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.


Introduction
Trifluoromethyl (CF 3 ) groups are important functional groups in pharmaceutical and medicinal chemistry as excellent bioisosteres of methyl groups. CF 3 groups are incorporated into drugs to improve potency, binding selectivity, lipophilicity and facilitate their administration [1,2]. Improved pharmacological activities and properties are observed with numerous CF 3 group-containing drugs. This trend is also visible in recent FDA approvals for drugs containing a CF 3 group (Figure 1) [3].
Many small molecule drugs recently approved for cancer treatment have at least one fluorine atom, underlining the importance of fluorine atoms as effective bioisosteres. The installation of one or multiple fluorine atoms into small molecule compounds can significantly improve metabolic stability, increase bioavailability and cellular membrane permeability, thus resulting in drugs with better pharmacokinetics and enhanced efficacy [3][4][5][6][7].
The rapidly growing number of CF 3 -containing drugs has triggered novel synthetic methodologies for incorporating CF 3 -groups into small molecules. Trifluoromethylation reactions entail electrophilic, nucleophilic, and radical synthetic methods. Significant advances include better and more readily available CF 3 group sources and optimizing new chemistry for site-specific CF 3 group incorporation. The synthesis of CF 3 -containing compounds traditionally involves multi-step and long synthesis sequences utilizing various trifluoromethylated synthons. However, conventional CF 3 chemistry has been replaced by The rapidly growing number of CF3-containing drugs has triggered novel synthetic methodologies for incorporating CF3-groups into small molecules. Trifluoromethylation reactions entail electrophilic, nucleophilic, and radical synthetic methods. Significant advances include better and more readily available CF3 group sources and optimizing new chemistry for site-specific CF3 group incorporation. The synthesis of CF3-containing compounds traditionally involves multi-step and long synthesis sequences utilizing various trifluoromethylated synthons. However, conventional CF3 chemistry has been replaced by more practical transition metal-catalysed trifluoromethylation reactions. The most widely used transition metal for trifluoromethylation reactions is Cu, along with Pd and Ni.
The development of various stable CF3-containing sulfinate salts such as Langlois' reagent (CF3SO2Na) proved to be a flexible approach for trifluoromethylation reactions via a radical mechanism [8,9]. The emergence of difluoromethylated precursors for the rapid generation of CF3 groups is also an important strategy. Nucleophilic Ruppert-Prakash reagent (CF3SiMe3) and similar silyl reagents have also been integral in popularising trifluoromethylation reactions. The development of hypervalent iodine compounds (Togni reagents) and sulfonium salts (Umemoto reagents) for electrophilic CF3 group transfer has also significantly extended the possibilities of trifluoromethylation chemistry [10]. These nucleophilic, electrophilic, and radical CF3 transfer reagents were tested under different experimental conditions, such as standalone reagents for direct trifluoromethylation reactions or as part of transition metal-mediated methods for CF3 group transfer chemistry ( Figure 2).  The development of various stable CF 3 -containing sulfinate salts such as Langlois' reagent (CF 3 SO 2 Na) proved to be a flexible approach for trifluoromethylation reactions via a radical mechanism [8,9]. The emergence of difluoromethylated precursors for the rapid generation of CF 3 groups is also an important strategy. Nucleophilic Ruppert-Prakash reagent (CF 3 SiMe 3 ) and similar silyl reagents have also been integral in popularising trifluoromethylation reactions. The development of hypervalent iodine compounds (Togni reagents) and sulfonium salts (Umemoto reagents) for electrophilic CF 3 group transfer has also significantly extended the possibilities of trifluoromethylation chemistry [10]. These nucleophilic, electrophilic, and radical CF 3 transfer reagents were tested under different experimental conditions, such as standalone reagents for direct trifluoromethylation reactions or as part of transition metal-mediated methods for CF 3   The rapidly growing number of CF3-containing drugs has triggered novel synthetic methodologies for incorporating CF3-groups into small molecules. Trifluoromethylation reactions entail electrophilic, nucleophilic, and radical synthetic methods. Significant advances include better and more readily available CF3 group sources and optimizing new chemistry for site-specific CF3 group incorporation. The synthesis of CF3-containing compounds traditionally involves multi-step and long synthesis sequences utilizing various trifluoromethylated synthons. However, conventional CF3 chemistry has been replaced by more practical transition metal-catalysed trifluoromethylation reactions. The most widely used transition metal for trifluoromethylation reactions is Cu, along with Pd and Ni.
The development of various stable CF3-containing sulfinate salts such as Langlois' reagent (CF3SO2Na) proved to be a flexible approach for trifluoromethylation reactions via a radical mechanism [8,9]. The emergence of difluoromethylated precursors for the rapid generation of CF3 groups is also an important strategy. Nucleophilic Ruppert-Prakash reagent (CF3SiMe3) and similar silyl reagents have also been integral in popularising trifluoromethylation reactions. The development of hypervalent iodine compounds (Togni reagents) and sulfonium salts (Umemoto reagents) for electrophilic CF3 group transfer has also significantly extended the possibilities of trifluoromethylation chemistry [10]. These nucleophilic, electrophilic, and radical CF3 transfer reagents were tested under different experimental conditions, such as standalone reagents for direct trifluoromethylation reactions or as part of transition metal-mediated methods for CF3 group transfer chemistry ( Figure 2).  The rapidly increasing number of novel trifluoromethylation chemistry techniques and their impact on drug development and agrochemistry were summarised in detail in several recent review articles [1,2,4,7,[10][11][12][13]. In this review, we present recent results of significant [ 18 F]trifluoromethylation chemistry strategies for using [ 18 F]CF 3 -containing radiotracers in preclinical and clinical positron emission tomography (PET) imaging studies. The continuous growth of PET as a functional imaging technique in biomedical and clinical research and the increasing number of CF 3 -containing drug compounds are the primary drivers for developing novel [ 18 F]trifluoromethylation chemistry strategies.

