Cryptates

The synthesis of macropolycyclic cryptands generally involves stepwise, straightforward pathways (18, 20, 33) based on the successive construction of systems of increasing cyclic order macrocyclic, macrobicyclic, and so on. Newkome has recently reported a satisfactory quaternization-dealkylation procedure, facilitating the synthesis of 8 (34). Unlike the synthetic approaches to simple crown ethers (10, 

template-directed syntheses involving metal cation association have proved unsuccessful. Nevertheless, an unusual intramolecular 

As discussed in Section II,B, the nitrogen lone pairs of the [2] cryp-tands may be turned either inward or outward with respect to the molecular cavity, leading to three possible conformations exo-exo, exo-endo, and endo-endo (Fig. 2). The most favorable conformation for complex formation is the endo-endo form, in which the nitrogen lone pairs are directed inward toward the metal ion. A wealth of crystallographic data exists for [2] cryptates, primarily from Weiss s group 

Crystal structure of (K[2.2.21) I (reproduced with permission). 

The rubidium and cesium complexes of [2.2.2] are isomorphous with approximate D3 symmetry and a crystallographic twofold rotation axis (41). While the rubidium cation is complexed almost without strain, the Cs+ is accommodated only by enlarging the cavity, increasing the mean C—C torsion angle to 71° (compared with 54° for the potassium cryptate). The ligand deformations required to complex Na+ and Cs + are reflected in their lower solution stability constants with respect to the K+ and Rb+ cryptates (see Section III,D). 

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The macrocychc hexaimine stmcture of Figure 19a forms a homodinuclear cryptate with Cu(I) (122), whereas crown ether boron receptors (Fig. 19b) have been appHed for the simultaneous and selective recognition of complementary cation—anion species such as potassium and fluoride (123) or ammonium and alkoxide ions (124) to yield a heterodinuclear complex (120). 

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. 

Ca can be complexed by crown ethers and cryptate ligands and ia this form can be transported across natural and artificial membranes. 

Phase-tiansfei catalysis (PTC) is a technique by which leactions between substances located in diffeient phases aie biought about oi accelerated. Typically, one OI more of the reactants are organic Hquids or soHds dissolved in a nonpolar organic solvent and the coreactants are salts or alkah metal hydroxides in aqueous solution. Without a catalyst such reactions are often slow or do not occur at ah the phase-transfer catalyst, however, makes such conversions fast and efficient. Catalysts used most extensively are quaternary ammonium or phosphonium salts, and crown ethers and cryptates. Although isolated examples of PTC can be found in the early Hterature, it is only since the middle of the 1960s that the method has developed extensively. 

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. 

Complexes Tlie term cryptate is now accepted to mean the complex formed between a cryptand and a substrate. Tlie corresponding complex with a coronand would be a coronate, a term suggested some years ago by the same authors . Presumably, a complex between a podand and some substrate would be a podate . 

Of these terms, the names crown ether, cryptand and cryptate are in general usage. 

Dietrich, Lehn and Sauvage recognized not only the possibility of enclosing a cation completely in a lipophilic shell, but they also recognized the potential for using such systems for activating associated anions. This is made particularly clear in a paper which appeared some years later One of the original motivations for our work on cryptates rested on their potential use for salt solubilization, anion activation and phase transfer catalysis . This particular application is discussed below in Sect. 8.3. 

Cryptates the chemistry of macropolycyclic inclusion complexes. J. M. Lehn, Acc. Chem. Res.,... 

Alkali and alkaline earth metal cryptates. D. Parker, Adv. Inorg. Chem. Radiochem., 1983, 27, 1-26 (150). 

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