Metal-free ¹⁸F-labeling of aryl-CF₂H via nucleophilic radiofluorination and oxidative C–H activation

Abstract

A metal-free and selective method to form [¹⁸F]aryl-CF₂H through nucleophilic radiofluorination of benzyl (pseudo)halides and oxidative C–H activation of benzylic C–H bonds has been developed. The method is operationally simple and tolerates a variety of electron-neutral/deficient arenes and heteroarenes.

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Fluorine bearing functionalities are found in a wide range of materials and biologically active molecules.¹ Incorporation of fluorine-18 radionuclide into these molecules facilitates the application of noninvasive positron emission tomography (PET) imaging in quantifying biochemical and pharmacological processes. As a result, continuous efforts have been made to develop novel radiofluorination methods that provide a diverse range of ¹⁸F-labeled vectors to support molecular imaging research and drug discovery.² Isosteric replacement of phenolic hydroxyl group with an aromatic difluoromethyl functionality (Ar-CF₂H) has been demonstrated as a useful medicinal chemistry approach, leading to the discovery of bioactive molecules, including thiazopyr, fluxapyroxad, deracoxib, ZSTK474 etc. (Scheme 1).^1d,3

Scheme 1 Aryl-CF₂H containing bioactive molecules.

The success of this substitution has been attributed to the similar acidic and isopolar characteristics of CF₂H and OH. Moreover, CF₂H functionality features hydrogen-bond-donating capability comparable to that of OH and NH groups, but much reduced hydrophilicity.⁴ Compared with recent advances in the preparation of non-radioactive Ar-CF₂H,⁵¹⁸F-labeling of this group has not yet been fully exploited. Initial methods relied on the silver(I)-mediated fluorodecarboxylation of α-fluorarylacetic acids with [¹⁸F]Selectfluor bis(triflate), an electrophilic fluorinating reagent derived from [¹⁸F]F₂.⁶ An alternative ¹⁸F-incorporation method employed a silver(I)-mediated halogen exchange strategy with readily available [¹⁸F]fluoride under mild reaction conditions, but required aryl-CHFCl precursors prepared by multi-step synthesis.⁷ Our recent work disclosed a more practical method, starting from easily accessible aryl (pseudo)halides, via activation of a benzoyl auxiliary, to form [¹⁸F]Ar-CF₂H functionality with an excellent substrate scope.⁸ However, to date, syntheses of [¹⁸F]aryl-CF₂H have been restricted to low-to-moderate specific activity radiotracers, which may have limited their application in PET imaging studies, particularly for low density biological targets.

Recently we demonstrated the capability of silver-catalyzed oxidative activation of benzylic C–H bonds of aryl-CH₃ for the synthesis of aryl-CF₂H, which was further adapted to transition-metal-free conditions.⁹ Inspired by these discoveries, we speculated that a similar protocol could be applicable to the transformation of benzylic C–H to C–¹⁸F bond formation in the context of [¹⁸F]aryl-CFH₂ intermediates, which could be easily obtained via nucleophilic radiofluorination of benzyl-(pseudo)halides with [¹⁸F]fluoride (Scheme 2). Herein, we report a general and metal-free method to radiolabel [¹⁸F]aryl-CF₂H groups via a sequential nucleophilic radiofluorination of benzyl (pseudo)halides, followed by oxidative C–H activation.

Scheme 2 Radiolabeling of [¹⁸F]aryl-CF₂H.

The feasibility of our method was experimentally confirmed, and further optimized by the radiosynthesis of 4-([¹⁸F]difluoromethyl)-1,1′-biphenyl ([¹⁸F]3a). An illustrative workflow is depicted in Scheme 3. Near quantitative radiochemical conversion (RCC; 98 ± 2%, n > 10) of 1a (8.09 μmol) to [¹⁸F]2a was achieved in 10 min in MeCN with tetraethylammonium bicarbonate (TEAB, 10.4 μmol) at 130 °C (Table 1). Under these conditions, the milder base used herein, compared with tetra-n-butylammonium hydroxide (TBAH),¹⁰ resulted in a 2-fold increase in RCC. Isolated [¹⁸F]2a was subsequently subjected to oxidative reaction conditions to install the benzyl ¹⁸F-fluoride. This step was optimized by varying the temperature and stoichiometry of reagents including AgNO₃, Na₂S₂O₈, and Selectfluor (Table 2). Under the initial optimal reaction conditions (Table 2, entry 3, RCC = 41 ± 6%), removal of AgNO₃ gave superior RCC (50 ± 5%, entry 7), indicating that such C–H activation can be achieved under transition-metal-free conditions.^9b Finally, compound [¹⁸F]3a was obtained in 50 ± 5% RCC with Na₂S₂O₈ (33.6 μmol) and Selectfluor (56.5 μmol) at 120 °C for 10 min (Table 2, entry 7).