PET Imaging and Fluorine-18
PET is an important molecular imaging technique with unique diagnostic applications for the quantitative assessment of biochemical and physiological processes in vivo with specific radiotracers. PET is also used to assess the pharmacokinetics and pharmacodynamics of novel drug candidates [14][15][16][17][18][19]. As a high sensitivity imaging technique, a typical PET experiment uses radiotracer doses in the nanomolar to picomolar concentration range. Such low administered doses generally prevent pharmacological and toxicological effects making PET studies a safe diagnostic procedure in the clinic.
Fluorine-18 ( 18 F) is among the most commonly used radioisotopes for PET, as demonstrated by hundreds of fluorine-18-labelled radiotracers [16,17]. Prominent examples of clinically used PET radiotracers are depicted in Figure 3. The most widely and frequently used PET radiotracer by far is glucose analogue 2-[ 18 F]fluoro-2-deoxy-D-glucose ([ 18 F]FDG). [ 18 F]FDG is particularly useful in measuring enhanced glucose metabolism due to altered energy metabolism, also referred to as the Warburg effect, in many cancers [20].
is another example of a fluorine-18-labelled radiotracer used in the clinic for the visualization of tissue or organ hypoxia in cancer and other diseases [21]. The importance of 18  and their impact on drug development and agrochemistry were summarised in detail in several recent review articles [1,2,4,7,[10][11][12][13]. In this review, we present recent results of significant [ 18 F]trifluoromethylation chemistry strategies for using [ 18 F]CF3-containing radiotracers in preclinical and clinical positron emission tomography (PET) imaging studies. The continuous growth of PET as a functional imaging technique in biomedical and clinical research and the increasing number of CF3-containing drug compounds are the primary drivers for developing novel [ 18 F]trifluoromethylation chemistry strategies.

PET Imaging and Fluorine-18
PET is an important molecular imaging technique with unique diagnostic applications for the quantitative assessment of biochemical and physiological processes in vivo with specific radiotracers. PET is also used to assess the pharmacokinetics and pharmacodynamics of novel drug candidates [14][15][16][17][18][19]. As a high sensitivity imaging technique, a typical PET experiment uses radiotracer doses in the nanomolar to picomolar concentration range. Such low administered doses generally prevent pharmacological and toxicological effects making PET studies a safe diagnostic procedure in the clinic.
Fluorine-18 ( 18 F) is among the most commonly used radioisotopes for PET, as demonstrated by hundreds of fluorine-18-labelled radiotracers [16,17]. Prominent examples of clinically used PET radiotracers are depicted in Figure 3. The most widely and frequently used PET radiotracer by far is glucose analogue 2-[ 18 F]fluoro-2-deoxy-D-glucose ([ 18 F]FDG). [ 18 F]FDG is particularly useful in measuring enhanced glucose metabolism due to altered energy metabolism, also referred to as the Warburg effect, in many cancers [20]. 1H-1-(3-[ 18 F]Fluoro-2-hydroxypropyl)-2-nitroimidazole ([ 18 F]FMISO) is another example of a fluorine-18-labelled radiotracer used in the clinic for the visualization of tissue or organ hypoxia in cancer and other diseases [21]. The importance of 18 F-labeled radiotracers for clinical PET was recently demonstrated by the FDA approval of fluorine-18labelled amino acid [ 18 F]Fdopa for clinical PET imaging of Parkinson's disease [3]. Many organofluorine compounds and the available pharmacological data of trifluoromethylated drugs place CF3 groups among the most predictable functional groups in medicinal chemistry and drug development. The high popularity of using CF3 groups in many drug molecules is also interesting for developing radiotracers labelled with the short-lived positron emitter fluorine-18. Replacing one of the three fluorine atoms in a CF3 group of a given compound with a fluorine-18 atom would provide a versatile and broadly applicable class of radiotracers for PET imaging. Although some radiochemistry strategies have been reported for this conversion, they are far from optimal [22]. Compared to other commonly employed fluorine-18 radiochemistry methods such as aliphatic and aromatic nucleophilic radiofluorinations, [ 18 F]trifluoromethylations are still relatively underexplored.

PET and [ 18 F]CF3 Group Chemistry
The development of fluorine-18-labelled PET imaging agents benefits significantly from the availability of numerous marketed fluorinated drugs and drug analogues. The existing large number of fluorine-containing drugs and the available literature on their Many organofluorine compounds and the available pharmacological data of trifluoromethylated drugs place CF 3 groups among the most predictable functional groups in medicinal chemistry and drug development. The high popularity of using CF 3 groups in many drug molecules is also interesting for developing radiotracers labelled with the short-lived positron emitter fluorine-18. Replacing one of the three fluorine atoms in a CF 3 group of a given compound with a fluorine-18 atom would provide a versatile and broadly applicable class of radiotracers for PET imaging. Although some radiochemistry strategies have been reported for this conversion, they are far from optimal [22]. Compared to other commonly employed fluorine-18 radiochemistry methods such as aliphatic and aromatic nucleophilic radiofluorinations, [ 18 F]trifluoromethylations are still relatively underexplored.

