Mark D. Struble

Evidence for a Symmetrical Fluoronium Ion in Solution

Fluorine Learns to Share Though halides typically coordinate to just one carbon center, their transient coordination to a second carbon (forming a positively charged bridge) explains the spatial dynamics of many reactions. However, unlike chlorine, bromine, and iodine—which can all form such halonium ions—fluorine does not appear to engage in carbon-bridging behavior, presumably because of its very high electronegativity. Struble et al. (p. 57, see the Perspective by Hennecke) synthesized a rigid molecule, particularly well-poised to manifest fluoride bridging, and provide evidence for a fluoronium intermediate in a displacement reaction. Fluoride appears to form a dicoordinate carbon-bridging intermediate previously seen only for the heavier halogens. [Also see Perspective by Hennecke] Halonium ions, in which formally positively charged halogens (chlorine, bromine, and iodine) are equivalently attached to two carbon atoms through three-center bonds, are well established in the synthetic chemistry of organochlorides, bromides, and iodides. Mechanistic studies of these ions have generated numerous insights into the origins of stereoselectivity in addition and displacement reactions. However, it has not been clear whether fluorine can form a halonium ion in the same manner. We present chemical and theoretical evidence for the transient generation of a true symmetrical fluoronium ion in solution from an appropriately configured precursor.

Positioning a Carbon–Fluorine Bond over the π Cloud of an Aromatic Ring: A Different Type of Arene Activation

It is known that the fluoro group has only a small effect on the rates of electrophilic aromatic substitutions. Imagine instead a carbon-fluorine (C-F) bond positioned tightly over the π cloud of an aryl ring-such an orthogonal, noncovalent arrangement could instead stabilize a positively charged arene intermediate or transition state, giving rise to novel electrophilic aromatic substitution chemistry. Herein, we report the synthesis and study of molecule 1, containing a rigid C-F⋅⋅⋅Ar interaction that plays a prominent role in both its reaction chemistry and spectroscopy. For example, we established that the C-F⋅⋅⋅Ar interaction can bring about a >1500 fold increase in the relative rate of an aromatic nitration reaction, affording functionalization on the activated ring exclusively. Overall, these results establish fluoro as a through-space directing/activating group that complements the traditional role of fluorine as a slightly deactivating aryl substituent in nitrations.

Synthesis of a Tight Intramolecular OH···Olefin Interaction, Probed by IR, 1H NMR, and Quantum Chemistry

We have synthesized a molecule containing a tight hydrogen-bonding interaction between an alcohol and a nonconjugated π-system. The strength of this hydrogen bond results in a large red shift, nearly 189 cm(-1), on the alcohol stretching frequency in the IR spectrum in comparison to a free alcohol control. The interaction is notable in that it possesses a better defined intramolecular hydrogen bond compared to the usual molecules for which it is noted, such as syn-7-norbornenol. This interaction was studied through the use of IR and NMR spectroscopy, X-ray crystallography, and molecular modeling calculations.