Fluorination

Living Edition
| Editors: Jinbo Hu, Teruo Umemoto

BF3 Fluorination for Preparing Alkyl Fluorides

  • Chang-Hua DingEmail author
  • Xue-Long HouEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-981-10-1855-8_43-1

Introduction

Monofluorination is an important access to introduce fluorine atom into organic molecules, which is realized by the strategies employing diethylaminosulfur trifluoride (DAST), tris(diethylamino)sulfonium difluorotrimethylsilicate (TAS-F), hydrogen fluoride, etc. as reagents as shown in other entries of the book. BF3.OEt2 as an ubiquitous reagent is usually used as a Lewis acid in synthetic organic transformations. However, BF3·OEt2 may also serve as an effective nucleophilic fluoride source in the fluorination reaction owing to the property of boron to form ate complex with some Lewis bases and anions. The mechanism of BF3 for fluoride transfer possibly involves a migration of the fluoride via an ate complex readily generated, trifluoroborate with the form of BF3X (X = Lewis base). Because of its high fluoride content and easy handling in the reaction, the use of BF3·OEt2 as a fluorine source has attracted great attention of synthetic chemists. A comprehensive review has appeared regarding the application of boron trifluoride in the monofluorination reaction [1]. This entry will focus on some synthetically useful applications of BF3 in the synthesis of alkyl fluorides, and some recent examples are included.

