title: Crystal Structures of Diaryliodonium Fluorides and Their Implications for Fluorination Mechanisms.
abstract: The radiofluorination of diaryliodonium salts is of value for producing radiotracers for positron emission tomography. We report crystal structures for two diaryliodonium fluorides. Whereas diphenyliodonium fluoride (1 a) exists as a tetramer bridged by four fluoride ions, 2-methylphenyl(phenyl)iodonium fluoride (2 a) forms a fluoride-bridged dimer that is further halogen bonded to two other monomers. We discuss the topological relationships between the two and their implications for fluorination in solution. Both radiofluorination and NMR spectroscopy show that thermolysis of 2 a gives 2-fluorotoluene and fluorobenzene in a 2 to 1 ratio that is in good agreement with the ratio observed from the radiofluorination of 2-methylphenyl(phenyl)iodonium chloride (2 b). The constancy of the product ratio affirms that the fluorinations occur via the same two rapidly interconverting transition states whose energy difference dictates chemoselectivity. From quantum chemical studies with density functional theory we attribute the "ortho-effect" to the favorable electrostatic interaction between the incoming fluoride and the o-methyl in the transition state. By utilizing the crystal structures of 1 a and 2 a, the mechanisms of fluoroarene formation from diaryliodonium fluorides in their monomeric, homodimeric, heterodimeric, and tetrameric states were also investigated. We propose that oligomerization energy dictates whether the fluorination occurs through a monomeric or an oligomeric pathway.
authors.affiliation: Center for Molecular Modeling, Office of Intramural Research, Center for Information Technology, National Institutes of Health, Building 12A, Rm 2049, Bethesda, MD, 20892, USA.
authors.forename: Yong-Sok
authors.identifier: http://orcid.org/0000-0002-6222-1197
authors.initials: YS
authors.lastname: Lee
authors.affiliation: Molecular Imaging Branch, National Institute of Mental Health, National Institutes of Health, Rm. B3C346A, 10 Center Drive, Bethesda, MD, 20892, USA.|Present address: Department of Nuclear Medicine, Yonsei University College of Medicine, 50-1 Yonsei-ro, Seodaemun-gu, 03722, South Korea.
authors.forename: Joong-Hyun
authors.identifier: http://orcid.org/0000-0002-9665-7829
authors.initials: JH
authors.lastname: Chun
authors.affiliation: National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, MD, 20892, USA.|National Institute of Chemistry, Ljubljana, Slovenia.
authors.forename: Milan
authors.initials: M
authors.lastname: Hodošček
authors.affiliation: Molecular Imaging Branch, National Institute of Mental Health, National Institutes of Health, Rm. B3C346A, 10 Center Drive, Bethesda, MD, 20892, USA.
authors.forename: Victor W
authors.identifier: http://orcid.org/0000-0001-9032-2553
authors.initials: VW
authors.lastname: Pike
keywords: chemoselectivity; ortho-effect; quantum chemistry; radiofluorination; reaction pathway