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Evidence for a Symmetrical Fluoronium Ion in Solution

Science  05 Apr 2013:
Vol. 340, Issue 6128, pp. 57-60
DOI: 10.1126/science.1231247

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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.

Abstract

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.

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