Cyclization-Fluorination with BF3·OEt2

The Prins cyclization reaction is a well-known method for the stereoselective construction of tetrahydropyran rings. In some cases, BF3 may serve as nucleophile in the reaction to afford fluorinated product. Rychnovsky and co-workers observed the fluorine incorporation in the Prins cyclization of α-acetoxy ether 1, affording a 2,6-disubstituted 4-fluorotetrahydropyran 2 in 64% yield when the reaction was carried out using BF3·OEt2 and HOAc in trifluoromethylbenzene (Eq. 1) [2]. The cyclization forms three new contiguous stereogenic centers in a highly stereoselective manner, which can be rationalized based on a chair conformation of the intermediate oxocarbenium ion. With the same approach, a 2,5-disubstituted 4-fluorotetrahydropyran 4 was obtained in 55% yield by the treatment of homoallylic acetal 3 with BF3·OEt2 in CH2Cl2 (Eq. 2) [3]. A single isomer was isolated, in which three substituents were located in equatorial position.
The Prins cyclization reaction of homoallylic alcohols with aldehydes in the presence of an acid catalyst is one of the most convenient methods for the synthesis of substituted tetrahydropyrans. The reaction proceeds through a hemiacetal and then an oxocarbenium ion intermediates. A regio- and stereoselective synthesis of 2,3,6-trisubstituted 4-fluorotetrahydropyran was realized via the reaction of (5E,3R)-homoallylic alcohol 6 with aldehydes 5 promoted by BF3·OEt2, providing the corresponding (4S)-4-fluoro-2,3,6-trisubstituted tetrahydropyran derivatives 7 stereoselectively in satisfactory yield (Eq. 3) [4]. The chirality was well-transferred to the tetrahydropyran product. The reaction proceeded by allyl-transfer/[3.3]-sigmatropic rearrangement.
O’Hagan and co-workers extended the substrate scope of the oxa-Prins fluorination using BF3·OEt2 as the Lewis acid and fluorine source. With cyclohexanol 8 as the nucleophilic partner, the reaction with benzaldehyde and 4-nitrobenzaldehyde gave rise to the corresponding fluorinated bicyclic products 9 in moderate yields with good diastereoselectivity (10/1) (Eq. 4) [5].
BF3·OEt2-mediated aza-Prins fluorination reaction has also been developed by the group of O’Hagan, enabling the direct synthesis of 4-fluoropiperidines [5]. The reaction of N-tosyl homoallylamine 10 with various aldehydes 5 using BF3·OEt2 as the Lewis acid mediator and nucleophilic fluoride source afforded 2-substituted 4-fluoropiperidines 11 in moderate to high yields (Eq. 5). All the substrates gave poor diastereoselectivities, and lowering the reaction temperature to −20 °C couldn’t improve diastereoselectivity. Homoallylamine with other protecting groups such as benzoyl and Boc proved to be unsuitable substrate.
Cyclization-fluorination mediated by BF3·OEt2 was observed by Tietze, Schneider, and co-workers in the reaction of estrone-derived imines 12 with BF3·OEt2 (Eq. 6) [6, 7]. It was noted that the electron-deficient N-aryl groups on the imine 12 favored the formation of fluorinated homoestrone derivatives 13, while the electron rich N-aryl groups promoted the hetero-Diels–Alder reaction, which gave nonfluorinated products.
Dicobalt hexacarbonyl alkynyl stabilized (Nicholas) cations can be used as initiators for the intramolecular cyclization with a pendant alkene. The intermediate could be trapped by fluoride transferred from BF3·OEt2 to afford fluorine-containing cyclic molecules. Tyrrell and co-workers described the synthesis of fluorinated benzopyran derivatives 15 via a one-pot procedure involving the intramolecular cyclization of in situ generated dicobalt hexacarbonyl alkyne complexes bearing an alkene group and fluorination with BF3·OEt2, followed by an oxidative decomplexation using ceric ammonium nitrate (CAN) (Eq. 7) [8, 9, 10].
Within the same year, Bertrand et al. reported the 6-endo cyclization-fluorination of Nicholas cations generated from ω-ethylenic propargyl alcohols 16 in the presence of BF3·OEt2 to prepare diastereoisomeric mixtures of fluorinated cyclohexanes 17 (Eq. 8) [11].
A BF3·OEt2-mediated [4 + 3]-cycloaddition of alkynyl 1,4-diether hexacarbonyldicobalt complexes 18 and allyl stannylsilanes 19 was developed by Patel and Green, giving the fluorocycloheptyne complexes 20 (Eq. 9) [12]. Very slow addition of BF3·OEt2 and high dilution of reactants (10−3) delivered the highest yield with minimal competing elimination, while a rapid BF3·OEt2 addition resulted in inferior yield. Later, the group found that replacement of allyldimetal equivalents by allyltrimethylsilane enabled the formation of fluorinated carbocycles in better yields but with lower diastereoselectivities [13].
Zhang and co-workers described an intramolecular aminofluorination reaction of homoallylic amines 21 for the formation of 3-fluoropyrrolidines 22 (Eq. 10) [14]. Under the effect of BF3·OEt2 and hypervalent iodine(III) reagent PhIO at room temperature, various 3-fluoropyrrolidines 22 were obtained in moderate to high yields (Eq. 10). An excellent regioselectivity was observed for all cases, but the diastereoselectivities were substrate-dependent. The mechanism was proposed based upon the experimental evidence, which involved the formation of carbocation intermediate followed by trapping with F ion.
Li et al. reported a regioselective intramolecular aminofluorination of unactivated alkenes for the preparation of 3-fluoropiperidines. In the presence of 1.1 equiv of phenyliodonium(III) diacetate (PIDA) and 2.0 equiv of BF3·OEt2, 3-fluoropiperidine compounds 24 were isolated in moderate yields along with the formation of 3-acetoxy compounds as the by-products (Eq. 11) [15]. The reaction was completed in minutes. Conventional metal fluorides or fluorine sources could not give the corresponding fluoroamidation products.

