Cryptands modification

Similar pentaerythritol cryptands have been prepared by a slight modification of the above approach. Although the general strategy is as illustrated in Eq. (8.11), atfetal formation is accomplished by reaction of pentaerythritol with paraformaldehyde. This reaction leads to diacetal 12 which is hydrolyzed in dilute H2SO4 to yield the monoacetal, 13. The latter is then used in a fashion similar to that described in Eq. 8.10, above. 

We do not discuss in detail the cases of tautomerism of heterocycles embedded in supramolecular structures, such as crown ethers, cryptands, and heterophanes, because such tautomerism is similar in most aspects to that displayed by the analogous monocyclic heterocycles. We concentrate here on modifications that can be induced by the macrocyclic cavity. Tire so-called proton-ionizable crown ethers have been discussed in several comprehensive reviews by Bradshaw et al. [90H665 96CSC(1)35 97ACR338, 97JIP221J. Tire compounds considered include tautomerizable compounds such as 4(5)-substituted imidazoles 1///4//-1,2,4-triazoles 3-hydroxy-pyridines and 4-pyridones. 

The reduction of the stannyl radical (t-Bu2MeSi)3Sn with alkali metals produces a variety of structural modifications depending on the solvent used (Scheme 2.55). Thus, in nonpolar heptane, a dimeric stannyllithium species [58c Li ]2 (E = Sn) was formed, whereas in more polar benzene, the monomeric pyramidal structure 58c [Ti -Li (C6H5)] was produced. In the latter compound the Li+ ion was covalently bonded to the anionic Sn atom being at the same time n -coordinated to the benzene ring. A similar monomeric pyramidal CIP 58c [Li (thf)2] was prepared by reduction in polar THE the addition of [2.2.2]cryptand to this compound resulted in the isolation of the free stannyl anion 58c K+([2.2.2]cryptand), in which the ion lacked its bonding to the Sn atom. ... 

As discussed in Chapter 4, the selectivity of cage ligands (such as the cryptands) for particular guests tends to be more readily controlled by structural modification than is the case for the crowns. This is usually a reflection of the cavities being inherently better defined in the three-dimensional cage structures. 

A second important advance was made with the availability of cryptands (73), a family of bicyclic polyoxadiamines which have available a three-dimensional cavity for complexation.6 Again many structural modifications are possible and lead to numerous macropolycyclic ethers. 

The results (Table 10) show that the cryptands could act to produce carrier-mediated facilitated diffusion and there was no transport in the absence of the carrier. The rate of transport depended upon the cation and carrier, and the transport selectivity differed widely. The rates were not proportional to complex stability. There was an optimal stability of the cryptate complex for efficient transport, logKs 5, and this value is similar to that for valinomycin (4.9 in methanol). [3.2.2] and [3.3.3] showed the same complexation selectivity for Na+ and K+ but opposing transport selectivities. The structural modification from [2.2.2] to [2.2.C8] led to an enhanced carriage of both Na+ and K+ but K+ was selected over Na+. The modification changes an ion receptor into an ion carrier, and indicates that median range stability constants are required for transport. Similar, but less decisive, results have been found in experiments using open-chain ligands and crown ethers.498... 

Other modifications to the [ . . ] cryptands include replacing one or more of the polyether chains by o-phenanthroline and/or bipyridine, as in (46), to obtain photoactive compounds.165... 

The macrobicyclic cryptands also bind AEC very strongly. Ligand 9 displays a unique and very high preference for Sr2+ and Ba2+ over Ca2+. Suitable structural modifications allow control over the M2+/M+ selectivity from preference for AEC to preference for AC binding [2.31]. 

For a time one managed by choosing new descriptions and used intricate contractions and modifications of the available terms Some examples being clathrate complex, clathrate hydrate, hydrocarbon clathrate, gas hydrate, interlamellar sorbent, molecular compoimd, addition compound, loose addition complex, cascade complex, lock-and-key complex, super molecular complex, molecular complex associate, tweezer molecule complex, soccer molecule complex, hexapus molecule complex, octopus molecule complex as well as complexes or inclusion compounds of spherands, sepulchrands, coronands, cyclidenes, cryptands, cryptophanes, calixarenes, cucurbit-uril, annelides etc. 

While a number of different phase transfer catalysts such as tetrabutyl ammonium halides (Ref. 10, 11), tetrabutyl ammonium hydroxide (Ref. 16-18), tetrabutyl ammonium hydrogen sulfate (Ref. 21-22), Adogen-464 (Ref. 16, 18), tetrabutyl phosphonium bromide (Ref. 19, 20), 18-crown-6 (Ref. 12, 13, 19, 20), cryptand [222] (Ref. 21, 22), etc., have been used in the chemical modification of polymers, few systematic studies of the influence of the catalyst on the reactions have been done. It is presumed that the same considerations which govern the choice of a phase transfer catalyst for classical organic synthesis also apply in the case of reactions with polymers. 

Many catalysts can be used tetrabutylammonium halides, tetrabutylammonium hydroxide, tetrabutylammonium hydrogen sulfate, tetrabutylphosphonium bromide, 18-crown-6 ether, and cryptand[2.2.2]. There have been few studies on the influence of the catalyst on the reactions. However, Nishibuko et al carried out an excellent study on the influence of experimental conditions on phase transfer catalyzed polymer modification they showed that the nature of the catalyst and the type of phase transfer reaction (solid-liquid, liquid-liquid), as well as the polarity of the solvent are very important parameters. The purity of the system must be carefully controlled thus, the presence of traces of water may have a great influence on the conversion and the occurrence of side reactions. 

---