Synthesis and Reactivity of α‐Cumyl Bromodifluoromethanesulfenate: Application to the Radiosynthesis of [18F]ArylSCF3

A highly reactive electrophilic bromodifluoromethylthiolating reagent, α‐cumyl bromodifluoro‐methanesulfenate 1 , was prepared to allow for direct bromodifluoromethylthiolation of aryl boron reagents. This coupling reaction takes place under copper catalysis, and affords a large range of bromodifluoromethylthiolated arenes. These compounds are amenable to various transformations including halogen exchange with [¹⁸F]KF/K₂₂₂ , a process giving access to [¹⁸F]arylSCF₃ in two steps from the corresponding aryl boronic pinacol esters.     

Trifluoromethylation of Arylsilanes with [(phen)CuCF3]

A method for the trifluoromethylation of arylsilanes is reported. The reaction proceeds with [(phen)CuCF₃] as the CF₃ source under mild, oxidative conditions with high functional‐group compatibility. This transformation complements prior trifluoromethylation of arenes in several ways. Most important, this method converts arylsilanes formed by the silylation of aryl C?H bonds to trifluoromethylarenes, thereby allowing the conversion of arenes to trifluoromethylarenes. The unique capabilities of the reported method are demonstrated by the conversion of a C?H bond into a C?CF₃ bond in active pharmaceutical ingredients which do not undergo this overall transformation by alternative functionalization processes, including a combination of borylation and trifluoromethylation.     

Copper‐Catalyzed One‐Pot Trifluoromethylation/Aryl Migration/Carbonyl Formation with Homopropargylic Alcohols

A novel copper‐catalyzed one‐pot functionalization of homopropargylic alcohols that involves trifluoromethylation, aryl migration, and formation of a carbonyl moiety has been developed. This reaction constitutes the first direct conversion of homopropargylic alcohols into CF₃‐containing 3‐butenal or 3‐buten‐1‐one derivatives in a regioselective manner. Mechanistic studies indicate that the 1,4‐aryl migration proceeds through a radical pathway.     

Silver‐Mediated Oxidative Trifluoromethylation of Phenols: Direct Synthesis of Aryl Trifluoromethyl Ethers

Aryl trifluoromethyl ethers (ArOCF₃) are prevalent in pharmaceuticals, agrochemicals, and materials. However, methods for the general and efficient synthesis of these compounds are extremely underdeveloped and limited. Herein, we describe a highly efficient and general procedure for the direct O‐trifluoromethylation of unprotected phenols through a silver‐mediated cross‐coupling reaction using CF₃SiMe₃ as the CF₃ source and exogenous oxidants. This novel oxidative trifluoromethylation provides access to a wide range of aryl trifluoromethyl ethers from simple phenols. The mild process was also applied to the late‐stage trifluoromethylation of a medicinally relevant compound.     

[18F]CuCF3: A [18F]Trifluoromethylating Agent for Arylboronic Acids and Aryl Iodides

Positron emission tomography has emerged as the leading method for medical imaging with fluorine‐18 as the most widely used radioactive isotope. Here we report a semi‐automated method for the preparation of valuable [¹⁸F]trifluoromethylcopper, as well as its use for the radiosynthesis of [¹⁸F]trifluoromethylarenes and heteroarenes. Mild conditions of [¹⁸F]trifluoromethylation make this method particularly useful for the radiosynthesis of pharmacologically relevant [¹⁸F]trifluoromethylarenes and heteroarenes.     

A General Copper‐Mediated Nucleophilic 18F Fluorination of Arenes

Molecules labeled with fluorine‐18 are used as radiotracers for positron emission tomography. An important challenge is the labeling of arenes not amenable to aromatic nucleophilic substitution (S_NAr) with [¹⁸F]F^?. In the ideal case, the ¹⁸F fluorination of these substrates would be performed through reaction of [¹⁸F]KF with shelf‐stable readily available precursors using a broadly applicable method suitable for automation. Herein, we describe the realization of these requirements with the production of ¹⁸F arenes from pinacol‐derived aryl boronic esters (arylBPin) upon treatment with [¹⁸F]KF/K₂₂₂ and [Cu(OTf)₂(py)₄] (OTf=trifluoromethanesulfonate, py=pyridine). This method tolerates electron‐poor and electron‐rich arenes and various functional groups, and allows access to 6‐[¹⁸F]fluoro‐L ‐DOPA, 6‐[¹⁸F]fluoro‐m‐tyrosine, and the translocator protein (TSPO) PET ligand [¹⁸F]DAA1106.     

