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Physical properties of fluorine
Fluorine is the most electronegative element (3.98) and therefore has a really high electron affinity (328 kJ/mol). At standard pressure and temperature fluorine (F2) is a pale yellow gas that is extremely reactive due to the low F-F bond energy (36.6 kcal/mol).
Fluorine in nature
Despite fluorine being the 13th most abundant element in the Earth’s crust, only 21 biosynthesized natural molecules containing fluorine are known (a strong contrast to the thousand of natural products containing chlorine or bromine atoms). In nature, halogenations are normally catalyzed by haloperoxidase enzymes. However, no fluoroperoxidase is known, a consequence of the high oxidation potential of fluorine. Additionally, the high solvation energy of the fluoride ion in aqueous media results in a tightly bound hydration shell of water molecules around the ion that dramatically lowers its nucleophilicity. The first recognized fluorinating enzyme, 5’-fluoro-5’-deoxyadenosine synthase, was found in bacteria Streptomyces cattleya and probably dehydrates solvated fluorides in its active site.
O’Hagan, Nature2002, 416, 270
Dong, Nature2004, 427, 561
Importance of fluorine in “daily life”
Carbon-fluorine bonds have an integral role in our daily life. Fluorine uniquely affects the properties of organic molecules through strong polar interactions. For example, the introductions of fluorine into pharmaceuticals can make them more bioavailable, lipophilic and metabolically stable and can increase the strength of a compound’s interaction with a target protein. Approximately 30% of all agrochemicals and 20% of all phramaceuticals contain fluorine. In the well-known polymer polytetrafluoroethylene (Teflon) the fluorine atoms are responsible for the low coefficient of friction and hydrophobicity the material (non-sticking coating of cookware). Hydrochlorofluorocarbons (HCFC) are used for refrigeration and air conditioning but also for propellants and solvents. Finally, the natural isotope 18F is the most commonly used positron-emitting isotope for molecular positron emission tomography (PET) imaging in oncology. An effective introduction of 18F in organic compounds is necessary as its half-life time is only 110 minutes. The most used reagent for PET scans is 2-[18F]fluoro-2-deoxyglucose.
Common Fluorinating reagents and their reactions
Transfer of “F+” to an electron-rich center is the fundamental process of electrophilic fluorination, and many efforts were devoted to tame elemental fluorine and to develop new reagents based on O-F and N-F bonds.
Taylor Tetrahedron1999, 55, 12431.
Elemental fluorine (F2)
The low F-F bond energy and the high energy of bonds formed with other elements make reactions of F2 extremely exothermic, often explosive. It was reported in 1960s that reactions became controllable by diluting F2with an inert gas such as N2 or Ar.
Purrington Chem. Rev.1986, 86, 997.
Patrick JOC1988, 53, 5153.
O-Fluoro electrophilic reagents
An early example of O-F electrophilic reagent is fluoroxyltrifluoromethane (CF3OF) developed by Barton. Acetyl hypofluorite was introduced in 1953, but not widely used until Rozen developed the convenient synthetic route in 1981. While CF3OF can be stored at room temperature, acetyl and perfluoroacetyl hypofluorites are less stable and are usually generated in situ from their acetate salts and F2.
Rozen Chem. Rev.1996, 96, 1717.
Barton Chem. Commun.1968, 806.
Rozen JOC2001, 66, 7464.
N-Fluoro electrophilic reagents
Major progress in the field of electrophilic fluorination came with the advent of N-F reagents. The lower electronegativity of nitrogen compared to oxygen and the greater strength of the N-F bond compared to the O-F bond contribute to decrease the electrophilicity of these reagents and render them stable and convenient to handle. Some of them are now commercially available.
Examples of commercially available N-fluoro electrophilic reagents
Pez Chem. Rev.1996, 96, 1737.
This class of reagents has been explored by Umemoto. The key to the success was the discovery that non-nucleophilic counterions were essential to their stability. The electrophilicity of reagents can be tuned by changing substituents on the pyridine ring.
