Photocatalytic Perfluoroalkylation of Disulfides and Diselenides. Syntheses of Perfluoroalkyl Thio- and Seleno-ethers

The synthesis of alkyl(aryl)-fluoroalkyl sulfanyl [R(Ar)-S-RF] and aryl-fluoroalkyl selenolyl (Ar-Se-RF) ethers through visible-light photocatalysis has been successfully carried out. This process involves disulfides, and diselenides [R(Ar)-X-X-R(Ar), where X = S or Se], and fluoroalkyl iodides (RF-I) in the presence of a base as an additive under photocatalysis. The photocatalyst Eosin Y and green light-emitting diodes are utilized for irradiation of R(Ar)-S-RF and Ar-Se-RF syntheses. Our method integrates low-energy visible-light photocatalysis, commercially available perfluoroalkylating reagents (RF-I), and easily obtainable disulfides and diselenides as starting materials. Mechanistic studies and a detailed synthetic procedure for (Ar)-S-RF on a large scale are presented.

Synthesis of 18F-labeled Aryl Trifluoromethyl Sulfones, -Sulfoxides, and -Sulfides for Positron Emission Tomography

Positron emission tomography (PET) is becoming increasingly important in nuclear medicine and drug discovery. To date, the development of many potential PET tracers is hampered by the lack of suitable synthetic pathways for their preparation. This is particularly true for the highly desired radiolabeling of compounds bearing [18F]CF3-groups. For instance, S(O)nCF3-groups (n=0, 1, 2) serve as structural motif in a range of biologically active compounds, but their radiosynthesis remains largely unprecedented (for n=1, 2). Herein, we describe general methods for the radiosynthesis of 18F-labeled aryl trifluoromethyl sulfones, -sulfoxides, and -sulfides. All three methods are operationally straightforward, start from widely available precursors, i.e., sulfonyl fluorides and thiophenols, and make use of the recently established [18F]Ruppert-Prakash reagent. Further, the syntheses display good functional group tolerance as demonstrated by the 18F-labeling of more than 40 compounds. The applicability of the new method is demonstrated by the radiolabeling of three bioactive molecules, optionally to be used as PET tracers. In a broader context, this work presents a substantial expansion of the chemical space of radiofluorinated structural motifs to be used for the development of new PET tracers.

Synthesis of 18F-labelled aryl trifluoromethyl ketones with improved molar activity

A method for the radiosynthesis of 18F-labelled aryl trifluoromethyl ketones starting from widely available Weinreb amides using [18F]fluoroform is presented. The method uses potassium hexamethyldisilazane as base and delivers products in high molar activity (up to 24 GBq μmol-1) and excellent radiochemical conversions. The applicability for PET tracer synthesis is demonstrated by the radiosynthesis of ten (hetero)aryl trifluoromethylketones, bearing electron-withdrawing and -donating substituents including a derivative of bioactive probenecid.

Transition-metal-catalyzed C-H bond activation as a sustainable strategy for the synthesis of fluorinated molecules: an overview

The last decade has witnessed the emergence of innovative synthetic tools for the synthesis of fluorinated molecules. Among these approaches, the transition-metal-catalyzed functionalization of various scaffolds with a panel of fluorinated groups (XR_F, X = S, Se, O) offered straightforward access to high value-added compounds. This review will highlight the main advances made in the field with the transition-metal-catalyzed functionalization of C(sp²) and C(sp³) centers with SCF₃, SeCF₃, or OCH₂CF₃ groups among others, by C-H bond activation. The scope and limitations of these transformations are discussed in this review.