Aryl Sulfonium Salts for Site‐Selective Late‐Stage Trifluoromethylation

Incorporation of the CF₃ group into arenes has found increasing importance in drug discovery. Herein, we report the first photoredox‐catalyzed cross‐coupling of aryl thianthrenium salts with a copper‐based trifluoromethyl reagent, which enables a site‐selective late‐stage trifluoromethylation of arenes. The reaction proceeds with broad functional group tolerance, even for complex small molecules on gram scale. The method was further extended to produce pentafluoroethylated derivatives.     

Difluorocarbene‐Derived Trifluoromethylthiolation and [18F]Trifluoromethylthiolation of Aliphatic Electrophiles

The first trifluoromethylthiolation and [¹⁸F]trifluoromethylthiolation of alkyl electrophiles with in situ generated difluorocarbene in the presence of elemental sulfur and external (radioactive) fluoride ion is described. This transition‐metal‐free approach is high yielding, compatible with a variety of functional groups, and operated under mild reaction conditions. The conceptual advantage of this exogenous‐fluoride‐mediated transformation enables unprecedented syntheses of [¹⁸F]CF₃S‐labeled molecules from most commonly used [¹⁸F]fluoride ions. The rapid radiochemical reaction time (≤1 min) and high functional‐group tolerance allow access to a variety of aliphatic [¹⁸F]CF₃S compounds in high yields.     

Iron-Catalyzed Trifluoromethylation of Indole-Tethered Alkene Enables Synthesis of CF3-Containing Spiroindolenines and Tetrahydrocarbazoles

An iron-catalyzed trifluoromethylation of indole-tethered alkene with Togni's reagent to construct CF₃-containing spiro[indole-3,3′-pyrrolidine] and tetrahydrocarbazole derivatives under mild and convenient conditions has been disclosed. Mechanistic studies indicate that the reaction proceed through a CF₃ radical addition to the alkene, followed by sequential dearomatizing spiocyclization of the indole and oxidation to afford the spiro[indole-3,3′-pyrrolidine] derivatives. Meanwhile, when the substituent at the C2 position of the indole is hydrogen, the CF₃-containing tetrahydrocarbazole is obtained through trifluoromethylation of alkene and cyclization of indole.     

Exploring the Role of Coinage Metalates in Trifluoromethylation: A Combined Experimental and Theoretical Study

Despite the known nucleophilic nature of [M(CF₃)₂]⁻ (M=Cu, Ag, Au) complexes, their participation in trifluoromethylation reactions of aryl halides remains unexplored. Here, for the first time, selective access to a [Cu(CF₃)₂]⁻ species is reported, which is ubiquitous in Cu-mediated trifluoromethylations, and we rationalize its complex mechanistic scenario as well as its behavior compared to its silver and gold congeners through a combination of experimental and computational approaches.     

Trifluoromethylation of Alkyl Radicals: Breakthrough and Challenges†

Trifluoromethylation of alkyl radicals is emerging as a powerful tool for C(sp³)–CF₃ bond formations. Based on the hypothesis of CF₃ group transfer from Cu(II)–CF₃ to alkyl radicals, a number of trifluoromethylation reactions have been developed, including trifluoromethylation of alkyl halides, decarboxylative trifluoromethylation of aliphatic carboxylic acids, C(sp³)–H trifluoromethylation, amino‐ and carbo‐trifluoromethylation of alkenes, etc. Challenges in this intriguing field are also discussed.     

Synthesis of 18F‐Difluoromethylarenes from Aryl (Pseudo) Halides

A general method for the synthesis of [¹⁸F]difluoromethylarenes from [¹⁸F]fluoride for radiopharmaceutical discovery is reported. The method is practical, operationally simple, tolerates a wide scope of functional groups, and enables the labeling of a variety of arenes and heteroarenes with radiochemical yields (RCYs, not decay‐corrected) from 10 to 60 %. The ¹⁸F‐fluorination precursors are readily prepared from aryl chlorides, bromides, iodides, and triflates. Seven ¹⁸F‐difluoromethylarene drug analogues and radiopharmaceuticals including Claritin, fluoxetine (Prozac), and [¹⁸F]DAA1106 were synthesized to show the potential of the method for applications in PET radiopharmaceutical design.     