In a series of N-fluoroquinuclidines, the fluorinating ability increase with the electron-withdrawing power of the R group (CH3, C2H5, C8H7 < CH2Cl < CF3CH2). Selectfluor (R=CH2Cl) is commercially available and has become a very popular reagent with many applications.
Wong ACIE2005, 44, 192.
Gilicinski J. Fluorine. Chem.1992, 59, 157.
Banks J. Fluorine Chem.1998, 87, 1.
In contrast to the reagents above, the N-fluorosulfonamides are neutral N-F reagents. These are stable and can be used for a variety of selective fluorinations. N-Fluorobenzenesulfonimide (NFSI) reported by Differding is most widely used thanks to their balance of property between stability and reactivity.
Differding Synlett1991, 187.
Fluoride is strongly solvated in protic solvents and forms tight ion pair in most aprotic solvents, so that fluoride is a poor nucleophile in protic solvents. Ion pairing in aprotic solvents, on the other hand, must be overcome so as to take advantage of the potent nucleophilicity of fluoride.
One solution to increase nucleophilicity of fluoride is to use sterically demanding cation that reduces ion pairing by delocalizing positive charge (e.g. tetrabutylammonium fluoride, TBAF). Compared with other ammonium fluoride like tetramethylammonium fluoride (TMAF), preparing anhydrous TBAF is difficult due to their decomposition such as Hoffmann elimination even at room temperature. DiMagno reported convenient synthetic route to anhydrous TBAF using SNAr substitution of C6F6, and showed that this facilitates halogen exchange reactions at lower temperature (also see 3.1).
HF is highly corrosive and difficult to handle, but an addition of amines facilitates its handling (e.g. pyridinium poly(hydrogen fluoride), PPHF, Olah’s reagent). The presence of amine also reduces the nucleophilicity of fluoride and the activation of substrates is normally required.
Oxidative desulfurization-fluorination process has gained more attention as an alternative substrate activation mode in complex molecule synthesis.
Hiyama ACIE2005, 44, 214.
Pairing the hard Lewis base F- with soft Lewis acids is another solution to increase the nucleophilicity of fluoride. Among them, S-F reagents are widely used. Commercially available reagents, DAST and deoxofluor, are prepared from SF4, and the latter showed higher thermal stability.
Shreeve Synthesis2002, 17, 2561
Middeleton JOC1975, 40, 574.
Lal JOC1999, 64, 7048.
Lal Chem. Commun.1999, 215.
Synthesis of aryl fluorides
Balz & Schiemann Chem. Ber. 1927,60, 1186.
Halogen Exchange (Halex) Process
This is a common method for the synthesis of fluorinated aromatics in industry, in which halogens serve as leaving groups and inexpensive, inorganic fluoride sources are used as nucleophiles.
Cationic copper-mediated fluorination of aryl iodides
Hartwig JACS2012, 134, 10795.
Fluorination of phenol
Ritter JACS 2011,133, 11482.
C(sp3)-F bond formations
-Fluorination of carbonyl compounds
The difficulty of this type of reactions stems from the fact that monofluorinated compounds are more readily deprotonated than starting materials. Many examples utilize -branched substrates to avoid this problem.
First example of enantioselective fluorination using chiral N-fluorosultams as fluorinating reagents
Lang TL1988, 29, 6087.
Catalytic enantioselective fluorination
First example of asymmetric fluorination of -ketoester catalyzed by Ti complex
Togni ACIE2000,39, 4359.
Enantioselective organocatalytic fluorination of -nonbranched aldehydes
MacMillan JACS2005, 127, 8826.
Transition metal catalyzed allylic substitution is a powerful and well-explored method with both carbon and heteroatom nucleophiles. The difficulty of catalytic allylic fluorination, however, was documented. Togni reported that the attack of fluoride on a -allyl Pd complex was thermodynamically unfavorable, making catalytic process difficult. In addition, allylic fluoride works as a leaving group, which also renders this process difficult.
Togni Eur. J. Inorg. Chem.2006, 1397; Gouverneur & Brown ACIE2009, 48, 1296.