Recent Advances in Synthetic Methodologies to Form C-18F Bonds

Positron emission tomography (PET) is an important technique for the early diagnosis of disease. Due to the specific physical and chemical properties of Fluorine-18, this important isotope is widely used in PET for labelling and molecular imaging, and its introduction into medicine molecules could produce PET tracers. Developing with the development of organic synthetic methodologies, the introduction of Fluorine-18 into drug molecules efficiently and rapidly under mild conditions, and the formation of C-¹⁸F chemical bonds, has become one of the leading topics in both organic synthetic chemistry and radiochemistry. In this mini-review, we review a series of recent advances in the organic synthesis of C-¹⁸F bonds (2015–2021), including non-catalytic radiofluorinations via good leaving functional groups, transition metal-catalyzed radiofluorinations, and photo- or electro-catalytic synthetic radiofluorinations. As a result of the remarkable advancements in this field, organic synthetic methods for forming C-¹⁸F bonds are expected to continue growing.

Advances in [18F]Trifluoromethylation Chemistry for PET Imaging

Positron emission tomography (PET) is a preclinical and clinical imaging technique extensively used to study and visualize biological and physiological processes in vivo. Fluorine-18 (¹⁸F) is the most frequently used positron emitter for PET imaging due to its convenient 109.8 min half-life, high yield production on small biomedical cyclotrons, and well-established radiofluorination chemistry. The presence of fluorine atoms in many drugs opens new possibilities for developing radioligands labelled with fluorine-18. The trifluoromethyl group (CF₃) represents a versatile structural motif in medicinal and pharmaceutical chemistry to design and synthesize drug molecules with favourable pharmacological properties. This fact also makes CF₃ groups an exciting synthesis target from a PET tracer discovery perspective. Early attempts to synthesize [¹⁸F]CF₃-containing radiotracers were mainly hampered by low radiochemical yields and additional challenges such as low radiochemical purity and molar activity. However, recent innovations in [¹⁸F]trifluoromethylation chemistry have significantly expanded the chemical toolbox to synthesize fluorine-18-labelled radiotracers. This review presents the development of significant [¹⁸F]trifluoromethylation chemistry strategies to apply [¹⁸F]CF₃-containing radiotracers in preclinical and clinical PET imaging studies. The continuous growth of PET as a crucial functional imaging technique in biomedical and clinical research and the increasing number of CF₃-containing drugs will be the primary drivers for developing novel [¹⁸F]trifluoromethylation chemistry strategies in the future.

Closing the gap between 19F and 18F chemistry

Positron emission tomography (PET) has become an invaluable tool for drug discovery and diagnosis. The positron-emitting radionuclide fluorine-18 is frequently used in PET radiopharmaceuticals due to its advantageous characteristics; hence, methods streamlining access to 18F-labelled radiotracers can make a direct impact in medicine. For many years, access to 18F-labelled radiotracers was limited by the paucity of methodologies available, and the poor diversity of precursors amenable to 18F-incorporation. During the last two decades, 18F-radiochemistry has progressed at a fast pace with the appearance of numerous methodologies for late-stage 18F-incorporation onto complex molecules from a range of readily available precursors including those that do not require pre-functionalisation. Key to these advances is the inclusion of new activation modes to facilitate 18F-incorporation. Specifically, new advances in late-stage 19F-fluorination under transition metal catalysis, photoredox catalysis, and organocatalysis combined with the availability of novel 18F-labelled fluorination reagents have enabled the invention of novel processes for 18F-incorporation onto complex (bio)molecules. This review describes these major breakthroughs with a focus on methodologies for C-18F bond formation. This reinvigorated interest in 18F-radiochemistry that we have witnessed in recent years has made a direct impact on 19F-chemistry with many laboratories refocusing their efforts on the development of methods using nucleophilic fluoride instead of fluorination reagents derived from molecular fluorine gas.