Scheme 3 Synthesis of [¹⁸F]2a and [¹⁸F]3a. Synthesis of [¹⁸F]2a: (a) trap and release ¹⁸F⁻ into a V-vial via a solution of TEAB in 1 mL MeCN/H₂O (v/v 7 : 3); (b) azeotropic drying with anhydrous MeCN (3 × 1 mL, 3 min for each cycle); (c) addition of 1a, MeCN, 130 °C, 10 min; (d) dilution and subsequent purification of the reaction mixture (∼12 min); (e) concentration of the eluent via a C-18 cartridge (∼2 min); (f) release of [¹⁸F]2avia 1 mL MeCN (∼1 min). Synthesis of [¹⁸F]3a: (g) addition of Na₂S₂O₈, Selectfluor and EtOH (∼1 min); (h) 120 °C, 10 min; (i) dilution and subsequent purification of the reaction mixture (∼12 min).

Table 1 Optimization for the synthesis of 4-([¹⁸F]fluoromethyl)-1,1′-biphenyl ([¹⁸F]2a)

Entry	1a (mg)	Base (mg)	Time (min)	Temp. (°C)	2a RCC^a (%)
a Determined by radio-TLC integration.
1	2.0	TBAH (3.1)	10	130	49 ± 3 (n = 2)
2	2.0	TEAB (0.5)	10	130	50 ± 2 (n = 2)
3	2.0	TEAB (1.0)	10	130	85 ± 2 (n = 2)
4	2.0	TEAB (2.0)	10	130	98 ± 2 (n = 10)

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Table 2 Optimization for the preparation of 4-([¹⁸F]difluoromethyl)-1,1′-biphenyl ([¹⁸F]3a)

Entry	AgNO3 (mg)	Na2S2O8 (mg)	Selectfluor (mg)	Time (min)	Temp. (°C)	RCC^a (%)
a Determined by radio-HPLC integration of product peaks, relative to [^18F]fluoride and ^18F-side products.
1	8.0	4.0	20.0	10	120	20 ± 2 (n = 2)
2	4.0	4.0	20.0	10	120	38 ± 5 (n = 2)
3	4.0	8.0	20.0	10	120	41 ± 6 (n = 2)
4	4.0	12.0	20.0	10	120	23 ± 3 (n = 2)
5	4.0	8.0	5.0	10	120	0 (n = 2)
6	4.0	8.0	15.0	10	120	33 ± 3 (n = 2)
7	0.0	8.0	20.0	10	120	50 ± 5 (n = 2)

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The scope of this methodology was then explored with more challenging substrates (Scheme 4). Under the optimized reaction conditions, all ¹⁸F-monofluorinated products were synthesized quantitatively, except [¹⁸F]2e (85 ± 5%) and [¹⁸F]2f (70 ± 3%). The isolated ¹⁸F-labeled compounds ([¹⁸F]2b–2i) were then subjected to the second step under optimized reaction conditions as described above. The final products [¹⁸F]3b–3h were obtained with moderate-to-good RCCs, ranging from 10% to 45%. As shown in Scheme 4, para-substituted electron-deficient ketone substrates [¹⁸F]2b and [¹⁸F]2c gave 10% and 15% RCCs, respectively. Ester substitution in the para-position of [¹⁸F]2d also yielded 12% RCC. Substrate [¹⁸F]2e exemplified the applicability of this method for a multi-substituted, sterically hindered compound with 28% RCC. In addition, a bromide motif in the product [¹⁸F]3e could allow further coupling reactions with various counterparts to provide more complex molecules. This method was also found to give good conversion for substrate [¹⁸F]2f (RCC = 45%) possessing a primary amide. The success of 2-arylpyridine substrate [¹⁸F]2g showed the compatibility of this labeling strategy for N-heteroatom bearing compounds. It is also noteworthy that substrate [¹⁸F]2h demonstrated the introduction of a second fluorine atom onto a tertiary carbon. Furthermore, product [¹⁸F]3h, bearing a (1,1-difluoroethyl)benzene fragment, is a key structural motif in a series of potent kidney urea transporter inhibitors,¹¹ analogous to the compound shown in Scheme 1. The electron-rich arene 1i led to no desired product [¹⁸F]3i, as [¹⁸F]2i immediately decomposed to [¹⁸F]fluoride under our standard oxidative conditions as well as alternative oxidation reagents, for example 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo [2.2.2] octane bis(hexafluorophosphate) (F-TEDA-PF₆).¹² It is possible that the increased reactivity brought by the electron donating groups makes [¹⁸F]2i more vulnerable to the fluorinating reagents and/or radical defluorination.

Scheme 4 Scope of oxidative benzylic C–H activation in preparing the [¹⁸F]aryl-CF₂H products.

