Crown ethers hole sizes

Crown Ether (hole size, A) Favored Cation (ionic diameter, A) ... 

Table 4.4 Comparison of ionic diameters and crown ether hole sizes ...

We saw in Fig. 6-30 the conversion of ethylene oxide to crown ethers upon reaction with appropriate metal salts, and demonstrated that the hole sizes of the products corresponded to the ionic radius of the template ion. However, lest we become over-confident, it should be pointed out that the major product from the reaction of ethylene oxide with caesium salts (r = 1.67 A) is not the expected 21-crown-7 with a hole size of about 1.7 A) but 18-crown-6 (hole size, 1.4 A) (Fig. 6-34). The reason for this lies in the structure of the complex formed. We have always assumed that the metal ion will try to lie in the middle of the bonding cavity of the macrocycle. There is no real reason why this should be. Caesium could form a complex with 21-crown-7 in which all of the oxygen atoms lie approximately planar with the metal in the centre of the cavity. It is also apparent that caesium could not occupy the middle of the cavity in 18-crown-6. However, a different type of complex can be formed with 18-crown-6, in which a caesium ion is sandwiched bet-... 

Crown ethers are cyclic poly ethers and their structure permits a conformation with certain sized holes in which cations can be trapped by co-ordination with the lone... 

The operation of the mismatch effects may be seen to best advantage when a range of products is possible from a single reactant or set of reactants. The reaction of ethylene oxide with metal salts results in the formation of crown ethers (Fig. 6-30). Obviously, a whole range of different cyclic oligomers and acyclic polymers could be formed from ethylene oxide. If we specifically wanted to obtain 18-crown-6, with a hole size of about 1.4 A, we would expect to use a potassium ion as template (r = 1.38 A). In fact, 18-crown-6 is obtained in good yield from the reaction of ethylene oxide with potassium tetra-fluoroboratc. In contrast, if we wanted 12-crown-4, with a hole size of about 0.8 A, it... 

Let us consider the [2+2] macrocyclic ligand 6.39, which is prepared by the non-template condensation of 1,2-diaminobenzene with 2,6-diformylpyridine. The hole size of this ligand is about 1.3 A, so we would expect it to be too large to bind first-row transition metal dications. As a matter of interest, the ligand binds K+, with an ionic radius of 1.38 A, more strongly than does the crown ether, 18-crown-6. In the absence of any che-... 

The ability of crown ethers to bind selectively to particular Group IA and Group IIA metal ions, because of the relationship between hole size and metal ion radius, has led to considerable interest in them in relation to membranes (models for selective ion transport), antibiotics (similar polyether structure), organic synthesis [solubilization of inorganic reagents leading to milder routes for oxidation (122), nucleophilic substitution (123), fluoridation (90)] and extraction of alkali... 

Several TTF-crown ether derivatives, such as LI, have been prepared and investigated as potential metal ion sensor systems [180]. Cyclic voltammetry studies revealed in all cases two one-electron reversible oxidation steps with Ei/2( ) = 0.48 and E]/2(i) = 0.64 V (vs. SCE), respectively. The addition of controlled amounts of metal ions such as Li", Na", K", and Ag leads to shifts of Ei/2H) up to 80 mV toward more anodic potentials, as expected for a repulsive coulombic interaction. The magnitudes of the observed shifts are dependent on the hole size in the crown ether and on the concentration of the metal ion, up to a certain limit. The synthesis and spectroscopic and electrochemical studies of unsym-metrical annelated TTF-crown ethers bound to Na, Ag", and Ba " have been recently described [181]. 

Kobayashi et al. recently developed the Pb(OTf)2-crown ether 56 complex as an efficient chiral catalyst of asymmetric aldol reactions in aqueous media (Scheme 10.51) [146]. This catalyst system achieves good to high yields and high levels of diastereo- (syn-selective) and enantioselectivity in the aldol reaction of a variety of aldehydes with propiophenone TMS enolate. The hole size of 56 is essential because 57 and 58 show no chiral induction. The unique structure of the Pb(OTf)2-56 complex as a chiral catalyst has been revealed by X-ray diffraction. 

Cation Binding by Crown Ethers The Hole-Size Relationship ... 

The hole-size relationship between cations and crown ethers has been a part of the lore in the cation binding area for nearly two decades. Although, to our knowledge, no formal definition of this principle has ever been offered, the general concept seems to be that cation binding will be optimized when the cation diameter and macrocycle cavity size are identical. A simple consequence of this concept is the notion that 15-crown-5 is selective (binds more strongly) for Na+ over K+. We have measured the homogeneous (equilibrium) stability constants for the reaction... 

Crown ethers are cyclic polyethers and their structure permits a conformation with certain sized holes in which cations can be trapped by co-ordination with the lone pair electrons on the oxygen atoms. These are used as phase transfer catalysts. The cyclic polymers of ethylene glycol (0CH2CH2) are named as X-crown-Y. X refers to total number of atoms in the ring and Y to the total number of oxygens in the ring. 

The concept of matching ligand hole size to the size of the metal ion has played a role in discussions of the apparent selectivity of particular ligands for particular metal ions. The selectivity (such as that discussed above for [M(18-crown-b)]" complexes, equation 11.20) is based on measured stabihty constants. It has, however, also been pointed out that the stability constants for [KL] complexes are often higher than for corresponding [ML] complexes where M = Li, Na, Rb or Cs, even when hole-matching is clearly not the all-important factor. An alternative explanation focuses on the fact that, when a crown ether binds M" ", the... 

Crown ethers or cryptates 493) are cyclic polyethers that selectively solvate alkali-metal ions, although depending on the size of the hole, other ions can be extracted by these reagents. Some recent work on extraction using these crown ethers has been reported 494-496) and more results can soon be expected from the use of crown ketones 408, 409). 

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