Palladium-catalyzed asymmetric allylic fluorination of acyclic and cyclic halides
Doyle JACS2010,132, 17402; JACS2011, 133, 15902.
Fluorinations involving alkyl radical
Computational studies showed that NFSI and Selectfluor have low N-F homolytic bond dissociation energies, which are independent of the dielectric properties of the medium. These reagents reacted with alkyl radicals generated from corresponding peroxides under either thermal or photolytic condition.
DFT-Calculated N-F Bond Dissociation Energies
Sammis JACS2012,134, 4026.
Decarboxylative fluorination of aliphatic carboxylic acids was reported with catalytic amount of Ag(I) and Selectfluor.
Li JACS2012,134, 10401.
Fe(III)/NaBH4-medicated hydrofluorination of unactivated alkenes was reported with Selectfluor as a source of fluoride radical.
Boger JACS2012, 134, 13588.
Fluorination of aliphatic C-H
Cu(I)/KB(C6F5)4 catalysis with Selectfluor
Lectka ACIE 2012,51, 10580.
Mn/Porphyrin catalysis with AgF/TBAF•3H2O
Groves Science 2012,337, 1322.
The incorporation of a trifluoromethyl group (CF3) into organic compounds has become one of the fastest growing research areas in the last 20 years. The main reason for the interest in trifluoromethylated compounds is that the introduction of CF3 groups can substantially enhance the metabolic stability of a drug candidate compared to its non-fluorinated analogous. Furthermore the fluorinated derivatives often exhibit improved bioavailability and increased lipophilicity, which leads to a facilitated cell membrane penetration. Trifluoromethyl groups are distinct from other alkyl groups such as the methyl group, both in terms of electronic structure and reactivity. The CF3 group has the same electronegativity as chlorine (3.2) and is similar in size to an isopropyl (i-Pr) group (bioisostere). Therefore, the trifluoromethyl group should be considered as a distinct functional group rather than a substituted methyl group.
Early examples of trifluoromethylation reactions relied on the conversion of chlorinated compounds into their fluorinated derivatives. The most used reagents for this kind of transformation were anhydrous HF and antimony trifluoride (SbF3, Swarts method). The use of HF for trifluoromethylation processes is still used nowadays at times, because no by products are formed during this reaction (besides HCl). Another early example is the Cu-promoted coupling reaction between an aryl halide and trifluoroiodomethane, known as the McLoughlin-Thrower reaction.
Swarts, Bull. Acad. Roy. Belg.1892, 24, 309
Kobayashi, TL1969, 47, 4095
Approaches for the trifluoromethylation of organic substrates can be divided into three major categories: Nucleophilic, electrophilic and radical trifluoromethylation, depending on the used reagents.
The easiest way to achieve nucleophilic trifluoromethylation would be the use of organometallic reagents, such as trifluoromethyl magnesium iodide. However, the problem is that the generated trifluoromethyl anion decomposes rapidly and irreversibly to the fluoride salt and difluoromethyl carbene (α-elimination). Therefore, other reagents for the nucleophilic trifluoromethylation had to be developed.
Trimethyl(trifluoromethyl)silane is an acid and water stable liquid and was first prepared by the group of Ruppert in 1984. However, the use of TMSCF3 as a nucleophilic trifluoromethylation reagent was mainly developed by Prakash, who also improved its synthesis. TMSCF3 is not only important as a direct trifluoromethylating reagent, but also as a starting material for the preparation of a manifold of other trifluoromethylating reagents and as a source of CF3 in catalytic C-CF3 bond formation. Aldrich: 30 CHF / g.
Ruppert, TL1894, 25, 2195
Prakash, Org. Synth.1995, 72, 232
Addition to carbonyl compounds
The first example of a nucleophilic trifluoromethylation with TMSCF3 was reported by Prakash in 1989. The trifluoromethylation of different carbonyl compounds upon activation of TMSCF3 with a fluoride initiator was described. The pre-coordination of the silicon atom to the carbonyl compound, followed by transfer of the CF3 group makes this reaction possible and avoids generation of unstable CF3 anion.