The Role of Ate Complexes in the Copper‐Mediated Trifluoromethylation of Alkynes

Trifluoromethylation reactions have recently received increased attention because of the beneficial effect of the trifluoromethyl group on the pharmacological properties of numerous substances. A common method to introduce the trifluoromethyl group employs the Ruppert–Prakash reagent, that is, Si(CH₃)₃CF₃, together with a copper(I) halide. We have applied this method to the trifluoromethylation of aromatic alkynes and used electrospray‐ionization mass spectrometry to investigate the mechanism of these reactions in tetrahydrofuran, dichloromethane, and acetonitrile as well as with and without added 1,10‐phenanthroline. In the absence of the alkyne component, the homoleptic ate complexes [Cu(CF₃)₂]^? and [Cu(CF₃)₄]^? were observed. In the presence of the alkynes RH, the heteroleptic complexes [Cu(CF₃)₃R]^? were detected as well. Upon gas‐phase fragmentation, these key intermediates released the cross‐coupling products R?CF₃ with perfect selectivity. Apparently, the [Cu(CF₃)₃R]^? complexes did not originate from homoleptic cuprate anions, but from unobservable neutral precursors. The present results moreover point to the involvement of oxygen as the oxidizing agent.     

Synthesis,characterization and catalytic activity of new aminomethyldiphosphine–Pd(II) complexes for Suzuki cross‐coupling reaction

A new range of CF₃‐substituted aminomethyldiphosphine (P―C―N) ligands ((C₆H₅)₂PCH₂)₂NR (R = ―C₆H₄(2‐CF₃) ( 1 ), ―C₆H₄(3‐CF₃) ( 1b ) has been synthesized from 2‐(trifluoromethyl)aniline and 3‐(trifluoromethyl)aniline with diphenylphosphine. The aminomethyldiphosphine ligands were reacted with Pd(cod)Cl₂ to give corresponding metal complexes, PdLCl₂ ( 2a , 2b ). The aminomethyldiphosphine–palladium compounds were characterized by utilizing several methods including NMR (¹H, ¹³C, ³¹P) and elemental analysis. These compounds were used as catalysts in Suzuki cross‐coupling reaction of aryl chlorides and bromides. The effect of base was also investigated in this current project. CF₃‐substituted aminomethyldiphosphine–palladium complexes were found to be efficient catalysts in Suzuki cross‐coupling reaction of activated and deactivated aryl boronic acids. Copyright © 2013 John Wiley & Sons, Ltd.     

Nickel‐Mediated Trifluoromethylation of Phenol Derivatives by Aryl C−O Bond Activation

The increasing pharmaceutical importance of trifluoromethylarenes has stimulated the development of more efficient trifluoromethylation reactions. Tremendous efforts have focused on copper‐ and palladium‐mediated/catalyzed trifluoromethylation of aryl halides. In contrast, no general method exists for the conversion of widely available inert electrophiles, such as phenol derivatives, into the corresponding trifluoromethylated arenes. Reported herein is a practical nickel‐mediated trifluoromethylation of phenol derivatives with readily available trimethyl(trifluoromethyl)silane (TMSCF₃). The strategy relies on PMe₃‐promoted oxidative addition and transmetalation, and CCl₃CN‐induced reductive elimination. The broad utility of this transformation has been demonstrated through the direct incorporation of trifluoromethyl into aromatic and heteroaromatic systems, including biorelevant compounds.     

Selective fluorination of m-tyrosine by OF2

Fluorine-18 labeled aromatic amino acids are routinely used as tracers in positron emission tomography (PET) to study in vivo metabolic processes. The most versatile method for the production of such radiotracers is electrophilic fluorination of the aromatic amino acid with [¹⁸F]F₂, which is most commonly produced by the gas-phase nuclear reaction ¹⁸O(p, n)¹⁸F. Although [¹⁸F]F₂ is the major product, considerable amounts of [¹⁸F]OF₂ (up to 20%) are also produced. Electrophilic fluorination reactions of l-phenylalanine, 3-nitro-l-tyrosine, 4-nitro-dl-phenylalanine, 3,4-dihydroxyphenyl-l-alanine (l-DOPA), 3-O-methyl-l-DOPA, 3,4-dimethoxy-l-phenylalanine, p-tyrosine and o-tyrosine in H₂O and of m-tyrosine in anhydrous HF (aHF), CF₃SO₃H, CF₃COOH, CH₃COOH, HCOOH and H₂O using OF₂ were investigated. Although F₂ is an efficient fluorinating agent in aHF, electrophilic fluorination reactions using OF₂ were shown to be most efficient in less acidic media such as H₂O. In addition, and contrary to reports that OF₂ and F₂ have similar reactivities, m-tyrosine was the only aromatic system studied that was fluorinated by OF₂ and this was optimum in H₂O for the fluorinated m-tyrosine isomers (total yield, 4.35 ± 0.04%). The presence of [¹⁸F]OF₂ byproduct has no significant impact on the fluorination of aromatic amino acids investigated in this study and the subsequent production of their corresponding ¹⁸F-labeled radiotracers for patient use.     