Radical-Promoted Distal C-H Functionalization of C(sp3 ) Centers with Fluorinated Moieties

Due to their unique properties, fluorinated scaffolds are pivotal compounds in pharmaceuticals, agrochemicals, and materials science. Over the last years, the development of versatile strategies for the selective synthesis of fluorinated molecules by direct C-H bond functionalization has attracted a lot of attention. In particular, the design of novel transformations based on a radical process was a bottleneck for distal C-H functionalization reactions, offering synthetic solutions for the selective introduction of fluorinated groups. This Minireview highlights the major contributions in this blossoming field. The development of new methodologies for the remote functionalization of aliphatic derivatives with various fluorinated groups based on a 1,5-hydrogen atom transfer process and a β-fragmentation reaction will be showcased and discussed.

Enantioselective preparation of atropisomeric biaryl trifluoromethylsulfanes via ring-opening of cyclic diaryliodoniums

Two convenient and practical methods for the synthesis of axially chiral biaryls bearing the trifluoromethylthio group are reported. A Cu-catalyzed enantioselective ring-opening reaction of cyclic diaryliodoniums with CsSCF₃ enables the direct synthesis of trifluoromethylthiolated biaryl atropisomers in high yields and enantioselectivity. For unsymmetric cyclic diaryliodoniums bearing an adjacent group to the C-I bond, a two-step procedure is required to achieve good regio- and enantioselectivity.

A Unified Strategy for Arylsulfur(VI) Fluorides from Aryl Halides: Access to Ar-SOF3 Compounds

A convenient protocol to selectively access various arylsulfur(VI) fluorides from commercially available aryl halides in a divergent fashion is presented. Firstly, a novel sulfenylation reaction with the electrophilic N-(chlorothio)phthalimide (Cl-S-Phth) and arylzinc reagents afforded the corresponding Ar-S-Phth compounds. Subsequently, the S(II) atom was selectively oxidized to distinct fluorinated sulfur(VI) compounds under mild conditions. Slight modifications on the oxidation protocol permit the chemoselective installation of 1, 3, or 4 fluorine atoms at the S(VI) center, affording the corresponding Ar-SO2 F, Ar-SOF3 , and Ar-SF4 Cl. Of notice, this strategy enables the effective introduction of the rare and underexplored -SOF3 moiety into various (hetero)aryl groups. Reactivity studies demonstrate that such elusive Ar-SOF3 can be utilized as a linchpin for the synthesis of highly coveted aryl sulfonimidoyl fluorides (Ar-SO(NR)F).

Fluorinated Ylides/Carbenes and Related Intermediates from Phosphonium/Sulfonium Salts

Owing to the special effects of the fluorine element, including high electronegativity and small atomic radius, the incorporation of a fluorinated group into organic molecules may modify their physical, chemical, and biological properties. Fluorine-containing compounds have found widespread application in a variety of areas, and thus, the development of efficient reagents and methods for the incorporation of fluorinated groups has become a subject of significant interest.Described in this Account are our recent discoveries in the chemistry of fluorinated ylides/carbenes and related intermediates generated from phosphonium/sulfonium salts. Initially, we obtained the (triphenylphosphonio) difluoroacetate, Ph₃P⁺CF₂CO₂^- (PDFA), which was proposed as a reactive intermediate but had never been successfully synthesized. PDFA, shelf-stable and easy to prepare, is not only a mild ylide (Ph₃P⁺CF₂^-) reagent, but also an efficient difluorocarbene source. It can directly generate difluorocarbene, via the first generation of ylide Ph₃P⁺CF₂^-, simply under warming conditions without the need for any additive. Interestingly, difluorocarbene chemistry was then discovered by using PDFA as a reagent. Difluorocarbene can be oxidized to CF₂═O, can react with elemental sulfur to afford CF₂═S, and can be trapped by NaNH₂ or NH₃ to give CN^-. The development of these processes into synthetic tools allowed us to achieve various reactions, including the challenging ¹⁸F-trifluoromethylthiolation and cyanodifluoromethylation. It was found that a substituent on the cation of a phosphonium salt can be directly transferred as a nucleophile despite the cation's high electrophilicity. This transfer process is like an "umpolung" of the cation, which may provide more opportunities for the synthetic utilities of phosphonium salts. The investigation of this transfer process led us to find that iodophosphonium salts, active intermediates which can be easily generated, may efficiently promote deoxygenative functionalizations of aldehydes and alcohols. Dehydroxylative substitution of alcohols by this protocol permits the use of unprotected amines with higher pK_a values as nucleophiles, which is an attractive feature compared with the Mitsunobu reaction. On the basis of the ylide-to-carbene process (Ph₃P⁺CF₂^- → :CF₂), we further developed sulfonium salts as precursors of fluorinated ylides and fluorinated methyl carbenes. In particular, the studies on difluoromethylcarbene, remaining largely unexplored, may deserve more attention. The discoveries may find utility in the synthesis of biologically active fluorine-containing molecules.

