Crown cationic guests

Crown ethers are cyclic polyethers designated [n]crown-m where n is the ring size and m the number of oxygen atoms, for instance [18] crown-6 1. They show a high affinity for cationic guest molecules, especially alkali metal cations, where the cation is commonly complexed within the cavity of macrocycle or sand-... 

Moyer, B. A. (1996) Basic Principles of Extraction and Liquid-Liquid Systems Employing Crown Ethers and Related Metal-Ion Receptors, in Atwood, J. L., Davies, J. E. D., McNicol, D. D., Vogtle, F., Lehn, J.-M. (eds.), Molecular Recognition Receptors for Cationic Guests, Pergamon, New York, pp 325-365 and references cited therein. 

Fig. 5 Crown ether coinplexation of a range of cationic guests (see text for details).

Macrocyclic polyethers (crown ethers) bind with cationic guests (e.g. primary ammonium cations) [14]. Meanwhile, anionic substrates (e.g. carboxylates) or... 

Selectivity of crown ether complexation. The ability of a host macrocyclic polyether to recognize, discriminate and selectively bind a particular guest metal cation in the presence of other different cationic guests is of prime importance to the development of sensor devices. A measure for the selectivity of a particular crown ether towards two different cations and M2 can be... 

A watershed observation was reported by Pedersen in 1967 based on studies performed at DuPont Central Research. Pedersen noted that the cyclic polyether called 18-crown-6 formed remarkably tight complexes with simple cations like K. The "crown" nomenclature arose because of the shape of the complex, with six oxygens binding the cation in a characteristic macrocyclic shape (Figure 4.6). The seminal observations of Pedersen were brilliantly exploited by other workers, most notably Cram and Lehn, and these three shared the 1987 Nobel Prize in chemistry for this work. "Host-guest" chemistry was born, with the crown ether serving as a molecular host to the cationic guest. 

The system exploiting -ir donor-acceptor effects is shown in Figure 4.16. Early host-guest studies along the lines of those discussed above established that a cyclophane crown could bind cationic guests such as the bipyridinium compound (also known as paraquat. 

The well-known discovery of crown ethers by Charles Pedersen in the 1960s led to the manifestation of a new field of molecular recognition, now fundamental to physical organic chemistry. These macrocyclic polyethers (hosts) encircle alkali metal cations (guests) through ion-dipole interactions between the metal ion and oxygen atoms (Figure 8.1). 

In the both cases. Substitution of KCN with NaCN did not affect the consumption of 5, but did the conversion yields of 3a (increased) and 3c (decreased) This reflects the tolerance of PEG for the size of the cationic guests incorporated in it compared with crown ether,... 

New diester/diamide-calix[4]aienediquinones and a diamide-benzo-15-crown-5-calix[4]arene receptor molecule have been prepared and shown to complex Group 1 and 2 metal, ammonium and alkylammonium cations with association constants up to 4.8 X 10 with Ba + and (3) in acetonitrile solution. The redox-active quinone containing receptors electrochemically recognise these cationic guest species, including for the first time, the amperometric detection of an alkyl ammonium cationic guest species by a redox-active ionophore. 

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