Lewis Acids with a Difference

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Science  20 Sep 2013:
Vol. 341, Issue 6152, pp. 1348-1349
DOI: 10.1126/science.1244423

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Lewis acids are electron-pair acceptors that readily bind electron-rich molecules and anions. Examples include fluorinated organoboranes such as 1 (see the first figure), which are widely used in synthesis, small-molecule activation, and catalysis (14), and silylium cations such as 2+,which are powerful anion abstractors (5). Pentavalent group 15 fluorides such as PF5 are another class of strong Lewis acids (6, 7), but they react with water to release hydrogen fluoride, making them corrosive. In principle, these inconvenient properties could be tamed by introducing organic substituents, a strategy validated in the development of boron Lewis acids such as 1. By analogy with silylium cation chemistry, the Lewis acidity of group 15 derivatives could be further enhanced if they were cations. Putting these ideas into practice, Caputo et al. report, on page 1374 of this issue, the synthesis and properties of the Lewis acid [(C6F5)3PF]+ [3+ (see the first figure)] (8).