Anticrowns catalysis

Metalloboranes, 4 172 exopolyhedral, 4 208-210 main group element, 4 207-208 transition element, 4 205-207 Metallo-carbohedrene clusters, 4 648 Metallocarboranes, 4 170 as catalysts, 4 217-218 economic aspects, 4 229 exopolyhedral, 4 215-216 f-block element, 4 225-226 host-guest chemistry-carborane anticrowns, 4 216-217 structural systematics, 4 176-179 transition metal, 4 210-215 Metallocene catalysis, MAO in, 16 92-93. 

Anticrowns are peculiar antipodes of crown ethers and their thia and aza analogues. They contain several Lewis acidic centers in the macrocyclic chain and so are able to efficiently bind various anions and neutral Lewis bases with the formation of unusual complexes, wherein the Lewis basic species is simultaneously bonded to all Lewis acidic atoms of the macrocycle. This remarkable property of anticrowns, being reminiscent of the behavior of conventional crown compounds in metal cation binding, makes them prospective aids in the areas of molecular recognition, ion transport, as well as organic synthesis and catalysis. 

In the present article, host-guest chemistry of anticrowns as well as available data on their applications in catalysis and as ionophores will be briefly reviewed. Strictly speaking, only macrocycles with three and more Lewis acidic centers in the chain can be considered as genuine anticrowns. Nevertheless, the binding properties of some macrocycles containing only two Lewis acidic atoms in the ring will also be discussed. 

The first example of the successful application of anticrowns in phase-transfer catalysis was reported in 1989, when it was shown that nonfluorinated poiymer-curamacrocycle 3 is able to catalyze the azo-coupling reaction between benzenediazonium halides PhN2" X (X-Cl, Br) and p-naphthol in the two-phase H2O-CH2Br2 system. Diphenylmercury proved to be totally inactive in this reaction. 

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