Cryptate compounds

Podates AcycHc analogues of crown ethers /coronands and cryptands (podands, eg, (11) (30) are also capable of forming inclusion compounds (podates) with cations and uncharged organic molecules, the latter being endowed with a hydrogen bond fiinctionahty. Podates normally are less stable than coronates and cryptates but have favorable kinetics. 

Other complexing agents sometimes advocated are cryptates, especially the compound dubbed [2.2.2] (Kryptofix 222) [23978-09-8] (see Chelating agents). Crown ethers were originally advocated for reactions in the presence of soHd reagents (Uquid-soHd PTC). It is now known, however, that onium salts are equally suitable in many cases. 

The crown ethers and cryptates are able to complex the alkaU metals very strongly (38). AppHcations of these agents depend on the appreciable solubihty of the chelates in a wide range of solvents and the increase in activity of the co-anion in nonaqueous systems. For example, potassium hydroxide or permanganate can be solubiHzed in benzene [71 -43-2] hy dicyclohexano-[18]-crown-6 [16069-36-6]. In nonpolar solvents the anions are neither extensively solvated nor strongly paired with the complexed cation, and they behave as naked or bare anions with enhanced activity. Small amounts of the macrocycHc compounds can serve as phase-transfer agents, and they may be more effective than tetrabutylammonium ion for the purpose. The cost of these macrocycHc agents limits industrial use. 

There is no essential difference between quenching via a MMCT state or a LMCT state. The latter occurs, for example, in Eu(III) if the LMCT state is either at low energy or if this state shows a large offset in the configurational coordinate diagram [23, 35]. The latter occurs in glasses [123], certain cryptates [124] and lanthanum compounds [125]. 

Similarly, by Schiff-base condensation reactions have been used to generate free cryptands from triamines and dicarbonyls in [2+3] condensation mode. These ligands react with silver(I) compounds to give dinuclear or trinuclear macrocyclic compounds where Ag Ag interactions may be present. Thus, with a small azacryptand a dinuclear complex with a short Ag- Ag distance (55) is found.498 With bigger azacryptand ligands also dinuclear complexes as (56) are achieved but without silver-silver interaction. 65,499-501 A heterobinuclear Ag1—Cu1 cryptate has also been... 

The effect of cryptands on the reduction of ketones and aldehydes by metal hydrides has also been studied by Loupy et al. (1976). Their results showed that, whereas cryptating the lithium cation in LiAlH4 completely inhibited the reduction of isobutyraldehyde, it merely reduced the rate of reduction of aromatic aldehydes and ketones. The authors rationalized the difference between the results obtained with aliphatic and aromatic compounds in terms of frontier orbital theory, which gave the following reactivity sequence Li+-co-ordinated aliphatic C=0 x Li+-co-ordinated aromatic C=0 > non-co-ordinated aromatic C=0 > non-co-ordinated aliphatic C=0. By increasing the reaction time, Loupy and Seyden-Penne (1978) showed that cyclohexenone [197] was reduced by LiAlH4 and LiBH4, even in the presence of [2.1.1]-cryptand, albeit much more slowly. In diethyl ether in the absence of... 

K (26). The former value leads to an interlonlc distance i equal to 4.6 A according to the classical Fuoss equation. This value Is too small compared to the results obtained for cryptated living polypropylene sulfide (8) and for cryptated tetraphenyl-borides In THF (24). This might mean that either K is not located inside the cavity of the ligand or the oxanion can penetrate into the cavity of the cryptand. This last explanation is consistent with comparative conductivity data made on model compounds (17) as shown in Table III. 

I would like to extend Prof. Simon s characterizations of these beautiful new molecules to include a description of the effects on lipid bilayers of his Na+ selective compound number 11, which my post-doctoral student, Kun-Hung Kuo, and I have found to induce an Na+ selective permeation across lipid bilayer membranes [K.-H. Kuo and G. Eisenman, Naf Selective Permeation of Lipid Bilayers, mediated by a Neutral Ionophore, Abstracts 21st Nat. Biophysical Society meeting (Biophys. J., 17, 212a (1977))]. This is the first example, to my knowledge, of the successful reconstitution of an Na+ selective permeation in an artificial bilayer system. (Presumably the previous failure of such well known lipophilic, Na+ complexing molecules as antamanide, perhydroan-tamanide, or Lehn s cryptates to render bilayers selectively permeable to Na+ is due to kinetic limitations on their rate of complexation and decomplexation). 

As noted earlier, the similarities between H+ and alkali metal cations have led to the use of the former as a probe in biological studies, including studies with various macrocydic ligands, especially those with oxygen donor atoms. The thallium(I) cryptates behave kinetically like the potassium compounds, and the binding constants to 18-crown-6 have been measured by 205T1 NMR methods.347 Several Tl1 compounds with crown ethers (L) have been prepared in... 

Sodium or potassium ions can also participate in the phase-transfer process when they are converted to lipophilic cations by complexation or by strong specific solvation. A variety of neutral organic compounds are able to form reasonably stable complexes with K+ or Na + and can act as catalysts in typical phase-transfer processes. Such compounds include monocyclic polyethers, or crown ethers (1), and bicyclic aminopolyethers (cryptates) (2). They can solubilize inorganic salts in nonpolar solvents and are particularly recommended for reactions of naked anions. Applications of these compounds have been studied.12,21-31... 

Crown ethers and cryptates are phase transfer catalysts but the use of these compounds in PTC reactions is limited to cases in which TAA salts are unsuitable.42... 

Cavilaics include crown rnacroring inclusion compounds (coronates), cryptates, podates. cytlophane host inclusion compnunds. calixarene inclusion compounds, cyclodextrin and amylose inclusion compounds, cucurbituril inclusion compounds, molecular deft inclusion compounds, and anionic guest inclusion compounds. 

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