of FAAs and FAR was previously overviewed. Detailed reports can be found in the following articles: synthesis and chemistry of Yarovenko reagent [1], Yarovenko and Ishikawa reagents [2], TFEDMA reagent [3], adducts of amines and 2-H-pentafluoropropene [4], and FAR reagents [5].

The synthesis of FAAs is straightforward and is based on the reaction of the corresponding secondary amine (linear, dimethyl or diethyl amines, or cyclic – pyrrolidine, piperidine, or morpholine) with the corresponding fluoroolefin. Due to high electrophilicity of double bond, the addition of amines to fluoroolefins proceeds under mild conditions and often in the absence of a solvent. Using this procedure, a variety of FAAs can be prepared, including adducts of chlorotrifluoroethylene and diethylamine (DEA) [1, 6] (Yarovenko reagent) and other amines [7], hexafluoropropene and DEA (Ishikawa reagent) [8, 9], dimethylamine (DMA) and tetrafluoroethylene (TFEDMA) [10, 11, 12], various amines with 2-H-pentafluoropropene [15], adducts of perfluoromethyl vinyl ether with DMA [13], DEA [14], and long-chain perfluorinated vinyl ethers various amines[16, 17, 18]. The synthetic procedure involves slow addition of fluoroolefin either into a solution of amine in inert organic solvent (THF, dioxane, ether etc.) or into pure amine at low temperature, leading to the corresponding FAAs in moderate to high yields. Typically, these reactions are exothermic and require careful temperature control of the reaction mixture. Due to high reactivity FAAs toward glass, preferred materials of construction for reactors are TeflonR, carbon or stainless steel, polyethylene, polypropylene, etc.

The composition of FAAs depends strongly on the structure of starting fluoroolefin and reaction conditions. For example, a reaction of liquefied dimethylamine with tetrafluoroethylene (TFE) leads exclusively to saturated adduct (HCF2CF2N(CH3)2, TFEDMA) in high yield [12], while the reaction TFE with DEA is slow at ambient temperature, requires heating, and leads to only moderate yield of adduct, due to substantial tar formation [12].

Similar reaction of hexafluoropropene and diethylamine leads to a mixture of almost equal amount of CF3CFHCF2N(C2H5)2 and CF3CF = CFN(C2H5)2 [19]. In case of 2-H-pentafluoropropene, it was shown that the structure of amine influences the composition of the product significantly [15]. While reaction of this olefin with DMA in ether solvent gave predominantly saturated product CF3CH2CF2N(CH3)2, the reaction with DEA, piperidine, and morpholine led to predominant formation of unsaturated adducts [4, 15]. This phenomenon is likely to be related to the basicity of the corresponding amine, since saturated adduct, formed as a result of nucleophilic addition of amine across the double bond, can undergo dehydrofluorination under the action of amine, producing unsaturated material (Eq. 1):

All known FAAs are liquids, which can be distilled either at atmospheric or preferably at reduced pressure, with boiling points varied from 33 °C/6 mm Hg (Yarovenko reagent) to 32 °C/127 mm Hg (TFEDMA [5]. Most of FAAs are prepared under anhydrous conditions and used without additional purification. Due to high reactivity of FAA toward glass (especially at elevated temperature), it should be avoided as material of construction during preparation and purification of FAAs [12].

Although Ghosez reagents of general structure R(R′)C = CFNR2″ formally are products of amine reaction with R(R′)C = CF2 usually, they are prepared using a sequence involving a reaction of the corresponding amide with phosgene, treatment of the iminium chloride with base, which leads to R(R′)C = CClNR2″ [20] and it’s conversion into R(R′)C = CFNR2″ by treatment with KF or CsF at elevated temperature [21].

Stability of FAA at ambient temperature varies, depending on the structure. For example, Yarovenko reagent (ClCFHCF2N(C2H5)2) is the least stable and has to be either stored refrigerated or used freshly prepared [1], while TFEDMA has unlimited shelf life at ambient temperature, if it is stored, in the absence of moisture, in the vessels made of steel or TeflonR.

Most of FAA’s fume being exposed to moist air, react violently with water and are not compatible with hydroxyl-containing solvents. It also should be pointed out that the majority of FAAs are potent producers of hydrogen fluoride and should be handled by trained personnel, using personal protective equipment recommended for handling hydrogen fluoride.

Deoxyfluorination of Alcohols

The ability of FAAs to replace hydroxyl group of alcohols by fluorine was first reported by Yarovenko and Raksha [6] for HClCFCF2N(C2H5)2 (Yarovenko reagent). This reagent gained popularity after several research groups [6, 22, 23, 24] demonstrated its utility for the conversion of various steroids [25] and certain alkaloids [2] into the corresponding fluorides. Yarovenko reagent was extensively used for fluorination of steroids, and a review by Sheppard and Sharts [1] has an excellent overview of these reactions. The yields of the corresponding fluorinated steroids can vary broadly, depending on the structure and reaction conditions.