PET and [ 18 F]CF 3 Group Chemistry
The development of fluorine-18-labelled PET imaging agents benefits significantly from the availability of numerous marketed fluorinated drugs and drug analogues. The existing large number of fluorine-containing drugs and the available literature on their pharmacokinetics/pharmacodynamics and toxicological profile provide an excellent starting point for the design and synthesis of respective fluorine-18-labelled compounds. Among the plethora of fluorine-containing drugs, compounds with CF 3 groups represent exciting leads for developing fluorine-18 radiotracers.
There have already been several examples utilizing trifluoromethylated drugs as PET probes. Prominent examples include fluorine-18-labelled COX-2 inhibitor [ 18 F]Celecoxib [23] and hypoxia imaging agent [ 18 F]TFMISO ( Figure 4) [24].  However, a particular challenge in synthesizing [ 18 F]trifluoromethylated com is the typically obtained relatively low molar activities of the radiotracers. This is due to undesired 18 F-19 F isotopic exchange reactions contributing to the radioactiv uct and lowering molar activity. Therefore, the development of novel [ 18 F]trifluo ylation methods primarily aims to increase molar activities of the final radiotrac achieving reasonable radiochemical yields.

Major Strategies for the Generation of [ 18 F]Trifluoromethyl Groups
In general, there are four strategies to synthesize [ 18 F] CF3 groups ( Figure 5).  19 F/ 18 F isotopic exchange reactions have many advantages over other st mainly the ease of substrate availability. The commercial availability of many dr substrates containing CF3 groups makes this strategy a versatile synthesis approa broad operational flexibility. However, the selective C-F bond cleavage represent lenge as the harsh conditions required for C-F bond cleavage significantly reduc However, a particular challenge in synthesizing [ 18 F]trifluoromethylated compounds is the typically obtained relatively low molar activities of the radiotracers. This is mainly due to undesired 18 F-19 F isotopic exchange reactions contributing to the radioactive product and lowering molar activity. Therefore, the development of novel [ 18 F]trifluoromethylation methods primarily aims to increase molar activities of the final radiotracer while achieving reasonable radiochemical yields.

Major Strategies for the Generation of [ 18 F]Trifluoromethyl Groups
In general, there are four strategies to synthesize [ 18 F] CF 3 groups ( Figure 5). pharmacokinetics/pharmacodynamics and toxicological profile provide an excellent starting point for the design and synthesis of respective fluorine-18-labelled compounds. Among the plethora of fluorine-containing drugs, compounds with CF3 groups represent exciting leads for developing fluorine-18 radiotracers.
There have already been several examples utilizing trifluoromethylated drugs as PET probes. Prominent examples include fluorine-18-labelled COX-2 inhibitor [ 18 F]Celecoxib [23] and hypoxia imaging agent [ 18 F]TFMISO ( Figure 4) [24]. However, a particular challenge in synthesizing [ 18 F]trifluoromethylated compounds is the typically obtained relatively low molar activities of the radiotracers. This is mainly due to undesired 18 F-19 F isotopic exchange reactions contributing to the radioactive product and lowering molar activity. Therefore, the development of novel [ 18 F]trifluoromethylation methods primarily aims to increase molar activities of the final radiotracer while achieving reasonable radiochemical yields.

Major Strategies for the Generation of [ 18 F]Trifluoromethyl Groups
In general, there are four strategies to synthesize [ 18 F] CF3 groups ( Figure 5).  19 F/ 18 F isotopic exchange reactions have many advantages over other strategies, mainly the ease of substrate availability. The commercial availability of many drugs and substrates containing CF3 groups makes this strategy a versatile synthesis approach with broad operational flexibility. However, the selective C-F bond cleavage represents a challenge as the harsh conditions required for C-F bond cleavage significantly reduces func-

19 F-to-18 F Isotopic Exchange Chemistry
19 F/ 18 F isotopic exchange reactions have many advantages over other strategies, mainly the ease of substrate availability. The commercial availability of many drugs and substrates containing CF 3 groups makes this strategy a versatile synthesis approach with broad operational flexibility. However, the selective C-F bond cleavage represents a challenge as the harsh conditions required for C-F bond cleavage significantly reduces functional group tolerance. Late-stage radiolabelling strategies are also relatively underdeveloped. Despite several challenges and limitations, isotopic exchange protocols have been frequently used for the radiolabelling CF 3 groups in the past, mainly due to the surprising tolerance to the presence of water [22]. The final radiolabelled compound and the starting compound would be chemically the same and pose a critical risk of diluting molar activity.

Nucleophilic Substitution/Addition Radiofluorinations
Nucleophilic substitution/addition radiofluorination chemistry is among the most popular radiochemistry strategies for synthesizing [ 18 F]CF 3 groups, as exemplified by numerous published synthesis protocols. The chemistry stems from the extensive experience and success with nucleophilic radiofluorinations, including nucleophilic addition of [ 18 F]fluoride across electron-deficient alkenes. Consequently, nucleophilic substitution/addition radiofluorination chemistry was explored more extensively than other strategies to synthesize [ 18 F]CF 3 groups. This success of this route could be because of the least probability of intrinsic molar activity dilution. The most challenging part of this chemistry is the synthesis of precursors that can undergo a facile nucleophilic substitution.

Difluorocarbene-Mediated Chemistry
Difluorocarbene-mediated chemistry is also a popular synthesis route for [ 18 F]CF 3 group generation. The simplicity in reaction design and availability of difluorocarbene sources helped to make this synthesis strategy a popular method for developing [ 18 F]CF 3containing radiotracers. Multiple difluorocarbene sources have been identified and utilized with varying success ( Figure 6) [25][26][27]. This route has been optimized for a wide variety of substrates such as aryl halides and sulfhydryl groups. The possibility of fluoride scrambling is a threat to molar activity. However, various reports have shown that this method is capable of [ 18 F]trifluoromethylation.
compound would be chemically the same and pose a critical risk of diluting mol ity.

