Concerted nucleophilic aromatic substitution with 19F− and 18F−

Abstract

Nucleophilic aromatic substitution (SNAr) is the most commonly used method to generate arenes that contain 18F for use in PET imaging; here, an unusual concerted SNAr reaction is presented that is not limited to electron-poor arenes. The nucleophilic aromatic substitution (SNAr) reaction is used routinely to generate the 18F-containing arenes used in positron-emission tomography (PET) imaging. This study presents an unusual concerted SNAr reaction that is not limited to electron-poor arenes and provides access to 18F-labelled compounds not accessible through conventional SNAr chemistry. Nucleophilic aromatic substitution (SNAr) is widely used by organic chemists to functionalize aromatic molecules, and it is the most commonly used method to generate arenes that contain 18F for use in positron-emission tomography (PET) imaging1. A wide range of nucleophiles exhibit SNAr reactivity, and the operational simplicity of the reaction means that the transformation can be conducted reliably and on large scales2. During SNAr, attack of a nucleophile at a carbon atom bearing a ‘leaving group’ leads to a negatively charged intermediate called a Meisenheimer complex. Only arenes with electron-withdrawing substituents can sufficiently stabilize the resulting build-up of negative charge during Meisenheimer complex formation, limiting the scope of SNAr reactions: the most common SNAr substrates contain strong π-acceptors in the ortho and/or para position(s)3. Here we present an unusual concerted nucleophilic aromatic substitution reaction (CSNAr) that is not limited to electron-poor arenes, because it does not proceed via a Meisenheimer intermediate. We show a phenol deoxyfluorination reaction for which CSNAr is favoured over a stepwise displacement. Mechanistic insights enabled us to develop a functional-group-tolerant 18F-deoxyfluorination reaction of phenols, which can be used to synthesize 18F-PET probes. Selective 18F introduction, without the need for the common, but cumbersome, azeotropic drying of 18F, can now be accomplished from phenols as starting materials, and provides access to 18F-labelled compounds not accessible through conventional chemistry.