The Ishikawa group demonstrated application of hexafluoropropene diethylamine adduct for conversion of alcohols and carboxylic acids into alkyl and acyl fluorides [19], and later this reagent was used for deoxyfluorination of fatty [26], terpene [27], and halogenated [28] alcohols, α-hydroxy esters [29], nitro alcohols [30], and monoesters of ethylene glycol [31].

Despite the fact that adduct of tetrafluoroethylene and dimethylamine (TFEDMA) was first prepared in the late 1950's [10, 32], it found application as fluorinating agent only recently. Similar to Yarovenko and Ishikawa reagents, this compound was demonstrated to be a potent reagent for the conversion of hydrocarbon and polyfluorinated alcohols into the corresponding alkyl fluorides [12].

At the beginning of this century, the Koroniak research group introduced adducts of various amines with 2-H-pentafluoropropene and studied their reaction of various alcohols [15], and reported data [15] suggest slightly lower reactivity of this reagent compared to Yarovenko, Ishikawa, or TFEDMA agents.

The influence of the ratio FAA to alcohol was studied in case of CF3CH = CFN(C2H5)2 (DEPFPA), and it was shown that the best yields in the reaction of (R)-(-)-2-octanol, cholesterol, and t-butanol can be achieved using excess of fluorinating agent (2:1 or 3:1 molar ratio of DEPFPA to alcohol) [15].

Fluorination of Alcohols

Primary Alcohols

In general, the reaction of primary alcohols with FAAs is straightforward, resulting in the formation of primary alkyl fluoride, along with small amount of by-products, such as α-olefins, as a result of side reaction – water elimination. It should be pointed out that deoxyfluorination process leads to the formation of equimolar amounts of the corresponding amide of fluorinated acid and HF (in case of reagents, containing –CF2NR2 unit), along with the corresponding alkyl fluoride (Eq. 2) [12]:

In most cases, deoxyfluorination of primary alcohols is carried out at elevated temperature (40–65 °C), optionally, in inert solvent, such as HCCl3, CH2Cl2, ether, etc. While the isolation of the products is not difficult in cases when column chromatography can be used (high boiling point or crystalline products), it can be problematic in case of materials with boiling points close to those of RfC(O)NR2 by-products. On lab scale, the product isolation is usually carried out by washing reaction mixture with water. It should be pointed out that this method is not very effective in case of Yarovenko and Ishikawa reagents, due to high partion coefficient of amides in organic phase. TFEDMA reagent offers some advantage, since HCF2C(O)N(CH3)2 has significantly higher solubility in water [33].

It also should be pointed out that a drawback of Yarovenko reagent is the formation of chlorine-containing by-products during fluorination process, especially at elevated temperatures [2]. Chlorine in this case derives from the decomposition of the reagent resulting in the formation of chlorinated impurities, which sometime may be significant [34].

Ghosez-type reagents offer certain advantages. Since this reagents do not produce additional amount of HF in reaction with alcohols, deoxyfluorination reaction proceeds under neutral conditions, which allows to achieve higher selectivity in case of acid-sensitive materials [35]. Additional examples of fluorination reactions using (CH3)2C = CFN(i-Pr)2 can be found in ref. [5, 36].

Fluorinated primary alcohols containing two- or three-carbon spacer between fluoroalkyl group and -OH group undergo clean fluorination under the action of TFEDMA reagent giving the corresponding fluorides in high yield (Eq. 3) [12]:

Secondary and Tertiary Alcohols

Secondary alcohols are more reactive toward FAAs. The fluorination reaction usually proceeds under milder condition (0–20 °C) and leads to the formation of secondary alkyl fluorides. Yields of fluorides are lower (typically 40–70%), due to olefin by-product formation as a result of competitive dehydration process.

Fluorination of optically active alcohols by FAAs usually proceeds with inversion of configuration, with ee's varying between 40% and 99%. For example, the reaction of R-(-)-octanol-2 and DEPFPA proceeds with complete inversion leading to pure S-(+)-2-fluorooctane in yields up to 85% [15], while fluorination of S-(+)-octanol-2 using Yarovenko reagents resulted in the formation of 2-fluorooctane with ~88% optical purity (40% yield, 78:22 mixture of F-octane/octenes) [37].