Nucleophilic Substitution/Addition Radiofluorinations
Nucleophilic substitution/addition radiofluorination chemistry is among t popular radiochemistry strategies for synthesizing [ 18 F]CF3 groups, as exemplified merous published synthesis protocols. The chemistry stems from the extensive exp and success with nucleophilic radiofluorinations, including nucleophilic add [ 18 F]fluoride across electron-deficient alkenes. Consequently, nucleophilic s tion/addition radiofluorination chemistry was explored more extensively than oth egies to synthesize [ 18 F]CF3 groups. This success of this route could be because of probability of intrinsic molar activity dilution. The most challenging part of this ch is the synthesis of precursors that can undergo a facile nucleophilic substitution.

Difluorocarbene-Mediated Chemistry
Difluorocarbene-mediated chemistry is also a popular synthesis route for group generation. The simplicity in reaction design and availability of difluoro sources helped to make this synthesis strategy a popular method for developing [ containing radiotracers. Multiple difluorocarbene sources have been identified lized with varying success ( Figure 6) [25][26][27]. This route has been optimized for variety of substrates such as aryl halides and sulfhydryl groups. The possibility of scrambling is a threat to molar activity. However, various reports have shown t method is capable of [ 18 F]trifluoromethylation.

Electrophilic Radiofluorinations
Electrophilic fluorination reactions are the dominant synthesis method of gen organofluorine compounds. Electrophilic fluorinations are accomplished via dire ination with fluorine gas or by electrophilic fluorinating reagents such as Select chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate)). In of trifluoromethylation reactions, electrophilic fluorination chemistry holds sev vantages. Hydrated [ 18 F]fluoride is mostly ineffective in nucleophilic attacks; it aqueous and other protic solvents is cumbersome. In the case of fluorine-18 radio try, electrophilic fluorine-18 transferring reagents such as [ 18 [2 major concern is the low molar activity associated with this method. Most of the philic syntheses start with carrier-added fluorine-18, which severely dilutes the m tivity.