Ring-Opening/Fluorination Reactions of Epoxides and Aziridines with BF3·OEt2

Ring-opening reaction of epoxides with fluorinating reagents is the most extensively employed method for preparation of vicinal fluorohydrins. Alk4NF, Bu4N+H2F3, and amine-HF adducts such as pyridine·9HF (Olah’s reagent), diisopropylamine trihydrofluoride, and triethylamine trihydrofluoride are among the commonly used fluorinating reagents. BF3·OEt2, though widely used as a Lewis acid catalyst in a variety of reactions of oxiranes, such as polymerization, nucleophilic ring-opening, and rearrangement, is also used as a fluorination reagent in ring-opening reaction of oxirane. The earliest example was reported in the 1957 by Henbest and Wrigley during their studies on the reaction of steroidal epoxides with BF3·OEt2 [16]. They found that the ring-opening fluorination product 26a was afforded in 62% yield by the reaction of 3β-acetoxy-5α,6α-epoxycholestane with BF3·OEt2 (Eq. 12). It was found that the presence of the 3β-OAc substituent on the A-ring of steroid was crucial for ring-opening fluorination, while the reaction did not afford the corresponding fluorohydrin product in the absence of a 3-substituent on the A-ring. Bowers and Ringold observed that the ring-opening fluorination of 3β-acetoxy-5α,6α-epoxypregane-20-one with a 1:1 mixture of BF3·OEt2 and HBF4·OEt2 gave fluorohydrin 26b in 81% yield (Eq. 12) [17], while only 39% yield was obtained using BF3·OEt2 alone under the same conditions. These results suggest that the BF4 ion may play the role of the active fluoride donor. To investigate the effect of electronic and steric factors influencing the reactivity of 5α,6α-epoxides toward BF3·OEt2, Bowers and co-workers performed the reaction of 5α,6α-epoxypregnan-3,20-dione, in which the steric influence of the 3-oxo substituent was negligible with BF3·OEt2, and fluorohydrin 26c was produced in 44% isolated yield (Eq. 12) [18]. These results indicated the substrate containing a 3-substituent that exerts an negative inductive effect (−I effect) favored the ring-opening fluorination of 5α,6α-epoxide. They reasoned that the 3,3-ethylenedioxy motif in a 5α,6α-epoxide might function in an similar manner as the 3-oxo substituent of 5α,6α-epoxypregnan-3,20-dione. Indeed, 3,3-ethylenedioxy-5α,6α-epoxide, upon treatment with BF3·OEt2, afforded fluorohydrins 26d in 48% yield (Eq. 12) [18].
The ring-opening fluorination of oxirane was successfully extended by Henbest and Wrigley to 3β-acetoxy-5β,6β-epoxycholestane 27 [16]. Fluorohydrin 28 was produced in 73% yield by treating it with BF3·OEt2 in benzene at room temperature (Eq. 13). As before, the presence of a 3β-acetoxy group was also critical for fluorine incorporation. The regioselectivity of the attack of fluoride [at C(5)] is reversed compared with that of 5α,6α-epoxycholestane [at C(6)], which was rationalized with a trans-diaxial ring-opening according to the Fürst–Plattner rule [19].
Blackett et al. reported ring-opening fluorination of 5β,6β-epoxycholestane 29 lacking the electron-withdrawing C(3)-substituent (Eq. 14) [20]. 5β,6β-Epoxycholestane 29 reacted with a large excess amount of BF3·OEt2 in Et2O at room temperature to provide fluorohydrin 30 in 57% yield, along with a regioisomeric fluorohydrin 31 in 6% yield. In contrast, a mixture of nonfluorinated rearrangement products was obtained when benzene was the solvent. These results demonstrated the crucial role of the solvent in dominating the reaction course of steroidal epoxides with BF3·OEt2. Similar phenomena were observed that the use of Et2O as an additive or cosolvent could greatly improve the yield of the ring-opening fluorination of 5,6-epoxysteroids with BF3·OEt2 [21, 22].
The ring-opening fluorination of nonsteroid oxiranes with BF3·OEt2 has also been documented. Early examples used structurally complex tetrasubstituted epoxides. For example, 7,11-epoxyisogermacrone 32 was transferred into the corresponding fluorohydrin 33 as the major product in 68% yield upon treatment with BF3·OEt2 in Et2O (Eq. 15) [23].

cis-2,3-Di-tert-butyloxirane 34, a symmetrical 1,2-dialkyl-substituted epoxide without the problem of regioselectivity in the reaction, was used for the ring-opening fluorination by Coxon and co-workers [24]. cis-2,3-Di-tert-butyloxirane 34 was treated with a large excess amount of BF3·OEt2 in Et2O at room temperature for 3 days to deliver syn-fluorohydrin 35 in 80% yield (Eq. 16). Again, the utility of Et2O as solvent was crucial for ring-opening reaction of fluoride. Fragmentation of epoxide occurred in the non-Lewis basic solvent CCl4.