Cyclic alcohols react with FFAs under mild conditions (as low as −25 °C), and the result of reaction strongly depends on the size of cycle of alcohol and type of fluorinating agent. In case of TFEDMA, fluorination of cyclopentanol and cycloheptanol leads the corresponding fluorides as predominate products (ratio fluoride/olefin 78:22 in both cases), while cyclooctanol gives the predominantly cyclooctyl fluoride (ratio fluoride/olefin 55:45). Cyclohexene forms as a major product in case of cyclohexanol and TFEDMA reaction (ratio fluoride/olefin 9:91) [12], while in case of Ishikawa reagent, cyclohexene was reported to be a single product [19]. On the other hand, (CH3)2C = CFN(i-Pr)2 was reported to convert cyclopentanol selectively into the corresponding fluoride (86% yield) [35]. The absence of significant amount of cyclopentene in this case may be attributed to the absence of hydrogen fluoride in the system, which can promote dehydration of alcohol.

Reactivity of tertiary alcohols toward FAAs strongly depends on the structure of alcohol. For example, the reaction of Ishikawa reagent and t-butanol at ambient temperature gives t-butyl fluoride, along with small amount of isobutylene (yields are 78% and 9%, respectively) [19]. Similar results were reported for fluorination of C4F9(CH2)2CR(CH3)OH (R = H or CH3) using TFEDMA. In both of these cases, the corresponding fluorides are dominant products, forming along with smaller amount of olefin by-products (ratio 97:3 and 78:22, respectively) [5, 12], but in the reaction of TFEDMA with hydrocarbon analogs (heptanol-2 and 2-methylheptanol-2), the formation of olefins is more pronounced (ratio of fluoride/olefin are 65:35 and 55:45, respectively) [5, 12]. Surprisingly, tertiary fluorinated alcohols without –CH2- spacer undergo clean dehydration to give the corresponding olefins by the action of Yarovenko reagent [38, 39] or TFEDMA [5, 12] (Eq. 4):
In case when tertiary alcohols are not able to undergo water elimination, yields of fluorides usually are higher. Despite of relatively high reaction temperature (60–140 °C), fluorination of various 1- and 2-adamantanols using Ishikawa reagent [19] or TFEDMA [12] led to the corresponding fluorides in 60–80% yield, and bicyclo[2.2.2]octanols were converted into tertiary fluorides using Yarovenko reagent [2, 24, 40] at elevated temperature (Eq. 5):

Allylic and Propargylic Alcohols

Fluorination of allylic alcohols by FAAs proceeds under mild conditions and is accompanied by allylic rearrangement, leading to a mixture of two isomeric fluorides (Eq. 6):
Propargylic alcohols in reaction with Ishikawa reagent (in the presence of base – (i-Pr)2NEt) do not underdo fluorination, but produce 2-fluoro-2-trifluoromethyl-3,4-alkadienamides in 68–85% yield; in case of chiral alcohols, products with rather high diastereoselectivity (92–96%) were prepared [41] (Eq. 7).

Allenes in this case form as a result of Claisen-type rearrangement involving intermediate “A” (Eq. 7) [41]. Similar reaction leading to the corresponding unsaturated fluorinated amides was also reported for the reaction of Ishikawa reagent [42] and [(C2H5)2NCF = CHCF3, DEPFPA] [43] with allylic alcohols, as a result of Eschenmoser-Claisen-type rearrangement.

Benzylic Alcohols and α-Hydroxy Carbonyl Compounds

Primary and secondary benzylic alcohols typically undergo fluorination at ambient temperature, producing the corresponding benzyl fluorides. For example, benzyl alcohol can be converted into benzyl fluoride using Yarovenko reagent [22], TFEDMA [12], DEPFPA or (CH3)2NCF2CH2CF3 [15] in 40%, 90%, 64%, and 95% yield, respectively (Eq. 8):

1-Phenylethanol produces 1-fluoro-1-phenylethane in reaction with Ishikawa or Yarovenko reagents [44] in 50% yield. Fluorination of optically active 1-phenylethanol (using Yarovenko reagent) was shown to proceed with inversion, leading to the corresponding stereoisomer with 55% ee [44]. Detailed summary on fluorination of various benzylic alcohols using Yarovenko and Ishikawa reagents can be found in Ref. [2].

While the reaction of methyl ester α-hydroxy iso-butyric acid with Yarovenko reaction proceeds with low yield formation of the ester of α-fluoro-iso-butyric acid [22], hydroxy esters activated by α-aryl group are much more active in this reaction and produce the corresponding fluorides upon treatment with Yarovenko or Ishikawa reagents or TFEDMA [33] in 55–80% yield under mild conditions (Eq. 9) [2]:

It should be pointed out that these reactions are extremely selective toward hydroxyl, while carbonyl group in all cases stays intact.

Fluorination of optically active α-hydroxy carbonyl compounds by FAAs proceeds in stereoselective fashion with inversion at stereocenter. Typically ees in this reactions are in a range 55–80% [2] for reactions of Yarovenko and Ishikawa reagents with optically active ethyl mandelate [2] and 26% for reaction of TFFEDMA and methyl mandelate [33]; TFEDMA was reported to convert optically active derivatives of proline into the corresponding fluorides in 85% ee [5], but much higher ee (up to 97%) was reported recently for the fluorination of pure enantiomers of lactic acid esters using TFEDMA [45].