Rapid and Efficient Synthesis of [11C]Trifluoromethylarenes from Primary Aromatic Amines and [11C]CuCF3

Prior studies have shown that trifluoromethylarenes can be labeled in high molar activities (A m > 200 GBq/μmol) with positron-emitting carbon-11 (t 1/2 = 20.4 min) by the reaction of the copper(I) derivative of [11C]fluoroform [11C]CuCF3, with several types of precursors, such as aryl iodides, arylboronic acids, and aryldiazonium salts. Nonetheless, these precursors can be challenging to synthesize, and in the case of diazonium salts, are unstable. Methods that reduce challenges in precursor preparation for the synthesis of [11C]trifluoromethylarenes are desirable to enhance possibilities for developing biologically relevant 11C-labeled compounds as radiotracers for biomedical imaging with positron emission tomography (PET). Here, we explored the production of no-carrier-added [11C]trifluoromethylarenes from commercially available primary aromatic amines through reactions of [11C]CuCF3 with diazonium salts that were generated in situ. Moderate to high isolated decay-corrected radiochemical yields (RCY) (32-84%) were obtained rapidly (within 2 min) for many para-substituted and meta-substituted primary aromatic amines bearing a halo, methoxy, thiomethyl, hydroxy, nitro, nitrile, carboxyl, ethylcarboxy, or trifluoromethyl substituent. Null to low RCYs (0-13%) were observed only for ortho bromo-, nitro-, or nitrile-substituted precursors. This new radiosynthetic method usefully expands options for producing PET radiotracers bearing a [11C]trifluoromethyl group, especially from aryl amine precursors.

Perfluoroalkylation of Thiosulfonates: Synthesis of Perfluoroalkyl Sulfides

A practical synthesis of perfluoroalkyl sulfides is described. The method employs stable and readily accessible thiosulfonates as new electrophiles with commercial nucleophilic perfluoroalkylating reagents. The mild reaction conditions allow access to a wide variety of both aryl- and alkyl-substituted perfluoroalkyl sulfides amenable to pharmaceutical development. Furthermore, the reaction operation is straightforward, odorless, does not produce toxic wastes, and, therefore should appeal to practitioners in industrial-scale productions.

Radical C–H 18F-Difluoromethylation of Heteroarenes with [18F]Difluoromethyl Heteroaryl-Sulfones by Visible Light Photoredox Catalysis

The 18F-labeling of CF2H groups has been recently studied in radiopharmaceutical chemistry owing to the favorable nuclear and physical characteristics of the radioisotope 18F for positron emission tomography (PET). Following up on the reported efficiency of the [18F]difluoromethyl benzothiazolyl-sulfone ([18F]1) as a 18F-difluoromethylating reagent, we investigated the influence of structurally-related [18F]difluoromethyl heteroaryl-sulfones in the reactivity toward the photoredox C–H 18F-difluoromethylation of heteroarenes under continuous-flow conditions. In the present work, six new [18F]difluoromethyl heteroaryl-sulfones [18F]5a–[18F]5f were prepared and, based on the overall radiochemical yields (RCYs), three of these reagents ([18F]5a, [18F]5c, and [18F]5f) were selected for the fully automated radiosynthesis on a FASTlabTM synthesizer (GE Healthcare) at high level of starting radioactivity. Subsequently, their efficiency as 18F-difluoromethylating reagents was evaluated using the antiherpetic drug acyclovir as a model substrate. Our results showed that the introduction of molecular modifications in the structure of [18F]1 influenced the amount of fac-IrIII(ppy)3 and the residence time needed to ensure a complete C–H 18F-difluoromethylation process. The photocatalytic C–H 18F-difluoromethylation reaction with the reagents [18F]5a, [18F]5c, and [18F]5f was extended to other heteroarenes. Radical-trapping experiments demonstrated the likely involvement of radical species in the C–H 18F-difluoromethylation process.

