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Highly efficient no-carrier-added Fluorine-18-functionalization of electron rich arenes and applications of hypervalent iodine
Positron Emission Tomography (PET) is a valuable imaging technique which provides a 3-dimensional real-time image of a biochemical process in vivo, non-invasively. The scope of PET is currently limited by the inability to provide clinically relevant radiotracers to appropriately diagnose the phenotype characteristic of a respective disease. Many clinically relevant radiotracers require functionalization of electron-rich arenes with [18F]-fluoride. Due to the short half-life of [18F] (t1/2 = 109.5 min), the late-stage incorporation of [18F]-fluoride is highly desirable. However, direct nucleophilic aromatic substitution is extremely difficult due to inherent nucleophilic properties of fluoride anion and conditions required for radiochemical syntheses. The work in this thesis provides the first general late-stage methodology of functionalizing electron rich arenes with [18F]fluoride through reductive elimination of diaryliodonium salts. Significant precedence exists for the preparation of diaryliodonium salts; however, most are limited by functional group tolerance or the use of toxic heavy metals. Regiospecific functionalization of highly activated aromatic systems by direct electrophilic aromatic substitution (EAS) or nucleophilic aromatic substitution (SNAr) is often difficult or impossible to achieve without the use of other accessory groups, which requires multiple steps and painstaking purifications. In this thesis, we show an electrophilically "tunable" I (III) reagent is mild enough to prepare regioisomerically pure diaryliodonium salts by direct EAS. Furthermore, regioselective iodination of electron rich arenes can be achieved from reductive elimination of the corresponding diaryliodonium salt. Therefore, we propose this simple, mild process to be a convenient alternative to the classic, Sandmeyer-type reaction.
Neumann, Kiel D, "Highly efficient no-carrier-added Fluorine-18-functionalization of electron rich arenes and applications of hypervalent iodine" (2012). ETD collection for University of Nebraska - Lincoln. AAI3504204.