Prakash, JACS1989, 111, 393
Prakash, JOC1991, 56, 984
Addition to other electrophiles
TMSCF3 found application in the trifluoromethylation of many different electrophiles. The trifluoromethylation of azirines and highly activated imines were reported by Laurent and Petrov.
Laurent, TL1994, 35, 3303
Petrov, TL2000, 41, 6959
A general method for the trifluoromethylation of unactivated imines was published in 2008 by the group of Dilman. The reason for the success of this reaction was the switch from basic to acidic conditions: The in situ generated anhydrous HF (from KHF2 and TFA) protonates the imine nitrogen, which leads to an increased electrophilicity of the iminium carbon.
Dilman, Eur. J. Org. Chem.2008, 5226
Bode, ACIE 2012, 51, 9173
Diastereo- and enantioselective examples
The addition of TMSCF3 to chiral sulfinylimines allowed for the preparation of enantioenriched trifluoromethylated amines.
Prakash, ACIE 2001, 40, 589
Prakash, JACS2002, 124, 6538
Several groups have been working on the asymmetric trifluoromethylation of aldehydes and ketones. The asymmetric induction was achieved by using chiral fluoride salts for the initiation of TMSCF3. The most successful approach, using a combination of tetramethylammonium fluoride and a cinchona alkaloid, was published by the group of Shibata.
Kobayashi, TL 1994, 35, 3137
Caron, Synthesis2003, 11, 1693
Shibata, OL 2007, 9, 3707 and OL 2010, 22, 5104
Considering the inherent electronegative character of the trifluoromethyl group, the idea of a reagent in which the CF3 group is positively charged might appear at first to be nonsensical. However, during the last 25 years several shelf-stable reagents were commercialized which allowed for the trifluoromethylation of a variety of nucleophiles (Umpolung of the CF3 anion).
First example by Yagupolskii
The first electrophilic trifluoromethylating reagent was synthesized by Yagupolskii in 1984: Treatment of aryltrifluoromethyl sulfoxide with SF3SbF6 and subsequent reaction of the sulfonium salt with an electron-rich arene provided the diaryl(trifluoromethyl)sulfonium salt. Yagupolskii’s reagent was then reacted with p-nitro-thiophenolate to give the corresponding trifluoromethyl sulfide in good yield.
To broaden the substrate scope of electrophilic trifluoromethylations, Umemoto synthesized several reagents based on a dibenzothiophenium core (also available with Se and Te). The aromatic moieties of the reagents were substituted with EDG or EWG in order to fine-tune their electrophilicity. Matching the power of the trifluoromethylating agent with the nucleophile made several trifluoromethylations possible that were not accessible before. Aldrich: 180 CHF / g.
Umemoto, TL1990, 31, 3579; JACS1993, 115, 2156 and J. Fluorine Chem.1999, 98, 75
A major limitation the sulfur-based electrophilic trifluoromethylating reagents is that only soft nucleophiles can be reacted and that there is no possibility of preparing N-CF3 or O-CF3 compounds (HSAB principle). To tackle this problem, Umemoto and co-workers synthesized a O-(trifluoromethyl)oxonium salt which acts as a source of a hard electrophilic trifluoromethyl group. The reagent was prepared at very low temperature by photochemical decomposition of a diazonium salt and reacted in situ with the electrophile (the reagent decomposes already at -70 ºC!). Despite the elegance of this approach, the method suffers from several shortcomings (e.g. all the reactions were run in a NMR tube and the products were never isolated).
In 2006, the group of Togni at ETH reported a new family of hypervalent iodine compounds in which the CF3 group is bonded directly to the iodine atom. As the reagent is prepared from TMSCF3 (!), the overall reaction can be considered a formal Umpolung of the CF3 group (from CF3- to CF3+). This new reagent could be used for the trifluoromethylation of many nucleophiles (enolates, thiols, phosphines…). Aldrich: 450 CHF / g.
