Crown ethers—continued

Crown ethers continue to be one of the most useful parts of supramolecular chemistry/91 From the beginning computations of metal ions complexes with synthetic ionophores/101 which have been aptly reviewed/111 emphasized the importance of including explicitly solvation in free energy calculations, also with ab initio calculations on calixarene complexes/121 Molecular dynamics simulations of 18-crown-6 ether complexes in aqueous solutions predict too low affinities, but at least correctly reproduce the sequence trend K+ > Rb+ > Cs+ > Na+. However, only the selection of K+ over Rb+ and Cs+ is ascribed to the cation size relative to that of the crown cavity, whereas K+ appears in these calculations to be selected over Na+ as consequence of the greater free energy penalty involved in displacing water molecules ftomNa/1131... 

The host guest chemistry of crown ethers continues to be exploited for the development of new ionophores for the selective recovery of Hg(II). A novel crown ring system containing a redox switchable trithiadiazapentalene/trithiotriuret unit (17/18) allows control of Hg(II) in solvent extraction experiments between chloroform and water. The thiocar-bonyl sulfur donor sites outside the macrocycUc cavity of (18) are responsible for strong complexation and extractabiUty into chloroform. 

The original concept and report of crown ethers is now more than three decades old. Even so, the field remains vital, and many applications in modem science are found for crowns. They are used as complexing agents and as substmctural elements in complex chemical architectures. The number of literature reports involving crown ethers continues to increase, and undiscovered applications are still ahead. 

The key difference in applications between crown ethers and lariat ethers is that the side chain(s) may possess residues that confer unique properties on the system. The crown ether continues to be important as a eomplexation site, but the chemistry of the molecule may actually be dominated by the side-arm substituents. 

The unsaturation present at the end of the polyether chain acts as a chain terminator ia the polyurethane reaction and reduces some of the desired physical properties. Much work has been done ia iadustry to reduce unsaturation while continuing to use the same reactors and hoi ding down the cost. In a study (102) usiag 18-crown-6 ether with potassium hydroxide to polymerise PO, a rate enhancement of approximately 10 was found at 110°C and slightly higher at lower temperature. The activation energy for this process was found to be 65 kj/mol (mol ratio, r = 1.5 crown ether/KOH) compared to 78 kj/mol for the KOH-catalysed polymerisation of PO. It was also feasible to prepare a PPO with 10, 000 having narrow distribution at 40°C with added crown ether (r = 1.5) (103). The polymerisation rate under these conditions is about the same as that without crown ether at 80°C. 

The first three-dimensional macrobicyclic TTF derivatives have been obtained <96CC615> and work on TTF-containing crown ethers and thioethers has continued <96LA551, 96JCS(P1)1995> with structures of this type being used to obtain the first TTF-containing... 

In the first half of the nineties, there has been a continuing trend from synthetic studies of classical crown ethers towards the polyazamacromolecules and the introduction of multiple heteroatoms, including most recently the metal atom centers. 

Until now, i.e., in 25 years of research, only two reviews on liquid crystalline crown ethers have been published [8, 9]. As both reviews cover the field only partially and, e.g., the fascinating polymeric crown ethers as well as taper-shaped liquid crystalline crown ethers are not discussed, we decided to give the first comprehensive review. The present chapter will be structured according to the molecular structure of the mesogens. The discussion of each type of crown ether mesogen will start with small molecules, continue with polymeric compounds, and conclude with possible applications (where applicable). 

In the early 1990s, there existed several classes of extractants for actinides (CMPO), for cesium and more generally alkali cations, and for strontium and alkaline earth cations (crown ethers and cosan). The combination of these extractants and the grafting of these functions on calixarene platforms have led to new classes of extremely efficient and selective extractants, in particular calixarene-crown, which are presently applied in the United States to treat the huge amounts of waste at the SRS. Calixarenes/ CMPO, crown ethers/cosan, CMPO/cosan, and more recently calixarenes/CMPO/ cosan are promising compounds. It is desirable that these studies, conducted at the international level, continue in particular to obtain a better understanding of the complex mechanisms of extraction of these compounds.127187... 

Shinkai and coworkers prepared numerous novel amphiphilic crowns (Shinkai, 1990) and incorporated them into membranes, formed membranes from them, or used them in liquid crystalline assemblies to control properties (He et al., 1990). Interest in this area continues. Four chiral amphiphilic crown ethers were recently reported that recognize enantiomeric amino acids when examined as Langmuir films (Badis et al., 2004). Finally, it is interesting to note that liposomes formed from amphiphiles (e.g., crown ethers) having neutral headgroups (i. e., niosomes) have been studied as drug delivery vehicles (Uchegbu and Vyas, 1998). 

Benzenedithiols are traditionally prepared by reductive dealkylation of 1,2-C6R4(SR )2, which in turn are obtained by treatment of dibromobenzenes with alkali metal or cuprous thiolates. The methodology continues to be used, for example, for crown ether-appended derivatives (15). A newer and more powerful synthesis of 1,2-benzenedithiol and its derivatives has been developed (16). This method (17) involves reaction of the benzenethiol with 2 equiv of BuLi to give 2-LiC6H4(SLi), which reacts with elemental sulfur to give the dithiolate (Eq. 1). 

---