Aza-thia crowns

Most of the liquid crystals discussed in this section bear a diaza[18]crown-6 40, a 4,4 -diaminodibenzo[18]crown-6 41, or a thia crown ether 42 in their center (Scheme 23). Substitution in 40 and 41 is conveniently feasible on the nitrogen atoms by formation of a Schiff base or an aza compound (in the case of 41) or by N-alkylation or -acylation (in the case of 41). O-Alkylation and -acylation of 42, which are difficult to obtain, open a path to thia crown centered liquid crystals. 

For the synthesis of aza-thia macrocycles, many of the methods that are available for the synthesis of aza cycles (Chapter 2) or aza-oxa crowns (Chapter 3) are also applicable. Thus the Richman-Atkins co-cyclisation of the ditoluenesulfonamide with an alkylditoluenesulfonate may be used for the synthesis of the [18]-N4S2 ring (Scheme 3.6). In this case, desulfonylation to afford the tetraamine 23 is best carried out with lithium in liquid ammonia.12... 

In the crown ethers (18) the interactions between the ligand and metal ion are considered to be more electrostatic in nature, rather than the covalent binding observed for the transition metal complexes of the aza, thia, and phospha macrocycles. The thermodynamic properties of these macrocycles have been extensively studied, with numerous reviews covering complexation, selectivity, and structural aspects, some with extensive tables of thermodynamic data. Considerable efforts have been made to correlate the interrelationship between cavity size of the macrocycles and stability of alkali and alkaline earth metal complexes. From X-ray and CPK models, cavity radii are determined as 0.86-0.92A for 15-crown-5 (64), 1.34-1.43 A for 18-crown-6 (65), and about 1.7 A for 21-crown-7 (66). For complex formation between the alkali metal ions and 18-crown-6, the maximum stability... 

The two most important thiacrown ethers are [14]aneS4 (12) and [12]aneS4 (13) (the thia equivalents of the aza-based crown ethers [14]aneN4 and [12]aneN4, respectively). Unlike oxygen-based crown ethers, sulfur-based crown ethers, thiacrown ethers, have a demonstrated ability to complex transition metals as opposed to alkali and alkaline earth metals. Nonetheless, interesting thiacrown ether complexes have been isolated with organoaluminum moieties. 

FIGURE 16 Crown ethers, (a) n = 0, 15-crown-5 n=1, 18-crown-6 n=2, 21-crown-7, (b) Thia-18-crown-6. (c) 1,10-Dithia-18-crown-6. (d) Aza-18-crown-6. (e) 1,10-Diaza-18-crown-6. (f) Benzo-18-crown-6. 

Some thia-crown ethers have been prepared from the reaction between caesium thiolates of appropriate l,w-dithiols and polyethylene glycol dibromides.The synthesis of mono-aza-crown ethers from dialkanolamines [equation (17)] can be performed without the need for a protecting group on... 

Redox chemistry of metal complexes of crown ethers and related aza and thia compounds 89CSR409 90CSR239. 

Anticrowns are peculiar antipodes of crown ethers and their thia and aza analogues. They contain several Lewis acidic centers in the macrocyclic chain and so are able to efficiently bind various anions and neutral Lewis bases with the formation of unusual complexes, wherein the Lewis basic species is simultaneously bonded to all Lewis acidic atoms of the macrocycle. This remarkable property of anticrowns, being reminiscent of the behavior of conventional crown compounds in metal cation binding, makes them prospective aids in the areas of molecular recognition, ion transport, as well as organic synthesis and catalysis. 

Figure 2.4 (Top) Aza and thia analogues of 18-crown-6. (Bottom) [NaSCN-H20-18-crown-6] with Na" complexed by the crown ether, a SCN and a water molecule (21). (By permission of International Union of Crystallography.)...

Table 2.2 Comparison of log values for the comp-lexation of 18-crown-6 and of some aza and thia analogues with and Ag" ...

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