Fluorination

Living Edition
| Editors: Jinbo Hu, Teruo Umemoto

Fluorination of Diaryliodonium Salts for Preparing Aryl Fluorides

  • Wenchao QuEmail author
  • Ximin Li
Living reference work entry

Later version available View entry history

DOI: https://doi.org/10.1007/978-981-10-1855-8_12-1

Introduction

Diaryliodonium salts are a class of hypervalent iodine (III) reagents that have been known since 1894 (Fig. 1) [14]. Although the term “salt” is commonly used, the T-shaped form shown by X-ray structures illustrates that these iodine (III) compounds have more covalent bond characteristics. “Diaryl-λ3-iodanes” is the nomenclature from IUPAC for this type of molecule [25, 36, 45, 46, 48]. Diaryliodonium salts with tetrafluoroborates, triflates, and tosylates as counter anions are frequently used due to their good solubility in general organic solvents, as well as the lack of nucleophilicity of these anions, compared with halide anion salts.
Fig. 1

General structure and T-shaped form of diaryliodonium salts

Because of their highly electron-deficient nature and excellent leaving-group ability, diaryliodonium salts are widely used as arylating agents in organic synthesis to assemble complex aromatic compounds, such as α-arylation of carbonyl compounds; metal catalyzed or metal-free cross-coupling reactions with arenes, heteroarenes, alkynes, alkenes, and many other substrates; and arylation with heteroatom nucleophiles (O, S, Se, N, F, etc.) [25, 36, 45, 46]. In the past two decades, introduction of a fluorine atom into the aromatic (or heteroaromatic) ring by employing aryliodonium salts has become an active research area (Fig. 2) [4], especially with the high demand for the development of fluorine-18 (18F, radioactive isotope of fluorine-19, half-life t1/2 = 110 min) radiolabeled organic molecules as radiopharmaceuticals and molecular imaging agents using the technique of Positron Emission Tomography (PET) [31, 38, 46]. This chapter will focus on fluorination and radiofluorination of diaryliodonium salts in preparation of fluorinated and radiofluorinated aromatic compounds and their application as clinically interesting radiotracers for PET imaging.
Fig. 2

Fluorination of diaryliodonium salts for preparing aryl fluoride

Synthesis of Diaryliodonium Salts

There are numerous synthetic routes currently available for the preparation of diaryliodonium salts. One common strategy involves two to three steps, with initial oxidation using inorganic oxidants under acidic conditions to convert aryl iodides to aryliodine (III) compounds. Then the aryliodine (III) intermediate, such as commonly prepared hydroxy(tosyloxy)iodoarene (ArI(OH)OTs, Koser’s reagent) or (diacetoxyiodo)arene (ArI(OAc)2), undergoes ligand exchange with an arene or a nucleophilic arylating reagent (e.g., arylborates, arylstannanes, or arylsilanes) to obtain the diaryliodonium salt. The subsequent anion exchange step may be necessary (Fig. 3). Shorter synthetic routes are performed for the preparation of symmetric iodonium salts using inorganic iodine(III) reagents. A detailed summarization regarding the synthesis of diaryliodonium salts was well presented by Merritt and Olofsson in 2009 [25].
Fig. 3

Synthesis of diaryliodonium salts including steps of oxidation, ligand exchange and anion exchange

Recently, one-pot synthetic methods from arenes and iodoarenes have greatly simplified the preparation of diaryliodonium salts by eliminating the isolation of the iodine (III) intermediate. For example, symmetric and asymmetric diaryliodonium triflates, including N-heterocyclic iodonium triflates can be synthesized from both electron-deficient and electron-rich substrates (Fig. 4ac) [3]. The regiospecific route employing arylboronic acids provides a complimentary synthetic pathway for other substitution patterns (Fig. 4d) [2]. Other oxidants, such as potassium persulfate (K2S2O8) [16], potassium peroxymonosulfate (Oxone) [36], and urea-hydrogen peroxide [32], have also been utilized in the one-pot synthetic method. In addition, different anions, such as triflates, tosylates, tetrafluoroborates, and trifluoroacetates [5], have been incorporated into diaryliodonium salts as well.
Fig. 4

One-pot synthetic methods for diaryliodonium triflates and tetrafluoroborates

Fluorination of Diaryliodonium Salts for Preparing Aryl Fluorides

The Discovery and Development of the Synthesis of Fluoroarenes from Diaryliodonium Salts

To the best of our knowledge, the first synthesis of aryl fluorides by nucleophilic fluorination of diaryliodonium salts was reported by Michael Van Der Puy in 1982. After mixing diaryliodonium salts and KF and heating the mixture to a molten state, both electron-poor and electron-rich fluoroarenes were obtained with varying yields (Fig. 5) [39].
Fig. 5