Using a slightly modified Togni reagent and in the presence of zinc(II) salts it is possible to convert primary and secondary aliphatic alcohols into the corresponding trifluoromethyl ethers. However, tertiary alcohols and phenols could not be O-trifluoromethylated (check a recent paper from Ritter for an elegant access to O-trifluoromethylated phenols). Recently, Togni also reported the trifluoromethylation of various azoles. Despite the significant interest of these N-CF3compounds, the reaction is not easy to execute and isomeric mixtures of products are normally obtained.
Togni, ACIE2009, 48, 4332 and ACIE2012, 51, 6511
Ritter, JACS2011, 133, 13308
One of the few examples of enantioselective trifluoromethylation was disclosed by MacMillan in 2010. Togni reagent in combination with the condensation of an aldehyde with a chiral imidazolidinone catalyst provided the α-formyl CF3 product in good yield and with excellent enantioselectivity.
MacMillan, JACS2010, 132, 4986
The addition of a CF3 radical to organic substrates, especially to alkenes, is an attractive way to introduce trifluoromethyl groups. Due to their high electronegativity, the F atoms exert a strong σ-inductive effect on the carbon radical (CF3 radical adapts a pyramidal configuration). Therefore it can be considered an electrophilic radical with a low-lying SOMO (single occupied molecular orbital), which reacts faster with electron-rich alkenes possessing high-lying HOMOs.
Early examples with trifluoroiodomethane (CF3I)
Haszeldine already reported in the late 1940s that CF3 radicals can be generated from CF3I upon heating or irradiation. This type of intermolecular iodine atom transfer/radical addition (ATRA) using CF3I has performed successfully with various alkenes and radical initiators.
Haszeldine, J. Soc. Chem.1949, 2856
Utimoto, TL1990, 31, 6391
Ojima, TL1984, 25, 303
Trifluoromethylsulfonyl chloride (CF3SO2Cl)
From a practical point of view, experiments with gaseous CF3I can be inconvenient and the concentration of the reagent is not easy to control. Trifluoromethylsulfonyl chloride turned out to be a valuable replacement as a source of CF3 radicals. In the presence of RuCl2(PPh3)3 as the catalyst various alkenes, but also electron-rich arenes, could be trifluoromethylated.
Sawada, J. Chem. Soc. Perkin Trans. 11991, 627
Kamigata, Chem. Lett.1990, 649
Based on the work of Kamigata, the trifluoromethylation of different heteroarenes was described. MacMillan showed that the photoredox catalyst Ru(phen)3Cl2 can generate the CF3 radical already at room temperature and high yields of the trifluoromethylated heterocycles were obtained.
Langlois showed that sodium trifluoromethanesulfinate (bench-stable, inexpensive solid) can be used to generate CF3 radicals upon oxidation with a catalytic amount of a Cu or Fe salt and in the presence of tert-butyl hydroperoxide as a terminal stoichiometric oxidant. A wide range of electron-rich benzene derivatives could be trifluoromethylated (“C-H activation”), however the yields were low and mixtures of regioisomers were obtained.
Langlois, TL1991, 51, 7525
Recently, Baran published an elegant study on the radical C-H trifluoromethylation of different heterocycles with Langlois reagent. Compared to the seminal report of Langlois, no metal additives were needed, although trace metals in the commercially available Langlois reagent could be responsible for the reaction initiation. In most cases the site selectivity of the trifluoromethylation is governed by the electronics of the heterocycle (innate functionalization). The conditions are mild and the reaction shows remarkable functional group tolerance and broad substrate scope. Therefore, this method can be considered as the state of the art for trifluoromethylation of heterocycles.
Baran, PNAS2011, 108, 14411
In the course of their study, Baran and co-workers realized that zinc sulfinate salts show remarkably enhanced reactivity compared with their sodium-derived analogues. Therefore, they synthesized several zinc sulfinate salts, e.g. (CF3SO2)2Zn, (CHF2SO2)2Zn and (CH2CF3SO2)2Zn, as a toolkit for the C-H functionalization of heterocycles. The reagents are all commercially available from Aldrich: 130 CHF / g.
