Tetrabenzo crown

A dramatic cesium effect was found by Weber in the case of the synthesis of the tetrabenzo crown ether 31, which is of interest for applications in ionselective electrodes [39]. The cyclization reactions starting from the cesium phenolate obtained from 29 and the tosylate 30 under high dilution conditions in DMF led to 31 in 37 % yield. This is a dramatic improvement of the formerly obtained yield of only 6 % in the system KOH/ -butanol/ethanol/DMF [40]. 

Fig. 5 Layer structure of (tetrabenzo [24] crown-8)(dibenzylammonium)(PF6) held together by weak C-H...F hydrogen bonding between the pseudorotaxane and PFg. Only N-H hydrogens shown [32]...

Dual-host approaches have also been used to good effect for CsN03 extraction. Nitrate is a common anion found upon nitric acid extraction of 137Cs+ in the nuclear industry. Cs+ is conveniently com plexed by a large crown ether such as tetrabenzo[24] crown-8 while simple tripodal amide hosts of type 5.31 (where R is a long alkyl group to impart lipophilicity) are effective at binding N03 . Extraction efficiency of Cs+ from water into 1,2-dichloroethane was found to be enhanced by a factor of up to 4.4 in the presence of the anion host.28... 

Polyaza macrocycles [njaneNm n = Number of ring atoms Nm = Number of nitrogen atoms Polyoxa macrocycles n-crown-m n = Number of ring atoms m = Number of oxygen atoms TRI = Tribenzo[fc,/,y][l,5,9]triazacyclododecine TAAB = Tetrabenzo[fc,/,y,n][l,5,9,13]tetraazacyclohexa-decine. 

The crown ether tetrabenzo-24-crown-8 shows selective binding of large alkali metals Cs and Rb" in preference to the smaller On complexation of Cs ... 

Fig. 1 Space-filling diagram showing the binding of 1,2-dichloroethane to Cs within a molecular cleft from the crystal structure of [Cs(tetrabenzo-24-crown-8)(l,2-dichloroethane)2] (NOslTHiO). Minor disorder of the 1,2-dichloroethane was omitted. This diagram was adapted from Ref [6].

Levitskaia. T.G. Bryan. J.C. Sachlehen. R.A. Lamb. J.D. Moyer, B.A. A surprising host-guest relationship between 1.2-dichloroethane and the cesium complex of tetrabenzo-24-crown-8. J. Am. Chem. Soc. 2000, 122. 

Finally, a complex molecule such as me5o-tetrabenzo-[15]-crown-5-porphyrin led us to obtain very poor results, both in yield and in optical purity, probably due to the high steric hindrance of the additive, which avoids the interaction with the enzyme. 

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