When unsymmetrical 1,2-dialkyl-substituted epoxides were subjected to the ring-opening fluorination, poor regioselectivity was observed. Coxon et al. reported that the reaction of trans-epoxide 36 with BF3·OEt2 in Et2O produced two regioisomeric fluorohydrins 37 and 38 in near 1:1 ratio (Eq. 17) [25]. Meanwhile, Meinwald rearrangement products, carbonyl compounds, were isolated as minor products. The configuration of the epoxide affected the reaction greatly. Under the same reaction conditions, Meinwald rearrangement for cis-epoxide 36 became dominated reaction course, while the fluorohydrins 37 and 38 were obtained in low yield (Eq. 18).
Regioselective attack of fluoride on the epoxides bearing a nucleofugal group was reported to give α-fluorocarbonyl. For example, α-nitro-α-phenylthio epoxides 39 reacted with BF3·OEt2 in toluene to afford α-fluoro phenylthio esters 40 in high yields (Eq 19) [26]. Herein, it was found that Et2O was not a suitable solvent, and other fluorine sources such as TBAF or KF led to unidentified by-products.
The ring-opening fluorination of an aryl epoxide with BF3·OEt2 was firstly described by House in 1956 [27, 28]. The chalcone oxide 41 was treated with 0.5 equiv of BF3·OEt2 in Et2O under reflux to provide fluorohydrin 42 in 59% yield (Eq. 20). The fluorohydrin 42 was proposed to be formed through the BF3–epoxide complex via a concerted epoxide cleavage and intramolecular fluoride transfer process. Later, Weber and co-workers extended the substrate scope of the ring-opening fluorination of aryl-substituted chalcone oxide derivatives 41 under House’s reaction conditions (Eq. 20) [29].
Davies and co-workers systematically studied the ring-opening fluorination of aryl epoxides with various substitution patterns and disclosed that only trans-β-methyl-aryl epoxides 43 provided synthetically useful yield [30]. The reaction only needed to use 0.33 equiv of BF3·OEt2, and fluorohydrins 44 were obtained as a single diastereoisomer in high yield (Eq. 21). The results demonstrates that all three fluorine atoms of BF3·OEt2 are transferable in the ring-opening reaction of oxirane (in accordance with the observation of Pericàs et al. [31]). In addition, inferior yield was obtained when greater than 0.33 equiv of BF3·OEt2 was used. This ring-opening fluorination tolerated a variety of synthetically versatile functional groups appended to the oxirane ring. The syn stereoselectivity of the products 44 was consistent with a SN1 type mechanism. The reaction was completed within a very short time (5−10 min).
The ring-opening fluorination of epoxides with BF3·OEt2 has demonstrated its utility in the synthesis of fluorinated analogues of natural products. BF3·OEt2 was used as a fluorination reagent in the ring-opening of anthracycline aglycone 8,9-epoxides 45, providing 8-fluoroanthracyclinones 46 in excellent yields with high regioselectivity (Eq. 22) [32], which was a key intermediate in the total synthesis of (8S)-8-fluoro-4-demethoxydaunorubicin [33], a fluorinated analogue of the naturally occurring anthracycline daunorubicin, which displays potent antitumor activity.
Compared with the well-developed ring-opening fluorination of epoxide, the corresponding version of aziridine is scarce. Voronkov and Fedotova described the first example of the ring-opening fluorination of an aziridine 47 with BF3·OEt2 in 1966. N-acetyl-N-(β-fluoroethyl)difluoroborazene 48 was obtained in 72% yield via the reaction of aziridine 47 with 1.0 equiv of BF3·OEt2 in Et2O at −78 °C, followed by acetylation (Eq. 23) [34].
This reaction did not receive much attention. Only several examples of ring-opening fluorinations of aziridines were reported as minor or rare cases. Nakayama group reported that N-Ts β-fluoro amine 50 was isolated in 70% yield from the reaction of N-Ts tetramethylaziridine 49 with 0.30 equiv of BF3·OEt2 in CHCl3 at low reaction temperature (Eq. 24) [35]. When reaction was carried out at room temperature, rearrangement of aziridine 49 occurred and no ring-opening fluorination was observed. The minor or rare cases of the ring-opening fluorinations of N-2-(trimethylsilyl)ethoxycarbonyl aziridine [36], N-Boc aziridine [37], and N-diethoxyphosphoryl aziridine [38] with BF3·OEt2 have also been described.
A general ring-opening fluorination of various aliphatic N-Ts aziridines 52 with BF3·OEt2 has been reported by Hou and co-workers. N-Ts β-fluoro amines 53 were afforded in good yield (Eq. 25) [39]. The fluoride regioselectively attacked the substituted position of the mono-substituted aziridine. As has also been observed with aryl epoxides [30, 31], all three fluorine atoms of BF3·OEt2 can be transferred in this reaction. The addition of alcohol as additive significantly accelerated the reaction. The iPrOH proved to be optimal, although its role remains unclear. When enantiomerically pure mono-substituted N-tosylaziridine 54 was used as a substrate, product 55 was obtained without the loss of ee value (Eq. 26). Obviously, this procedure provides an easy and simple but efficient way to chiral β-fluoro amine.