A plausible reaction mechanism is shown in Scheme 5. Homolytic cleavage of a peroxydisulfate anion produces two sulfate radical anions at elevated temperature, which oxidizes the benzylic C–H bond of [¹⁸F]aryl-CFH₂ (A) to a benzylic radical (B). Then, Selectfluor reacts through a radical mechanism,¹³ providing a fluorine radical for bond formation with the benzylic radical (B) to produce the desired [¹⁸F]aryl-CF₂H (C).

Scheme 5 Proposed mechanism for the radiosynthesis of [¹⁸F]aryl-CF₂H.

Specific activity is a very critical parameter which should be considered in both radiolabeling methods and PET imaging studies.¹⁴ Obtaining high specific activity is still a challenge for ¹⁸F-labeling of many multi-fluorinated compounds.^6a,7,8 As shown in Scheme 6, we measured the specific activity of the aryl-CF₂H product [¹⁸F]3a by our protocol (see the ESI† for details). Briefly, compound [¹⁸F]2a was synthesized and isolated using a commercial automated radiofluorination unit (GE TRACERlab™ FX_FN; Fig. S1, ESI†), starting from 1.7 GBq of [¹⁸F]fluoride, in 61% radiochemical yield (non-decay corrected) with >99% radiochemical purity and 51.8 GBq μmol⁻¹ specific activity at the end-of-synthesis (∼60 min synthesis time from [¹⁸F]fluoride). The isolated intermediate [¹⁸F]2a was formulated and eluted from a C-18 cartridge with 1 mL of MeCN. It was diluted with 1 mL of sterile water, from which 0.4 mL was used for the second step. Under the optimized conditions, the mixture was heated for 10 min, cooled and diluted using a mobile phase. The reaction mixture was injected into semi-preparative HPLC for purification. The purified [¹⁸F]3a was isolated in 38% radiochemical yield (non-decay corrected, based on the added activity of [¹⁸F]2a) with a specific activity of 22.2 GBq μmol⁻¹ at the end-of-synthesis (∼25 min synthesis time from the addition of [¹⁸F]2a). As shown in Table 3, the current method gives superior specific activity over other reported syntheses of [¹⁸F]aryl-CF₂H.^6a,7,8 Efforts were also made to further improve the specific activity by changing the counterion of the current fluorinating reagents to PF₆⁻ (F-TEDA-PF₆) and OTf⁻ (1-chloromethyl-4-fluoro-1,4-diazoniabicyclo [2.2.2]octane bis(trifluoromethane sulfonate; F-TEDA-OTf; see the ESI† for its preparation), though none of them gave the desired product [¹⁸F]3a. Since the tetrafluoroborate anion (BF₄⁻) has been proven to give high specific activity comparable to that of OTf⁻ in an analogous radiosynthesis of aryl-[¹⁸F]fluoride from (mesityl)(aryl)iodonium salt precursors,¹⁵ it is reasonable to postulate that there were limited exchange reactions between ¹⁹F and ¹⁸F under the present radiolabeling conditions.

Scheme 6 Specific activity determination for 4-([¹⁸F]difluoromethyl)-1,1′-biphenyl ([¹⁸F]3a).

Table 3 Summary of the radiolabeling of [¹⁸F]aryl-CF₂H

Ref.	[^18F]aryl-CF2H	Reaction type	Starting radioactivity (GBq)	Specific activity (GBq μmol^−)
a Specific activity after radioactive decay correction.
Gouverneur et al.^6a		Electrophilic	—	2.5^a
Gouverneur et al.^7		Nucleophilic	3.5	0.03
Liang, Vasdev, Ritter et al.^8		Nucleophilic	1.4	3.0
This work		Radical	1.7	22.2

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In conclusion, we have investigated a metal-free oxidative C–H activation strategy for the synthesis of [¹⁸F]aryl-CF₂H with good radiochemical yields and reasonably high specific activity. This method was successfully applied to mono- or multi-substituted electron-neutral and electron-deficient substrates, among which the amide group, N-containing heterocycle, and tertiary benzylic C–H bond were well tolerated. Further ongoing improvement resides in applying this method to electron-rich substituted arenes.

We would like to thank the staff at the radiochemistry program, Gordon Center for Medical Imaging, Nuclear Medicine and Molecular Imaging, Massachusetts General Hospital, MA, and Department of Chemistry & Chemical Biology, Northeastern University for their generous support. We also thank Drs Lee Collier and Hema Krishnan for helpful discussion. S. H. L is a recipient of NIH career development award from the National Institute on Drug Abuse (DA038000).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cc07913j

‡ Current address: Department of Chemistry, University of the West Indies, Mona, Kingston 7, Jamaica.

§ Current address: Department of Biochemistry, Microbiology and Immunology, University of Ottawa & University of Ottawa Heart Institute, 40 Ruskin Street, Ottawa, Ontario, USA.

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