Base-catalysed 18F-labelling of trifluoromethyl ketones. Application to the synthesis of 18F-labelled neutrophil elastase inhibitors

Copper(I)-free syntheses of [11C/18F]trifluoromethyl ketones from alkyl or aryl esters and [11C/18F]fluoroform

Herein, we report a copper(I)-free method for labeling the trifluoroacetyl group with positron-emitting carbon-11 (t1/2 = 20.4 min) or fluorine-18 (t1/2 = 109.8 min) as part of our exploration of radiolabeled fluoroforms to access new radiolabeled chemotypes of interest for tracer development. Treatment of alkyl esters and aryl esters, containing electron-donating or electron-withdrawing groups, with [11C/18F]fluoroform in the presence of strong base, gave [11C/18F]trifluoromethyl ketones as novel radiolabeling synthons in moderate to high yields within 15 minutes.

A review of frustrated Lewis pair enabled monoselective C-F bond activation

Frustrated Lewis pair (FLP) bond activation chemistry has greatly developed over the last two decades since the seminal report of metal-free reversible hydrogen activation. Recently, FLP systems have been utilized to allow monoselective C-F bond activation (at equivalent sites) in polyfluoroalkanes. The problem of 'over-defluorination' in the functionalization of polyfluoroalkanes (where multiple fluoro-positions are uncontrollably functionalized) has been a long-standing chemical problem in fluorocarbon chemistry for over 80 years. FLP mediated monoselective C-F bond activation is complementary to other solutions developed to address 'over-defluorination' and offers several advantages and unique opportunities. This perspective highlights some of these advantages and opportunities and places the development of FLP mediated C-F bond activation into the context of the wider effort to overcome 'over-defluorination'.

Synthetic Advantages of Defluorinative C-F Bond Functionalization

Much progress has been made in the development of methods to both create compounds that contain C-F bonds and to functionalize C-F bonds. As such, C-F bonds are becoming common and versatile synthetic functional handles. This review summarizes the advantages of defluorinative functionalization reactions for small molecule synthesis. The coverage is organized by the type of carbon framework the fluorine is attached to for mono- and polyfluorinated motifs. The main challenges, opportunities and advances of defluorinative functionalization are discussed for each class of organofluorine. Most of the text focuses on case studies that illustrate how defluorofunctionalization can improve routes to synthetic targets or how the properties of C-F bonds enable unique mechanisms and reactions. The broader goal is to showcase the opportunities for incorporating and exploiting C-F bonds in the design of synthetic routes, improvement of specific reactions and advent of new methods.

A Review on the Halodefluorination of Aliphatic Fluorides

Halodefluorination of alkyl fluorides using group 13 metal halides has been known for quite some time (first reported by Newman in 1938) and is often utilized in its crude stoichiometric form to substitute fluorine with heavier halogens. However, recently halodefluorination has undergone many developments. The reaction can be effected with a range of metal halide sources (including s-block, f-block, and p-block metals), and has been developed into a catalytic process. Furthermore, methods for monoselective halodefluorination in polyfluorocarbons have been developed, allowing exchange of only a single fluorine with a heavier halogen. The reaction has also found use in cascade processes, where the final product may not even contain a halide, but where the conversion of fluorine to a more reactive halogen is a pivotal reaction step in the cascade. This review provides a summary of the developments in the reaction from its inception until now.

1 Introduction

2 Stoichiometric Halodefluorination

2.1 Group 13 Halodefluorination Reagents

2.2 Other Metal Halide Mediated Halodefluorination

3 Catalytic Halodefluorination

4 Monoselective Halodefluorination

5 Cascade Reactions Involving Halodefluorination

6 Summary and Outlook

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.