Enantioselective catalytic synthesis of α-aryl-α-SCF3-β2,2-amino acids

We herein report a novel entry towards chiral α-SCF3-β2,2-amino acids by carrying out the ammonium salt-catalyzed α-trifluoromethylthiolation of isoxazolidin-5-ones. This approach allowed for high enantioselectivities and high yields and the obtained heterocycles proved to be versatile platforms to access other targets of potential interest.

Chemistry for Positron Emission Tomography: Recent Advances in 11 C-, 18 F-, 13 N-, and 15 O-Labeling Reactions

Positron emission tomography (PET) is a molecular imaging technology that provides quantitative information about function and metabolism in biological processes in vivo for disease diagnosis and therapy assessment. The broad application and rapid advances of PET has led to an increased demand for new radiochemical methods to synthesize highly specific molecules bearing positron-emitting radionuclides. This Review provides an overview of commonly used labeling reactions through examples of clinically relevant PET tracers and highlights the most recent developments and breakthroughs over the past decade, with a focus on 11 C, 18 F, 13 N, and 15 O.

Synthesis and Reactivity of α-Cumyl Bromodifluoromethanesulfenate: Application to the Radiosynthesis of [18 F]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 [18 F]KF/K222  , a process giving access to [18 F]arylSCF3 in two steps from the corresponding aryl boronic pinacol esters.

Synthesis and Derivatization of 1,1-[18 F]Difluorinated Alkenes

A general method for the synthesis of 1,1-[¹⁸ F]difluorinated alkenes from [¹⁸ F]fluoride is reported. This transformation is highly regioselective giving the desired ¹⁸ F-fluoroalkenes with radiochemical purities of up to 77 % within 20 minutes and a molar activity (A_m ) of 1 GBq μmol^-1 . The transformations are operationally simple to perform and were readily translated onto a commercial automated synthesis unit. The resultant 1,1-[¹⁸ F]difluorinated alkene motif is prevalent in numerous drug molecules, and this is the first general method to synthesize this motif with fluorine-18. ¹⁸ F-fluorinated alkenes are excellent building blocks and participate in a number of post-labeling transformations to access a range of ¹⁸ F-perfluorinated functional groups that have never before been radiolabeled with non-carrier-added [¹⁸ F]fluoride. This method considerably expands the range of ¹⁸ F-motifs accessible to radiochemists.

An electrophilic reagent for the synthesis of OCHFMe-containing molecules

Herein the synthesis of a novel and bench stable electrophilic reagent to construct the OCFHMe motif from O-nucleophiles has been described. This sulfonium salt, readily obtained in 5 steps, reacted with various phenols and alcohols. The resulting products, including complex molecules, were obtained in good yields. This reagent was also used for the functionalization of thiol derivatives.

Boron reagents for divergent radiochemistry

This review discusses boron reagents as precursors for divergent radiolabelling with a focus on carbon-11, fluorine-18 and iodine-123, -125, -131.

Domino cyclization/trifluoromethylation of 2-alkynylanilines using fluoroform-derived CuCF3: synthesis of 3-(trifluoromethyl)indoles

A domino cyclization/trifluoromethylation strategy for the construction of indole cores with concomitant installation of a CF₃ group is described.