Electrophilic Radiofluorinations
Electrophilic fluorination reactions are the dominant synthesis method of generating organofluorine compounds. Electrophilic fluorinations are accomplished via direct fluorination with fluorine gas or by electrophilic fluorinating reagents such as Selectfluor (1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate)). In the case of trifluoromethylation reactions, electrophilic fluorination chemistry holds several advantages. Hydrated [ 18 F]fluoride is mostly ineffective in nucleophilic attacks; its use in aqueous and other protic solvents is cumbersome. In the case of fluorine-18 radiochemistry, electrophilic fluorine-18 transferring reagents such as [ 18 [28][29][30][31]. A major concern is the low molar activity associated with this method. Most of the electrophilic syntheses start with carrier-added fluorine-18, which severely dilutes the molar activity.
In 2010, Kachur et al. reported an improved strategy for the synthesis of EF5 [33]. The study added catalytic amounts of iodine, and the authors observed that 1% iodine improved the radiochemical yields to 50%. Suehiro [27]. The described procedure generated [ 18 F]TFMISO in reasonable radiochemical yields (60%) but at a low molar activity (0.003 GBq/µmol). Riss et al. reported a nucleophilic alkene [ 18 F]fluorination method resulting in products at a high molar activity (86 GBq/µmol) [34]. The method was limited to the generation of 2,2,2-[ 18 F]trifluoroethyl groups, but the operational simplicity of the chemistry and the obtained high molar activities are remarkable. While the molar activity of the method is impressive, the radiochemical yield was low [35]. Van [38]. The strategy utilized electrophilic radiofluorination with [ 18 F]F 2 resulting in products with diminished molar activity in the range of 0.02 GBq/µmol. Gomez et al. reported a method for the synthesis of fluorine-18-labelled trifluoroacetamides [39]. The direct nucleophilic displacement of difluorobromides with [ 18 F]fluoride was not efficient. The addition of metal salts was also not successful, whereas the addition of base DBU proved to be crucial, and radiochemical conversion yields of 71% were observed. Further exploration of other nitrogen-based nucleophilic activators such as DBU confirmed the beneficial effects of nitrogen bases. The use of guanidine derivatives TBD and MTBD further improved the radiochemical conversions to 81% and 76%, respectively. The authors reported a moderate molar activity of 8.4 GBq/µmol for the reaction. Meyer and co-workers reported a nucleophilic halex reaction to synthesize α-CF 3 ketones [40]. The reaction used TBD (1,5,7-triazabicyclo-[4.4.0]dec-5-ene) as additive. They were able to obtain a 30% yield and a molar activity up to 1 [41]. The authors subjected the [ 18 F]CF 3 compound to hydride reduction to obtain their target molecule. The final product (a fluorine-18-labelled 1,4-benzodiazepine-2-one) was obtained at a molar activity of 0.037 GBq/µmol. In 2020, Szpera and co-workers reported an improved synthesis of 2,2,2-[ 18 F]trifluoroethanol [42]. The overall strategy of radiofluorination of α-diflurobromo ester to generate the radiolabelled ester and a further reduction of the ester to corresponding alcohol was similar to that of Johnstrom et al. The higher molar activity radiofluorination route disclosed by Gomez and co-workers was adapted to this synthesis, where DBU is used as an additive to facilitate the radiofluorination (Scheme 2).  3 labelling. Here, the CF 3 group is coordinated to an Au(III) complex. The Au(III) complex facilitates the C-F bond cleavage and migratory insertion to generate an isolable R-CF 2 -metal complex, which in turn provides easy access to the [ 18 F]CF 3 moiety [43]. This unique protocol enables the preparation of fluorine-18-labelled aliphatic CF 3 -containing radiotracers in 6% radiochemical yield and with a low molar activity in the range of 0.3 GBq/µmol. The feasibility of using bis(trifluoromethyl)Au(III) complexes for the isotopic exchange with [ 18  Here, the CF3 group is coordinated to an Au(III) complex. The Au(III) complex facilitates the C-F bond cleavage and migratory insertion to generate an isolable R-CF2-metal complex, which in turn provides easy access to the [ 18 F]CF3 moiety [43]. This unique protocol enables the preparation of fluorine-18-labelled aliphatic CF3-containing radiotracers in 6% radiochemical yield and with a low molar activity in the range of 0.3 GBq/μmol. The feasibility of using bis(trifluoromethyl)Au(III) complexes for the isotopic exchange with [ 18  and its subsequent analysis as a potential radiotracer for quantifying aggregated tau protein levels in Alzheimer's disease (AD) and progressive supramolecular palsy (PSP) [44]. The authors followed the nucleophilic addition chemistry route of n.c.a. [ 18 F]fluoride on a difluoromethylidene to generate the [ 18 F]CF3 group (Scheme 4). They observed the formation of a [ 18 F]difluoromethylidene compound as the primary side product during the reaction. The combined radiochemical yield was 14%, including the side product. Further optimization did not improve the radiochemical yield. Moreover, the authors found that the ratio of [ 18 F]difluoromethylidene increased, and the radiochemical yield of [ 18 F]LNS was not higher than 7%. and its subsequent analysis as a potential radiotracer for quantifying aggregated tau protein levels in Alzheimer's disease (AD) and progressive supramolecular palsy (PSP) [44]. The authors followed the nucleophilic addition chemistry route of n.c.a. [ [44]. This process afforded the radiotracer in radiochemical yields of 4.6% at high molar activities of 120 GBq/µmol. In the clinic, radiotracer [ 18 F]NML showed good brain uptake and favourable pharmacokinetics, but the brain retention in AD and PSP patients was low (Figure 7).  [44]. This process afforded the radiotracer in radiochemical yields of 4.6% at high molar activities of 120 GBq/μmol. In the clinic, radiotracer [ 18 F]NML showed good brain uptake and favourable pharmacokinetics, but the brain retention in AD and PSP patients was low (Figure 7).   [44]. This process afforded the radiotracer in radiochemical yields of 4.6% at high molar activities of 120 GBq/μmol. In the clinic, radiotracer [ 18 F]NML showed good brain uptake and favourable pharmacokinetics, but the brain retention in AD and PSP patients was low (Figure 7).    [48]. The procedure involved a fluorodesulphurization of a dithioester, which had to be subjected to another desulphurization reaction to furnish the [ 18 F]CF 3 group. Cheguillaume et al. reported a strategy for synthesizing perfluorinated alkyl motifs to develop novel nitroimidazole-based hypoxia PET markers [49]. In this report, the authors utilized a 1,3-dibromo-5,5-dimethylhydantoin (DBH)-activated radiofluorination of trithioorthoesters with [ 18 F]HF-pyridine followed by an additional DBH-activated fluorination step with an excess of HF-pyridine to yield [ 18 F]CF 3 -containing compounds.
part of a more extensive study involving electrophilic substitution reactions of iodanes with different electrophiles, including [ 18 F]XeF2. The reaction of [ 18 F]XeF2 with a cyclic perfluorinated iodane afforded a [ 18 F]CF3CF2-containing compound with a radiochemical yield of 9%. A carrier-added radiosynthesis reported by Josse et al. achieved a molar activity of 2.7 GBq/μmol [48]. The procedure involved a fluorodesulphurization of a dithioester, which had to be subjected to another desulphurization reaction to furnish the [ 18 F]CF3 group. Cheguillaume et al. reported a strategy for synthesizing perfluorinated alkyl motifs to develop novel nitroimidazole-based hypoxia PET markers [49]. In this report, the authors utilized a 1,3-dibromo-5,5-dimethylhydantoin (DBH)-activated radiofluorination of trithioorthoesters with [ 18 F]HF-pyridine followed by an additional DBHactivated fluorination step with an excess of HF-pyridine to yield [ 18  published a nucleophilic radiolabelling of [ 18 F]TMP195, a 5-[ 18 F]-trifluoromethyl-1,2,4-oxadiazole (TFMO) containing radiotracer [55]. They synthesized the radiotracer with a 4% radiochemical yield and a relatively low molar activity of 0.5 GBq/μmol (Scheme 7). For this experiment, the [ 18 F]fluoride was purchased from an external source and it underwent significant decay (>2 half-lives) before the experiment. Authors hypothesized this as a reason for the overall low molar activity. The authors used a direct nucleophilic radiofluorination reaction with n.c.a. [ 18 F]fluoride on difluoroarylbromide to yield the desired [ 18 F]CF 3 compound [53]. The described [ 18 F]CF 3 chemistry was part of a multi-step synthesis of a fluorine-18-labelled GABA uptake inhibitor. Hammadi et al. reported the nucleophilic displacement of a benzylic bromide to generate 4-chloro-α,α,α-[ 18 F]trifluoromethyl toluene in moderate radiochemical yields (~30%) but low molar activities in the range of 1 GBq/µmol [54]. The fluorine-18-labelled aryl chloride was further used for the synthesis of selective serotonin uptake inhibitor [ 18 F]-(S)-fluoxetine. The authors observed that achieving higher molar activity depends on a delicate balance between temperature and precursor concentration. Prabhakaran et al. described the synthesis of [ 18 F]celecoxib via direct nucleophilic substitution of the corresponding bromide [23]. The reaction provided radiotracer [ 18 F]celecoxib at moderate molar activities of 4.5 GBq/µmol, which was tested for PET imaging in baboons and rats ( Figure 8) [55]. They synthesized the radiotracer with a 4% radiochemical yield and a relatively low molar activity of 0.5 GBq/µmol (Scheme 7). For this experiment, the [ 18 F]fluoride was purchased from an external source, and it underwent significant decay (>2 half-lives) before the experiment. Authors hypothesized this as a reason for the overall low molar activity. A peripheral cannabinoid receptor-1 (CB-1R) specific antagonist was labelled with 18 F and tested in PET imaging experiments (Scheme 8). The study by Chang et al. demonstrated that radioligand [ 18 F]DBPR211 showed only low blood-brain barrier penetration capacity in mice during the PET imaging experiments [56]. The radiolabelling precursor contained an aryl difluorobromomethyl group which has already been established as a suitable group for nucleophilic substitutions with n.c.a. [ 18 F]fluoride to generate aryl [ 18 F]CF3 groups. The radiolabelling was carried out in the presence of two unprotected N-H bonds, and the final product was isolated in radiochemical yields of 7.6% at a molar activity of 40 GBq/μmol. A peripheral cannabinoid receptor-1 (CB-1R) specific antagonist was labelled with 18 F and tested in PET imaging experiments (Scheme 8). The study by Chang et al. demonstrated that radioligand [ 18 F]DBPR211 showed only low blood-brain barrier penetration capacity in mice during the PET imaging experiments [56]. The radiolabelling precursor contained an aryl difluorobromomethyl group which has already been established as a suitable group for nucleophilic substitutions with n.c.a. [ 18 F]fluoride to generate aryl [ 18 F]CF3 groups. The radiolabelling was carried out in the presence of two unprotected N-H bonds, and the final product was isolated in radiochemical yields of 7.6% at a molar activity of 40 GBq/μmol. A peripheral cannabinoid receptor-1 (CB-1R) specific antagonist was labelled with 18 [60]. The Cu-difluorocarbene-mediated radiolabelling proceeded with a 6% radiochemical yield and a low molar activity of up to 0.76 GBq/μmol (Scheme 11). King et al. developed a method for the labelling of 5-trifluoromethyl-2-deoxyuridine (trifluridine) with fluorine-18 [61]. The difluorocarbene-mediated reaction provided [ 18 F]trifluridine in radiochemical yields of 3% and a molar activity of 0.4 GBq/μmol. Radiotracer [ 18 F]trifluridine was used for subsequent PET imaging describing the biodistribution and clearance profile of the compound in a tumour-bearing mouse (Figure 9). Fu et al. reported a [ 18 F]trifluoromethylation reaction for the synthesis of [ 18 F]PTTP, a radiotracer that targets purinergic receptor P2X7 (P2X7R). P2X7R is an emerging molecular target for tracking and visualizing inflammation [62].
The radiosynthesis involved a Cu-difluorocarbene-mediated reaction with an aryl iodide. ClCF2CO2Me was used as the difluorocarbene source. The radiosynthesis afforded [ 18 F]PTTP in 7% radiochemical yield and a molar activity of 0.35 GBq/μmol suitable for PET imaging (Figure 10).  [60]. The Cu-difluorocarbene-mediated radiolabelling proceeded with a 6% radiochemical yield and a low molar activity of up to 0.76 GBq/µmol (Scheme 11). King et al. developed a method for the labelling of 5-trifluoromethyl-2deoxyuridine (trifluridine) with fluorine-18 [61]. The difluorocarbene-mediated reaction provided [ 18 F]trifluridine in radiochemical yields of 3% and a molar activity of 0.4 GBq/µmol. Radiotracer [ 18 F]trifluridine was used for subsequent PET imaging describing the biodistribution and clearance profile of the compound in a tumour-bearing mouse (Figure 9). Fu et al. reported a [ 18 F]trifluoromethylation reaction for the synthesis of [ 18 F]PTTP, a radiotracer that targets purinergic receptor P2X7 (P2X7R). P2X7R is an emerging molecular target for tracking and visualizing inflammation [62].
The radiosynthesis involved a Cu-difluorocarbene-mediated reaction with an aryl iodide. ClCF 2 CO 2 Me was used as the difluorocarbene source. The radiosynthesis afforded  In 2020, Kee et al. published an elegant method for the radiolabelling of unprotected native aromatic residues in peptides [26]. The novelty of this method lies in the generation of [ 18 F]trifluoromethane sulfinate, an efficient and proven trifluoromethylating agent. The reaction was conducted via a multi-component assembly involving a difluorocarbene  In 2020, Kee et al. published an elegant method for the radiolabelling of unprotected native aromatic residues in peptides [26]. The novelty of this method lies in the generation of [ 18 F]trifluoromethane sulfinate, an efficient and proven trifluoromethylating agent. The reaction was conducted via a multi-component assembly involving a difluorocarbene  In 2020, Kee et al. published an elegant method for the radiolabelling of unprotected native aromatic residues in peptides [26]. The novelty of this method lies in the generation of [ 18 F]trifluoromethane sulfinate, an efficient and proven trifluoromethylating agent. The reaction was conducted via a multi-component assembly involving a difluorocarbene In 2020, Kee et al. published an elegant method for the radiolabelling of unprotected native aromatic residues in peptides [26]. The novelty of this method lies in the generation of [ 18 F]trifluoromethane sulfinate, an efficient and proven trifluoromethylating agent. The reaction was conducted via a multi-component assembly involving a difluorocarbene source (PDFA), [ 18 F]fluoride and an SO 2 source. The ammonium salt of [ 18 F]trifluoromethane sulfinate was used for radiolabelling with n.c.a. [ 18 F]fluoride in the presence of a Fe(III) salt and TBHP as a stoichiometric oxidant. The chemistry tolerates water, which enables its direct use for the radiolabelling of peptides under aqueous conditions. The described method was particularly effective for the introduction of [ 18 F]CF 3 groups into tyrosine and tryptophan residues of the peptide backbone. A successful application of the methodology yielded octreotide[Trp(2-CF 2 18 F)] with a molar activity of 0.3 GBq/µmol (Scheme 12).  [31]. Electrophilic radiofluorinations using Selectfluor are stable towards water without the risk of inactivation. This carrier-added radiofluorination process yielded the target compounds in radiochemical yields greater than 95% and molar activities of 3.3 GBq/μmol.  [31]. Electrophilic radiofluorinations using Selectfluor are stable towards water without the risk of inactivation. This carrier-added radiofluorination process yielded the target compounds in radiochemical yields greater than 95% and molar activities of 3.3 GBq/µmol.  [31]. Electrophilic radiofluorinations using Selectfluor are stable towards water without the risk of inactivation. This carrier-added radiofluorination process yielded the target compounds in radiochemical yields greater than 95% and molar activities of 3.3 GBq/μmol.   [63]. The process uses a commercially available Jacobsen-type precatalyst and iodosobenzene as a stoichiometric oxidant. The authors reported good radiochemical yields and a molar activity in the range of 4 GBq/µmol. Among the earliest reports on the synthesis of [ 18 [30]. The group was successful in synthesizing and using the new reagent for the radiolabelling of a variety of cysteine residues with a wide range of radiochemical yields and molar activities. The installation of [ 18 F]CF3 group was accomplished via a nucleophilic halogen exchange reaction on a bromodifluoromethyl arylsulfide (Scheme 15). The molar activity of the reagent pre-cyclization (2-(trifluoromethylthio) biphenyl) was 0.08 GBq/μmol. The only reported molar activity of the final products was of the cyclic peptide cRADfC([ 18 F]CF3), 0.15 GBq/μmol with a radiochemical yield of 20%.