First report of nucleophilic fluorination of diaryliodonium salts

Over a decade later, Pike and Aigbirhio reported their investigation of radiofluorination of diaryliodonium salts to obtain [18F]fluoroarenes in 1995 (Fig. 6) [30]. Using K[18F] and K222 in acetonitrile, para substituted [18F]fluorobenzenes (with hydrogen, chloro, methyl, bromo and methoxy groups) were synthesized with radiochemical yields ranging from 7.5 to 88% (RCY, decay-corrected). In this study, fluoroarenes bearing more electron withdrawing groups were the major fluorination products when asymmetrical diaryliodonium salts were used. These results illustrated the access to fluorination of electron-rich arenes which is not easily achievable by the traditional SNAr reaction. This study also opened the door for using diaryliodonium salts as radiofluorination substrates for synthesizing PET imaging radiotracers.
Fig. 6

First report of radiofluorination of diaryliodonium salts

After that, several research groups made significant progress in developing aromatic fluorination and radiofluorination methodologies using diaryliodonium salts as precursors. In 2007, Carroll and colleagues reported the radiosynthesis of 3-[18F]fluoropyridine and 3-[18F]fluoroquinoline. By using 4-methoxyphenyl as the “dummy” ligand, both desired products were formed in good yields (Fig. 7a) [7]. In the same year, they found that the addition of radical scavengers, such as TEMPO, could improve the reproducibility of the fluorination of diaryliodonium salts (Fig. 7b) [6]. Coenen and his co-workers conducted a systematic investigation of radiofluorination methods based on using aryl(2-thienyl)iodonium salts to synthesize various [18F]fluoroarenes with non-carrier- added (NCA) [18F]fluoride (Fig. 7c) [33].
Fig. 7

Fluorination methodologies using diaryliodonium salts reported in 2007

DiMagno et al. found that using a low polarity aromatic solvent (i.e., benzene and toluene) to replace the polar aprotic solvent, CH3CN, combined with removing the inorganic salts by filtration, can largely improve the aromatic fluorination yields of diaryliodonium salts [40]. Resorting to “ortho-effect” directing power (the nucleophile favors attack at the ipso carbon in the equatorial ring of the trigonal bipyramidal iodonium complex) [22, 44] and utilizing microreactor technology, Chun et al. systematically investigated the reaction parameters of radiofluorination of diaryliodonium salts. The results provide a guideline for designing diaryliodonium salts for synthesizing ortho-substituted [18F]fluoroarenes [8]. In their following report, they further investigated the radiosynthesis of meta-substituted [18F]fluoroarenes from corresponding diaryliodonium tosylates using the microreactor technique [9].

In 2013, the Sanford group reported a copper-catalyzed nucleophilic fluorination of asymmetric diaryliodonium salts [18]. In contrast to the ortho-effect, this study showed that the less sterically hindered arene was fluorinated preferentially when using a bulky mesityl group as an auxiliary (Fig. 8). A Cu(I)/Cu(III) catalytic cycle was proposed based on DFT calculations. A year later, the Scott group and the Sanford group implemented this methodology in radiosynthesis using NCA [18F]fluoride and obtained protected versions of the PET tracers 4-[18F] fluorophenylalanine ([18F]FPA) and 6-[18F]- fluoro-L-DOPA ([18F]FDOPA) (Fig. 9) [17].
Fig. 8

Cu-catalyzed fluorination of diaryliodonium salts with KF

Fig. 9

Cu-catalyzed radiofluorination of diaryliodonium salts with NCA [18F]fluoride

Iodonium ylides are another class of hypervalent iodine (III) reagents that have also been used as precursors for aromatic radiofluorination to synthesize PET tracers [35]. A recent example is the development of spirocyclic iodonium ylides as radiofluorination precursors reported by Vasdev and Liang et al. (Fig. 10) [34]. Further discussion of this type of research is beyond the limit of this chapter and related work can be found in a recent review presented by Gouverneur et al. [31].
Fig. 10

Radiofluorination of spirocyclic iodonium ylides

The Applications of Synthesizing [18F]Fluoroarenes from Diaryliodonium Salts

Synthesis of Functionalized [18F]Fluoroarenes as Radiosynthons

Wüst et al. synthesized 4-[18F]fluoroiodobenzene using NCA [18F]fluoride and symmetric 4,4′-diiododiaryliodonium salts, and then used it as radiosynthon for palladium catalyzed Sonogashira reactions [43]. Two years later, they successfully synthesized two 18F–labelled cyclooxygenase-2 (COX-2) inhibitors using 4-[18F]fluoroiodobenzene as the radiosynthon (Fig. 11) [42]. Ermert et al. reported the synthesis of 4-[18F]fluorobromobenzene and 4-[18F]fluorophenol as potential radiosynthons using the corresponding symmetric diaryliodonium salts [13, 15].
Fig. 11

Synthesis of 18F–labelled COX-2 inhibitors using 4-[18F]fluoroiodobenzene as radiosynthon

To further broaden the scope of functionalized [18F]fluoroarenes, Chun and Pike synthesized a series of [18F]fluoroarenes bearing functional groups by direct radiofluorination of the corresponding diaryliodonium salts. Various functional groups, including azide, aldehyde, ester, ketone, chloromethyl, and even bromomethyl, all remained intact after a high-temperature radiofluorination process and were suitable for further transformation (Fig. 12) [10, 11]. The synthesis of meta-[18F]fluorobenzaldehyde using phenyl(3-formylphenyl)iodonium salts was also reported by Griffiths and his colleagues [1].
Fig. 12

