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Verkade Base
Verkade bases or Proazaphosphatrane, also called super-bases, are cage molecules whose interest is their exceedingly strong nonionic Bronsted and Lewis bases properties, although weakly nucleophilic. Surprizingly, the strongest basic center of the molecule is the Phosphorous atom (and not nitrogen atoms). The P-H bond need a base as strong as t-BuO- to be deprotonated. With pKa values spanning a large range (17 – 42), proazaphosphatranes and phosphazenes facilitate reactions previously restricted to ionic bases such as NaH, potassium t-butoxide, LDA and NaHMDS.
General structure of Verkade bases. They cas be imbedded in larger structures.
Verdade base were designed to be used as ‘double-ended’ ligands for exemple for connecting metal species via the bridgehead atoms into organometallic polymers. They have been discoverded by Verkade's groupin 1989 [1]. For a review, see [2].
[1] Lensink, C.; Xi, S.-K.; Daniels, L. M.; Verkade, J. G., J. Am.Chem. Soc. 1989, 111, 3478–3479.
[2] J. G. Verkade, P. B. Kisanga, Tetrahedron 2003, 59, 7819–7858.