Reactivity of 5-<Emphasis Type="Italic">endo</Emphasis>-hydroxy-4-azatricyclo[5.2.1.0<Superscript>2,6</Superscript>]dec-8-en-3-ones and their isomerization initiated by acids and bases

Study of isomerization of 5-endo-hydroxy-4-azatricyclo[5.2.1.0^2,6]dec-8-en-3-ones under the action of Lewis acids (MgBr₂, AlCl₃), CF₃COOH, and NaH showed that the optimum catalyst of the process was trifluoroacetic acid. In reaction of 4-benzyl-5-endo-hydroxy-4-azatricyclo-[5.2.1.0^2,6]dec-8-en-3-one with anhydrous AlCl₃ in benzene was unexpectedly isolated N-benzyl-3-(diphenylmethyl)bicyclo[2.2.1]hept-5-ene-2-carboxamide. A convenient method was developed for the preparation of 5-exo-alkoxy-4-alkyl(aryl)-4-azatricyclo[5.2.1.0^2,6]dec- 8-en-3-ones.     

Novel use of an isotope separator to determine the position of fluorine-18 in labelled 1,1,1,2-tetrafluoroethanes

A novel technique is described for measuring the site selectivity of methods for labelling the major CFC-alternative, 1,1,1,2-tetrafluoroethane (HFA 134a), with fluorine-18 (t_1/2 = 109.7 min). The carbon–carbon bond in radiofluorinated HFA 134a is broken in the ion source of an isotope separator. Radioactivity associated with the ion beam of the [CF₂ ¹⁸F]^+. fragment (m/z = 68) is collected, measured and divided by the integrated mass of the simultaneously collected ion beam for the [CF₃]^+. fragment (m/z = 69) to give the ‘specific radioactivity’ (in nCi nmol^–1) of the radiolabel in the 1-position. Similarly, the ‘specific radioactivity’ of the radiolabel in the 2-position is calculated from the measured radioactivity of the ion beam from the [CH₂ ¹⁸F]^+. fragment (m/z = 32) and the integrated mass of the simultaneously collected ion beam from the [CH₂F]^+. fragment (m/z = 33). The selectivity of the labelling procedure for a particular position is then given by the decay-corrected ratio of specific radioactivity at that position to the sum of specific radioactivities. The labelling of HFA 134a by the reaction of [¹⁸F] fluoride with trifluoroethylene was found to have 97% selectivity for the CF₃ group, whereas labelling by the reaction of [¹⁸F] fluoride with 2,2,2-trifluoroethyl p-toluenesulphonate was found to have 91% selectivity for the CH₂F group. This information is of value for tracer studies of the fate of HFA 134a in man following its inhalation as a drug propellant. The described technique is of potentially wider value for determining the position of fluorine-18 in labelled polyfluorinated molecules.     

Rhodium-Catalyzed Addition of Aryl Boronic Acids to 2,2-Disubstituted Malononitriles

β-Ketonitriles bearing a quaternary carbon at the 2-position were prepared through Rh-catalyzed addition of aryl boronic acids to 2,2-disubstituted malononitriles. In contrast to the previously described transnitrilative cyanation of aryl boronic acids with dialkylmalononitriles, the present reaction avoids retro-Thorpe collapse of the intermediate addition product through the use of a milder base. The reaction was amenable to a variety of aryl boronic acids and disubstituted malononitriles, providing a diverse array of β-ketonitriles. The products could be further derivatized to valuable chiral α,α-disubstituted-β-aminonitriles through addition reactions to the corresponding N-tert-butanesulfinyl imines.     

Palladium‐Catalyzed Cascade CH Trifluoroethylation of Aryl Iodides and Heck Reaction: Efficient Synthesis of ortho‐Trifluoroethylstyrenes

A palladium‐catalyzed selective C? H bond trifluoroethylation of aryl iodides has been explored. The reaction allows for the efficient synthesis of a variety of ortho‐trifluoroethyl‐substituted styrenes. Preliminary mechanistic studies indicate that the reaction might involve a key Pd^IV intermediate, which is generated through the rate‐determining oxidative addition of CF₃CH₂I to a palladacycle; the bulky nature of CF₃CH₂I influences the reactivity. Reductive elimination from the Pd^IV complex then leads to the formation of the aryl–CH₂CF₃ bond.