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Lewis Acidity of Organofluorophosphonium Salts: Hydrodefluorination by a Saturated Acceptor

Science  20 Sep 2013:
Vol. 341, Issue 6152, pp. 1374-1377
DOI: 10.1126/science.1241764

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The Pull of Phosphorus

Lewis acidity is primarily associated with compounds like boranes that lack a full complement of electrons in their coordination sphere and therefore attract electron donors (Lewis bases) to fill the gap. Caputo et al. (p. 1374; see the Perspective by Gabbaï) now show that a class of 4-coordinate phosphonium salts can act as surprisingly potent Lewis acids, despite their electronic saturation. The phosphorus cations, bearing fluorine and fluorinated aromatic substituents, can sever an alkyl carbon-fluorine bond by pulling away its fluoride—a process rendered catalytic through the use of a silane acceptor.

Abstract

Prototypical Lewis acids, such as boranes, derive their reactivity from electronic unsaturation. Here, we report the Lewis acidity and catalytic application of electronically saturated phosphorus-centered electrophilic acceptors. Organofluorophosphonium salts of the formula [(C6F5)3–xPhxPF][B(C6F5)4] (x = 0 or 1; Ph, phenyl) are shown to form adducts with neutral Lewis bases and to react rapidly with fluoroalkanes to produce difluorophosphoranes. In the presence of hydrosilane, the cation [(C6F5)3PF]+ is shown to catalyze the hydrodefluorination of fluoroalkanes, affording alkanes and fluorosilane. The mechanism demonstrates the impressive fluoride ion affinity of this highly electron-deficient phosphonium center.

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