Baran, Nature2012, 492, 95
Copper-assisted trifluoromethylation reactions of prefunctionalized materials (“cross coupling”) have become extremely popular in the last couple of years. The main reason for this is that copper can stabilize the CF3 anion via coordination to it and in doing so removing electron density from the carbon atom.
The oxidative trifluoromethylation of boronic acids was achieved by the groups of Qing and Buchwald. To suppress side reactions, copper had to be used in stoichiometric fashion (plausible mechanism shown below). In Buchwald’s case oxygen could be used as stoichiometric oxidant (in comparison with the use of stoichiometric silver salts in Qing’s chemistry). The mild reaction conditions allowed for high functional group compatibility.
Qing, OL 2010, 12, 5060
Buchwald, JOC2011, 76, 1174
One of the postulated intermediates in Cu-promoted trifluoromethylations is [(phen)CuCF3]. This complex could be synthesized, isolated and fully characterized by the group of Hartwig. The complex is stable at room temperature under nitrogen atmosphere for over one month. The high reactivity of [(phen)CuCF3] as trifluoromethylating reagent was demonstrated by the coupling with a broad range of aryl iodides.
Hartwig, ACIE 2011, 50, 3793
Recently, the group of Grushin showed that the simplest copper-CF3-complex CuCF3 can actually be synthesized via the direct cupration of fluoroform (CF3H). Fluoroform is a side-product of Teflon manufacturing and therefore really cheap and available in large quantities. This approach could be particularly attractive for preparing trifluoromethylated compounds on an industrial scale (Ruppert-Prakash reagent is too expensive for large scale operations). The coupling conditions are fairly mild and no additives are necessary.
In 2011, the groups of Buchwald and Wang simultaneously developed a Cu-catalyzed allylic trifluoromethylation of unactivated terminal olefins. The reaction could proceed via a radical or electrophilic pathway.
Buchwald, ACIE 2011, 50, 9120
Wang, JACS2011, 133, 16410
During the course of their studies, the group of Buchwald became interested in trapping the putative intermediate of the allylic trifluoromethylation by a nucleophile as a means of synthesizing diverse CF3 building blocks in a step-economical fashion. They found that olefins which are substituted with oxygen containing groups (e.g. carboxylic acids, alcohols or phenols) can undergo oxytrifluoromethylation reactions to provide the difunctionalized products in good yields.
The most challenging step in C-CF3 bond formation via Pd-catalyzed cross coupling is the reductive elimination. For a fast reductive elimination to occur there must be sufficient overlap between the Pd-C and Pd-CF3 σ-bond. Due to the difference in electronegativity, the CF3-Pd bond is strongly polarized towards the CF3 group and therefore electron density is missing in the region where it is required for C-CF3 bond formation.
Early studies by Grushin
In 2006, Grushin showed that reductive elimination from the palladium(II) complex XanthphosPd(Ph)CF3 is possible. The electron-donating capability and the wide bite-angle of the Xantphos-ligand, which brings the phenyl and CF3 group closer together and facilitates so the reductive elimination, are responsible for the success of this reaction. However, Grushin was never able to develop reaction conditions that allowed all three elementary steps – oxidative addition, transmetallation and reductive elimination – to proceed in the same reaction flask, as required for catalysis.
Grushin, JACS 2006, 128, 12644
First Pd-catalyzed Ar-CF3 bond-forming reaction
Four years after Grushin the group of Buchwald reported the first Pd-catalyzed Ar-CF3 bond forming reaction using the extremely bulky and electron-rich BrettPhos ligand. The reaction employs aryl chlorides and (trifluoromethyl)triethylsilane (TESCF3) as the CF3 source. The CF3 anion is slowly generated in situ by initiation with KF, thus reducing the potential of side reactions to occur.
Buchwald, Science 2010, 328, 1679
Ar-CF3 bond-forming reaction via C-H activation
Shortly after Buchwald, the first Ar-CF3 bond-forming reaction directly from C-H bonds using a directing group strategy was reported by Yu. The reaction could be performed with simple Pd(OAc)2 and Umemoto reagent as a CF3 source.