Other Fluorinations with BF3·OEt2

In addition to the above ring-opening fluorination and Prins cyclization fluorination reactions, BF3·OEt2 has also been found its application as a nucleophilic fluorination reagent. The reaction of α-diazo-β-keto esters with BF3·OEt2 has been reported by Hayes, Moody, and co-workers as an efficient method to access α-fluoro-β-keto ester. The treatment of α-diazo-β-keto ester 56 with BF3·OEt2 in Et2O gave α-fluoro-β-keto ester 57 in 25% yield. The yield increased to 45% by using CH2Cl2 as solvent (Eq. 27) [40]. As comparison, the use of CsF, TBAF, or Py·9HF (Olah’s reagent) resulted in partial decomposition and/or no reaction. The nucleophilic fluorination of ethyl 3-(1-adamantanyl)-2-diazo-3-oxopropanoate 58 with BF3·OEt2 in CH2Cl2 was also described by Ohno et al. (Eq. 28) [41].
The chlorofluorination of alkenes by using BF3·OEt2 combined with methyl hypohalites was reported by Heasley and co-workers. Fivefold excess of alkenes 60 was treated with a 1:1 mixture of MeOCl and BF3·OEt2 in CH2Cl2 to afford β-fluoro chloride 61 in moderated to high yield (Eq. 29) [42, 43]. The α-methoxy halide by-product was observed from competitive methoxy transfer. The reaction was postulated to involve fluoride transfer to a chloriranium ion intermediate from the MeOBF3 ion.

Conclusion

BF3 fluorination has become a clearly valuable reagent for nucleophilic fluorination and has demonstrated its power in the synthesis of alkyl fluorides. BF3 fluorination has shown its advantages such as low cost, high fluoride content, relative easier manipulation, and compatibility with standard borosilicate glassware. Since BF3·OEt2 has only been used as a fluorination reagent in a limited number of reactions so far, new development of fluorination methods with BF3·OEt2 and its more applications in organic synthesis are anticipated.

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Authors and Affiliations

  1. 1.State Key Laboratory of Organometallic ChemistryShanghai Institute of Organic Chemistry, Chinese Academy of SciencesShanghaiChina