The Balz-Schiemann synthesis of aryl fluorides via the thermal decomposition of aryl diazonium tetrafluoroborate salts represents both the earliest and the most proverbial example of the BF4 ion as a nucleophilic fluoride source. Mechanistic studies of the Balz-Schiemann reaction have accrued strong experimental support for an SN1 mechanism via the intermediacy of an aryl cation, which is quenched by fluoride transfer directly from the BF4 ion. A notable advance in this area has been the development by Ohmori and co-workers of a fluorinative variant of the Mitsunobu reaction, involving the electrooxidative generation and subsequent thermal decomposition of alkoxytriphenylphosphonium tetrafluoroborates, allowing for the conversion of alcohols to the corresponding alkyl fluorides with inversion of configuration. Although stereoselectivity during C-F bond formation for reactions proceeding via (putative) carbocation intermediates can be highly variable, the ring-opening of strained rings or ?iranium intermediates via fluoride transfer from fluoroborate ions typically proceeds with high stereoselectivity.
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