Electron-withdrawing groups drive fluorine-18 isotopic exchange on aromatic rings

Chemists screening the labeling of aromatic fluorine compounds have mapped, in detail, which functional groups and substitution positions make fluorine-19/fluorine-18 isotopic exchange work efficiently. The study, published in EJNMMI Radiopharmacy and Chemistry, screened 19 fluorobenzene derivatives under standardized conditions and found that strong electron-withdrawing substituents, particularly nitro and nitrile groups, strongly promote the exchange, while electron-donating groups such as methyl and methoxy resulted in no detectable conversion at all. The positional effects were equally pronounced: ortho-substituted substrates consistently showed the highest labeling efficiency, followed by para-substituted substrates, whereas meta-substituted compounds performed worse.

Isotopic exchange is a conceptually simple approach to positron emission tomography, or PET, tracer development. In this method, radioactive fluorine-18 swaps places with a non-radioactive fluorine-19 atom already present in the target molecule, so the target compound itself serves as the labeling precursor. This circumvents the need to design and synthesize a separate precursor molecule, a step that often consumes considerable time and resources in conventional radiochemistry. The trade-off is that isotopic exchange is limited by typically low molar activity and by the requirement that the fluorinated position be sufficiently electron-deficient to undergo nucleophilic exchange.

PET is an imaging technique that uses the decay of radioactive isotopes to visualize the real-time, three-dimensional biodistribution of compounds in the body. Fluorine-18 is widely used in PET imaging because it can substitute for fluorine-19 in a molecule, producing radiolabeled analogues that are chemically and biologically indistinguishable from their non-radioactive counterparts. This is particularly useful because fluorine atoms are common in drug molecules, where they can enhance metabolic stability and bioavailability. Fluorine-18 is well suited for labeling small-molecule PET tracers, with a moderate half-life of about 109.7 minutes and a high positron decay fraction of 97 percent.

There are two fundamental ways of covalently introducing fluorine-18 into a molecule: electrophilic and nucleophilic fluorination. Electrophilic fluorination uses an electrophilic fluorine source to add fluorine to an electron-rich site, but it suffers from low specific activity, poor functional group tolerance, poor regioselectivity, high oxidizing power, corrosiveness, and limited availability of the required fluorine gas. Nucleophilic fluorination, in which a fluoride ion displaces a leaving group in a site-specific manner, requires a purpose-designed precursor and often harsh reaction conditions, which can hinder late-stage fluorination of complex molecules. Isotopic exchange, predominantly carried out under nucleophilic conditions, sidesteps the precursor problem but depends strongly on substrate reactivity.

In the new work, the researchers performed the screening in two solvents: dimethyl sulfoxide at 175 degrees Celsius and dimethylformamide at 150 degrees Celsius, the lower temperature adjusted to account for dimethylformamide’s lower boiling point. The substrate set included mono-, di-, and trifluorobenzene derivatives as well as fluorobenzenes bearing nitro, nitrile, aldehyde, methoxy, and methyl substituents in different positions. In dimethyl sulfoxide, compounds bearing nitrile, nitro, and aldehyde substituents underwent efficient labeling. Trifluorobenzene showed moderate reactivity, with radiochemical conversion reaching up to 45 percent, while difluorinated analogues exhibited only minimal exchange. In dimethylformamide, the overall conversion was significantly reduced across all compounds, and the ortho-over-para-over-meta trend observed in dimethyl sulfoxide was not preserved.

To quantify these effects, the team examined the apparent kinetics of isotopic exchange for fluorobenzonitriles, nitrofluorobenzenes, and fluorobenzaldehydes in dimethyl sulfoxide, fitting conversion-time data with nonlinear regression to a one-phase association model. Ortho- and para-nitrofluorobenzenes showed the most rapid exchange, with observed rate constants of 1.59 and 1.41 per minute and half-lives of 0.44 and 0.49 minutes respectively, and high apparent plateaus of 89 to 94 percent. Fluorobenzonitriles followed a similar pattern, with rate constants of 1.14 to 1.38 per minute at ortho and para positions, while the meta isomer was slower at 0.46 per minute and plateaued at only 68 percent. The fluorobenzaldehydes showed lower apparent plateaus overall, between 65 and 71 percent, and no exchange was detected for 3-fluorobenzaldehyde. The authors caution that because of experimental variability inherent to fluorine-18 work, these parameters describe relative trends across substrates rather than intrinsic rate constants.