Synthesis of [18F]fluoroiodobenzene radiosynthons bearing various functional groups

Early in 2017, DiMagno’s group reported a detailed investigation of the radiofluorination of diaryliodonium salts derived from anilines. This study disclosed that the installation of an electron-withdrawing group for the nitrogen group is the key to achieve successful radiofluorination with good yields. The practical applicability of this methodology to the production of 18F–labelled PET tracers was demonstrated by successful synthesis of the protected [18F]flutemetamol (Fig. 13) [24].
Fig. 13

Synthesis of protected [18F]flutemetamol using a diaryliodonium salt

Synthesis of Clinically Interesting PET Tracers from Diaryliodonium Salts

Many applications of diaryliodonium salts in synthesis of clinically interesting PET tracers with NCA [18F]fluoride have been reported in recent years. For example, 6-18F-fluoro-3,4-dihydroxy-L-phenylalanine ([18F]FDOPA) has been in widespread clinical use for neurologic imaging as well as for detecting neuroendocrine tumors and monitoring their progression. In the early days, [18F]FDOPA was synthesized by an electrophilic demetalation aromatic fluorination from [18F]F2 and a corresponding organotin precursor (Fig. 14a) [26]. The difficulty of producing a large amount of this radiotracer and the low specific activity limited the clinical application of [18F]FDOPA.
Fig. 14

Three reported methods for clinical production of [18F]FDOPA

To circumvent the above disadvantages, much effort has been dedicated to developing a nucleophilic fluorination protocol for the production of [18F]FDOPA. In 2013, Libert and his collaborators reported a curie level production of [18F]FDOPA using the NCA [18F]fluoride-based nucleophilic fluorination method (Fig. 14b) [23]. This production process is composed of five reaction steps and two purification operations in total. Two years later, Kuik et al. described a production of [18F]FDOPA using a diaryliodonium salt NCA radiofluorination approach. The whole process only involves two reactions, nucleophilic fluorination and acidic deprotection, and can be easily adapted for commercially available automated production modules (Fig. 14c) [21]. In the same year, Edwards et al. also reported similar research work using a diaryliodonium salt NCA radiofluorination method to synthesize [18F]FDOPA [12].

In 2016, Warnier et al. presented a one-step, fully automated, and cGMP compliant production of [18F]UCB-H, a PET tracer for synaptic vesicle glycoprotein 2A (SV2A), which relies on the corresponding diaryliodonium salt. The desired product was produced at a curie level with high specific activity in a routine manner. This three-step operation approach demonstrated the advantage of using the diaryliodonium salt as a precursor with a shortened process, faster reaction time, and higher yield compared with the nitro precursor–based six-step production method (Fig. 15) [41].
Fig. 15

Comparison of two radiosynthesis methods of [18F]UCB-H

In the past years, researchers from different institutions worldwide have demonstrated the accessibility of many other clinically desired 18F–labelled PET tracers from the NCA radiofluorination of corresponding diaryliodonium salt precursors, such as [18F]DAA1106 [47], mGluR5 PET tracers [37], [18F]flumazenil [27], 4-[18F]FMHPG [19], 6-[18F]fluorodopamine ([18]FDA) [29], [18F]fluoropalonosetron [28], and 4–4-(3-[18F]fluorophenethoxy)pyrimidine [20] (Fig. 16). In addition, the copper catalyzed aromatic radiofluorination method using diaryliodonium salts as precursors also proved the potential for practical usage. In 2015, Zlatopolskiy et al. demonstrated successful access to three clinically interesting tracers, [18]FDA, [18F]FPhe and [18F]DAA1106, using copper-catalyzed radiofluorination of (mesityl)(aryl)iodonium salts [49].
Fig. 16

Clinically desired18F–labelled PET tracers from diaryliodonium salts

Conclusion and Future Directions

The application of diaryliodonium salts in fluorine chemistry has been explored extensively in the past two decades. Many simplified and efficient synthetic methods have been developed to access structurally diverse diaryliodonium salts. Challenging and previously unavailable fluorinated products were achieved by nucleophilic fluorination and radiofluorination. Diaryliodonium salts as radiofluorination precursors have been widely explored and successfully applied to produce clinically and preclinically interesting PET tracers.

Further development of synthetic methods for complex diaryliodonium salts, which are not tolerant to strong oxidation conditions or strong Lewis or Brønsted acids, still remains as a challenge for researchers. In addition, fine-tuning the reaction parameters and adapting diaryliodonium salt-based radiofluorination methods to the cGMP environment for automated, large scale, and routine production of desired PET tracers remains an ongoing project.

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© Springer Nature Singapore Pte Ltd. 2018

Authors and Affiliations

  1. 1.Citigroup of Biomedical Imaging CenterWeill Cornell MedicineNew YorkUSA
  2. 2.Avid RadiopharmaceuticalsPhiladelphiaUSA