A New Dimension in Fluorine Chemistry: Fluorine in Close Interactions
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Date
2016-07-20
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Johns Hopkins University
Abstract
This work attempts to shed some light on some of the fundamental chemistry utilized by C-F bonds in organofluorine compounds, with special care to noncovalent interactions. A fluorinated sesquinorbornane scaffold was the template for the majority of the studies owing to its rigid conformation and ability to be modified. Thanks to those two attributes, the properties of C-F bonds were observed under various conditions in a multitude of different conformations. Through that work, the C-F bond was observed to take part in SN1 reactions through anchimeric assistance. Even more interesting is the anchimeric assistance is facilitated through a positively charged fluoronium intermediate. In addition, the conformation of the scaffold, allows other functional groups to be placed in close proximity to the C-F bond. After making a series of molecules with different functional groups, the through-space interaction of the C-F bond with the proximal C-H was investigated. A correlation between proton-fluorine coupling constants and the interaction’s strength was observed. The positioning also allowed the observation of a strong hydrogen bonding interaction between the fluorine and an alcohol. While the hydrogen bond appears strong by most spectroscopic methods, IR spectroscopy shows little to no shift when compared to a control molecule. This appears to be a rare case of bond compression from steric strain cancelling out bond relaxation from hydrogen bonding, a so called “no-shift” bond. In an attempt to be sure our system can be used to generate hydrogen bonds, a hydrogen bond between an alcohol and a nonconjugated pi-system was investigated. Owing to the reduction of steric strain in the molecule a strong hydrogen bond was observed accompanied by a characteristic red shift in the IR spectrum. Interestingly the red shift was nearly 189 cm-1, which is much larger than other observed examples of this type of interaction. In order to bolster our claims about a fluoronium ion intermediate, an in depth investigation into the reactivity of the fluoronium system was reported. This was done through several rate studies, control reactions and isotopic labeling experiments. All of which indicate that the fluoronium is the most likely intermediate. Finally another example of fluorine acting as a directing group was investigated. Specifically, a C-F bond was found to influence a Diels-Alder reaction between a fluorinated dienophile and a borole so that one product was favored to a substantial degree.
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Keywords
Fluorine, Hydrogen Bonding, Fluoronium