[18F]CF 3 -Heteroatom Compounds
Verhoog et al. utilized [ 18 F]CF 3 Umemoto reagents to facilitate the electrophilic [ 18 F]trifluormethylation of unmodified cysteine residues [30]. The group was successful in synthesizing and using the new reagent for the radiolabelling of a variety of cysteine residues with a wide range of radiochemical yields and molar activities. The installation of [ 18 F]CF 3 group was accomplished via a nucleophilic halogen exchange reaction on a bromodifluoromethyl arylsulfide (Scheme 15). The molar activity of the reagent precyclization (2-(trifluoromethylthio) biphenyl) was 0.08 GBq/µmol. The only reported molar activity of the final products was of the cyclic peptide cRADfC([ 18 [30]. The group was successful in synthesizing and using the new reagent for the radiolabelling of a variety of cysteine residues with a wide range of radiochemical yields and molar activities. The installation of [ 18 F]CF3 group was accomplished via a nucleophilic halogen exchange reaction on a bromodifluoromethyl arylsulfide (Scheme 15). The molar activity of the reagent pre-cyclization (2-(trifluoromethylthio) biphenyl) was 0.08 GBq/μmol. The only reported molar activity of the final products was of the cyclic peptide cRADfC([ 18 F]CF3), 0.15 GBq/μmol with a radiochemical yield of 20%.    [27]. The described metho opened the path for trifluoromethylthiolation reactions starting from several commer cially available alkyl halides (Scheme 17).
Liang group followed up with a modified method for using PDFA in the context o transition metal-mediated [ 18 F]trifluoromethylthiolation of α-bromo carbonyl compound [66]. These new studies gave a clearer picture of the mechanism associated with the syn thesis pathway. This rapid method afforded compounds at the low molar activity of 0.07 GBq/μmol. [ 18 F]Trifluoromethyl cysteine as an amino acid mimicking radiotracer for imagin glioma was explored by Tang et al. [67]. The [ 18 F]trifluoromethylthiolation reaction wa achieved by employing PDFA and S8, on cyclic sulfamidates. The one-step assembly o [ 18 F]trifluoromethyl cysteine represents an interesting method for the synthesis of the flu orine-18-labelled amino acid mimics that was tested for PET imaging in C6 glioma-bear ing mice (Figure 11).  [27]. The described method opened the path for trifluoromethylthiolation reactions starting from several commercially available alkyl halides (Scheme 17).
Liang group followed up with a modified method for using PDFA in the context of transition metal-mediated [ 18 F]trifluoromethylthiolation of α-bromo carbonyl compounds [66]. These new studies gave a clearer picture of the mechanism associated with the synthesis pathway. This rapid method afforded compounds at the low molar activity of 0.074 GBq/μmol. [ 18 F]Trifluoromethyl cysteine as an amino acid mimicking radiotracer for imaging glioma was explored by Tang et al. [67]. The [ 18 F]trifluoromethylthiolation reaction was achieved by employing PDFA and S8, on cyclic sulfamidates. The one-step assembly of [ 18 F]trifluoromethyl cysteine represents an interesting method for the synthesis of the fluorine-18-labelled amino acid mimics that was tested for PET imaging in C6 glioma-bearing mice (Figure 11). Liang group followed up with a modified method for using PDFA in the context of transition metal-mediated [ 18 F]trifluoromethylthiolation of α-bromo carbonyl compounds [66]. These new studies gave a clearer picture of the mechanism associated with the synthesis pathway. This rapid method afforded compounds at the low molar activity of 0.074 GBq/µmol.
[ 18 F]Trifluoromethyl cysteine as an amino acid mimicking radiotracer for imaging glioma was explored by Tang et al. [67]. The [ 18 F]trifluoromethylthiolation reaction was achieved by employing PDFA and S 8 , on cyclic sulfamidates. The one-step assembly of [ 18 F]trifluoromethyl cysteine represents an interesting method for the synthesis of the fluorine-18-labelled amino acid mimics that was tested for PET imaging in C6 gliomabearing mice (Figure 11)