Fluorination of Steroids and Natural Products

FAAs are widely used for regio- and stereoselective fluorination of various steroids. For example, 5-cholestane-3-ol (cholesterol) was converted into the corresponding fluoride in 72–83% yield using Yarovenko and Ishikawa reagents [2], TFEDMA [12], or adducts of 2-H-pentafluoropropene and amines [15]. Independent of the reagent, the fluorination of cholesterol is completely stereoselective and proceeds with complete inversion at stereocenter leading to 3-β-fluorocholest-5-ene in 72–83% yield. In case of other steroids, yields of fluorinated products may vary broadly, depending mostly on the structure of starting steroid, while the nature of fluorinating agent seems to be less important. A comprehensive summary on the use of Yarovenko and Ishikawa reagents for the synthesis of fluorinated steroids can be found in review articles [1, 2].

FAAs in general are not active toward ring-bonded hydroxyls of carbohydrates; however, side chain –OH group can be replaced by fluorine [2].

Successful fluorination of gibberellins, kaurenoids, and brefeldin A leading to the corresponding fluoro derivatives was reported for Yarovenko reagent. Ishikawa reagents was used for the replacement of side chain OH group in some β-lactams and for preparation of antibacterial agent florfenicol [2].

Carbonyl Compounds and Acids

In general, FAAs are less potent fluorinating agents compared to sulfur tetrafluoride or DAST and would not convert C = O group into –CF2-. While the reaction of TFEDMA with propionic aldehyde at 100 °C was reported to give C2H5CF2H in moderate yield and selectivity [12], neither adamantanone-2 or ethyl pyruvate produced the corresponding difluorides under similar conditions [12] .

On the other hand, β-diketones were found to be active toward TFEDMA producing selectively RC(O)CH2CF2R (R = CH3, C2H5, C3H7) in 42–63% yield at elevated temperature. Two-step mechanism of this process involves the interaction of enolic form of β-diketone with TFEDMA leading to the formation of RC(O)CH = CFR and a mole of HF in first step, followed by the addition of HF across the double bond of RC(O)CH = CFR, leading to the corresponding difluoride [46]. It should be pointed out that this reaction is applicable to noncyclic, aliphatic β-diketones, since in case of cyclic β-diketones the reaction results in exclusive formation of cyclic β-diketones carrying difluoroacetyl group in 2-position [46].

FFAs are useful reagents for one-step conversion of acids into acyl fluorides. Ishikawa and Yarovenko reagents [19] and TFEDMA [12] can be used for the conversion of aliphatic, aromatic, and sulfonic acids into acyl (sulfonyl) fluorides in moderate to high yields. One disadvantage of this process, a formation of acidic impurities (HF), can be overcome by carrying out the reaction in the presence of NaF [47]. In this case the process leads to the formation of pure, HF-free acyl fluorides.

Mechanism of Deoxyfluorination Reaction

Despite the fact that several mechanisms of fluorination of alcohols by FAAs were proposed, at this point a mechanism, including cyclic transition state [12, 15], is probably the most consistent one. Overall two-step process involves the reaction of FAAs with alcohol (Eq. 10, primary alcohol is used as an example) with the formation of intermediate “B” in the first step, followed by its decomposition with the formation of alkyl fluoride and a complex of fluorinated amide with HF:
Indirect evidence for the formation of intermediate B was observed in the reaction of heptanol-1 and TFEDMA, when the reaction mixture was quenched by water after a short period [12], leading to the formation of equal amount of 1-fluoroheptane and HCF2C(O)OCH2C6H13, which likely derived from hydrolysis of alkoxy amine “B.” The decomposition of intermediate “B” leading to the alkyl fluoride and complex of amide with HF is likely to proceed through cyclic transition state (Eq. 11) [12]:

The mechanism involving cyclic transition state does explain the preferential formation of product with inverted stereochemistry in case of optically active alcohols and relatively high stereoselectivity of fluorination, since in this case the nucleophilic attack of fluoride anion on carbon will occur preferential from less hindered side. The formation of olefin by-products, especially in case of secondary and tertiary alcohols, may be a result of competitive process involving the formation of carbocation intermediate. However, in some cases, the formation of olefin may proceed without the formation of carbocation, since, in the reaction of TFEDMA with HCF2CF2C(CH3)2OH (toluene solvent, 100 °C), the formation of products of toluene alkylation was not observed, as it would be expected in case of the process involving the formation of free carbocation [12].

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© Springer Nature Singapore Pte Ltd. 2018

Authors and Affiliations

  1. 1.FluoroproductsChemours Co.WilmingtonUSA