The kinetic patterns are consistent with a nucleophilic aromatic substitution-type mechanism. Electron-withdrawing groups increase the electrophilicity of the aromatic ring, and ortho- and para-substitution positions allow these groups to stabilize the negatively charged Meisenheimer intermediate through resonance. Meta-substituents can withdraw electron density inductively but cannot provide resonance stabilization, leading to markedly reduced exchange efficiency. The fluorobenzaldehydes also showed qualitative signs of thermal degradation when heated in dimethyl sulfoxide, a problem not apparent in dimethylformamide, which the authors note may complicate interpretation of those results.

Because nitrile-containing compounds exchanged efficiently and the nitrile group is synthetically versatile, 2-fluorobenzonitrile was selected for molar activity experiments. Lowering the substrate amount from the 25 millimolar screening concentration down to 2.5 micromolar still yielded about 10 percent radiochemical conversion, suggesting that high molar activity was theoretically attainable. Scale-up experiments with roughly 1 gigabecquerel of starting radioactivity confirmed the feasibility: using 3.50 plus or minus 0.25 gigabecquerel of tetraethylammonium fluoride, the reaction yielded 166.67 plus or minus 78.36 megabecquerel of isolated 2-fluorobenzonitrile, corresponding to a molar activity of 121.39 plus or minus 36.50 gigabecquerel per micromole. The radiochemical yield was 5.84 plus or minus 2.23 percent, decay-corrected, and radiochemical purity was 88.55 plus or minus 3.90 percent, both of which the authors describe as modest. Radio-HPLC analysis consistently showed two early-eluting radioactive impurities whose identity was not investigated.

The conversion also dropped substantially under scale-up conditions, from approximately 90 percent to approximately 70 percent at 25 millimolar, and from approximately 50 percent to approximately 10 percent at 25 micromolar. The authors note that no efforts were made to optimize the conversion, since the experiment was intended only to assess the feasibility of achieving high molar activity. They also observed that the isolated product concentration did not match the theoretical amount of substrate added, possibly related to volatilization during the high-temperature reaction, as slight deformation of the reaction caps was seen after heating to 175 degrees Celsius, though this was not investigated experimentally. The molar activity experiments were further limited by the need to manually transfer the fluoride between vials, restricting the maximum radioactivity that could be used; an automated system could permit larger starting amounts and higher molar activity.

The study focused on simple aromatic systems, and kinetic fits for some substrates were poorly constrained. Because all substrates were evaluated under standardized conditions, the influence of reaction temperature on exchange efficiency was not separately investigated, and the high temperature and solvent choice may not suit all substrates. Nonetheless, the findings clarify how functional groups and their positions govern fluorine-19/fluorine-18 isotopic exchange and demonstrate that, under appropriate conditions, the method can deliver molar activities above 100 gigabecquerel per micromole, within the range required for PET radiopharmaceuticals, without the need for separately functionalized precursors. For tracers aimed at low-density or low-abundance targets, high molar activity is often critical, while other applications tolerate lower molar activity when the target is not saturable or the endogenous substrate is present in vast excess. By bypassing precursor development, condition optimization, and purification method development, isotopic exchange could accelerate the development of new PET tracers bearing suitable electron-withdrawing substituents, motivating future work on more complex and clinically relevant scaffolds.

Subject of Research: Functional group and positional effects on fluorine-19/fluorine-18 isotopic exchange in aromatic rings for PET tracer labeling

Article Title: The influence of functional groups and their relative position on isotopic exchange between 19F and 18F in aromatic rings

Article References: Högnäsbacka, A. A., Cortés González, M. A., Wiranegara, A. D., & Schou, M. (2026). The influence of functional groups and their relative position on isotopic exchange between 19F and 18F in aromatic rings. EJNMMI Radiopharmacy and Chemistry, 11(1), Article 63. https://doi.org/10.1186/s41181-026-00494-4

Image Credits: AI Generated

DOI: 10.1186/s41181-026-00494-4

Keywords: fluorine-18, isotopic exchange, PET tracers, radiochemistry, nucleophilic aromatic substitution, fluorobenzonitrile, molar activity, electron-withdrawing groups, radiopharmaceuticals, dimethyl sulfoxide, Meisenheimer intermediate, aromatic fluorination