Summary and Conclusions
[ 18 F]Trifluoromethylation chemistry is a rapidly evolving area in radiopharmaceutical sciences for PET imaging. Considered a challenging and exotic subset of 18 F radiofluorination reactions in the past, [ 18 F]trifluoromethylations are now becoming a more established radiofluorination technique in radiopharmaceutical chemistry. The increasing presence of CF3 groups in many drug molecules and the continuous growth of PET as a cutting-edge functional imaging technique in biomedical and clinical research are the primary drivers for developing novel [ 18 F]trifluoromethylation chemistry strategies. In comparison with traditional radiofluorinations, there are unique challenges associated with [ 18 F]trifluoromethylations. A major challenge is the low molar activity of <5 GBq/μmol of many [ 18 F]CF3 group-containing PET radiotracers. Traditional radiofluorination methodologies with n.c.a. [ 18 F]fluoride typically achieve high molar activities >40 GBq/μmol. There is only a handful of reported [ 18 F]trifluoromethylation methods that can achieve that. The low molar activity challenge of many [ 18 F]CF3 group-containing PET radiotracers is observed with the majority of the reported methods. However, some of the discussed four major synthesis strategies for the installation of a [ 18 F]CF3 group could offer opportunities to obtain radiotracers at a high molar activity.
One-pot isotopic exchange reactions are quite limited when it comes to producing radiotracers at higher molar activities due to the inherent challenge of removing the chemically and physically identical labelling precursor. A two-step isotopic exchange radiofluorination involving an isolable metal complex yielded the radiotracer at a low molar activity of 0.3 GBq/μmol. Nucleophilic [ 18 F]trifluoromethylation chemistry starting from n.c.a [ 18 F]fluoride is the most widely explored and successful method for [ 18 F]trifluoromethylation reactions. However, the synthesis of difluoromethyl groups with a suitable leaving group can pose a special synthetic challenge associated with this strategy. Both aliphatic and aromatic nucleophilic [ 18 F]trifluoromethylation reactions provide radiotracers in reasonable radiochemical yields and high molar activities in the range of 30-120 GBq/μmol. The obtained favourable radiochemical yields and high molar activities predestine nucleophilic [

Summary and Conclusions
[ 18 F]Trifluoromethylation chemistry is a rapidly evolving area in radiopharmaceutical sciences for PET imaging. Considered a challenging and exotic subset of 18 F radiofluorination reactions in the past, [ 18 F]trifluoromethylations are now becoming a more established radiofluorination technique in radiopharmaceutical chemistry. The increasing presence of CF 3 groups in many drug molecules and the continuous growth of PET as a cutting-edge functional imaging technique in biomedical and clinical research are the primary drivers for developing novel [ 18 F]trifluoromethylation chemistry strategies. In comparison with traditional radiofluorinations, there are unique challenges associated with [ 18 F]trifluoromethylations. A major challenge is the low molar activity of <5 GBq/µmol of many [ 18 F]CF 3 group-containing PET radiotracers. Traditional radiofluorination methodologies with n.c.a. [ 18 F]fluoride typically achieve high molar activities >40 GBq/µmol. There is only a handful of reported [ 18 F]trifluoromethylation methods that can achieve that. The low molar activity challenge of many [ 18 F]CF 3 group-containing PET radiotracers is observed with the majority of the reported methods. However, some of the discussed four major synthesis strategies for the installation of a [ 18 F]CF 3 group could offer opportunities to obtain radiotracers at a high molar activity.
One-pot isotopic exchange reactions are quite limited when it comes to producing radiotracers at higher molar activities due to the inherent challenge of removing the chemically and physically identical labelling precursor. A two-step isotopic exchange radiofluorination involving an isolable metal complex yielded the radiotracer at a low molar activity of 0.3 GBq/µmol. Nucleophilic [ 18 F]trifluoromethylation chemistry starting from n.c.a [ 18 F]fluoride is the most widely explored and successful method for [ 18 F]trifluoromethylation reactions. However, the synthesis of difluoromethyl groups with a suitable leaving group can pose a special synthetic challenge associated with this strategy. Both aliphatic and aromatic nucleophilic [ 18 F]trifluoromethylation reactions provide radiotracers in reasonable radiochemical yields and high molar activities in the range of 30-120 GBq/µmol. The obtained favourable radiochemical yields and high molar activities predestine nucleophilic [ 18  F]fluorinating reagents proceed with moderate radiochemical yields and low molar activities. The increasing number of drugs containing trifluoromethoxy (CF 3 O-) and trifluoromethylthio (CF 3 S-) groups also require innovative radiochemistry for the synthesis of respective fluorine-18-labelled radiotracers. The increasing number of innovative [ 18 F]trifluoromethylation chemistries is becoming a significant driving force for the development of novel fluorine-18-labelled imaging agents. Although the reports discussed in this review demonstrate the considerable progress in [ 18 F]trifluoromethylation chemistry over the last decade, fluorine-18 radiochemistry is still heavily dominated by synthesis strategies yielding mono-radiofluorinated radiotracers (aryl-18 F and alkyl-18 F) by using traditional nucleophilic and electrophilic radiofluorinations. Thus, further investments in the development of novel [ 18 F]trifluoromethylation chemistry are required. The development of innovative [ 18 F]trifluoromethylation chemistry will significantly impact future radiopharmaceutical chemistry and preclinical and clinical molecular imaging.