Crown Ether-Type Macrocycles

The functional groups outlined in Fig. 10 are all bifunctional ligands that can, at least from a theoretical point of view, form monomeric, dimeric, or oligomeric complexes with metal ions or a diorganoboryl group. 

So far, only for the first two of the six ligand types outlined in Fig. 10 have oligomeric structures with boron been reported. This may be in part due to the relatively small number of publications related to this field and the fact that a systematic attempt to prepare boron macrocycles with these ligands has not been realized so far. 

The following structures will show that macrocyclic rings between metal ions or BR2 units and acyloxy or acylamino groups have some relationship with rigid coronands [Ic]. 

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Fig.1. Rotaxane dendrimers from crown ether type macrocycle ring, bipyridinimn derivatives rod component and Frechet-type dendron stoppers...

By addition of a chiral alcohol, such as (—)-menthol or (S,)-2-mcthylbutanol, in the presence of triethylamine to a cyclic achiral precursor, two crown ether type macrocycles 69 and 70 containing a phosphite unit were obtained and have been further used in asymmetric hydroformylations71 (Section D.I.5.8.). 

JCS(P2)1081, 93SL905]. 2,2 -Bifuran subunits have also been incorporated into annulenes (88AGE411), sapphyrins (92T9661), and crown ether-type macrocycles (82JOC3038). 

Large differences in composition are, in principle, possible by changing the metal center in metallocenes combined with macrocyclic ethers or cryptands, but these were mostly limited to air- and water-stable compounds, which could be considered for further cation and anion sensing devices. So far ruthenocene, titanocene, mo-lybdocene, and tungstocene moieties associated with crown ether-type macrocycles have been described. 

An interesting footnote to the general synthesis of macrocyclic crown ether-type esters is found in the work of Ors and Srinivasan . These workers used cinnamate esters and diesters as precursors to large ring lactones. Depending on orientation, either truxil-... 

Different classifications for the chiral CSPs have been described. They are based on the chemical structure of the chiral selectors and on the chiral recognition mechanism involved. In this chapter we will use a classification based mainly on the chemical structure of the selectors. The selectors are classified in three groups (i) CSPs with low-molecular-weight selectors, such as Pirkle type CSPs, ionic and ligand exchange CSPs, (ii) CSPs with macrocyclic selectors, such as CDs, crown-ethers and macrocyclic antibiotics, and (iii) CSPs with macromolecular selectors, such as polysaccharides, synthetic polymers, molecular imprinted polymers and proteins. These different types of CSPs, frequently used for the analysis of chiral pharmaceuticals, are discussed in more detail later. 

Protons are relatively simple targets for sensor molecules and do not require engineered receptors, however, achievement of selective interactions with other chemical species requires much more elaborate receptors. In the most cases cations are bound via electrostatic or coordinative interactions within the receptors alkali metal cations, which are rather poor central ions and form only very weak coordination bonds, are usually bound within crown ethers, azacrown macrocycles, cryptands, podands, and related types of receptor moieties with oxygen and nitrogen donor atoms [8], Most of the common cation sensors are based on the photoinduced electron transfer (PET) mechanism, so the receptor moiety must have its redox potential (HOMO energy) adjusted to quench luminescence of the fluorophore (Figure 16.3). 

We will discuss alcohol and ether functionalised NHC first, followed by furan functionalised ones. After a brief glimpse into Bertrand s noncyclic biaryl amino carbenes, we will turn towards macrocyclic, crown ether type examples and finally to NHC bearing a carbonyl group in the sidechain. 

ENANTIOSEPARATION OF PHARMACEUTICALLY RELEVANT CHIRAL COMPOUNDS USING CYCLODEXTRIN, MACROCYCLIC ANTIBIOTIC. AND CROWN-ETHER TYPE CSPs... 

Templates based on ether tethers also have been examined. Improvements in product yield may be anticipated from the reduced steric congestion in the cyclization step due to the substitution of an -O linkage for a -CH2 linkage [43]. In addition, such oxygenated tethers might serve as crown-ether type substrates, sequestering ions that in turn serve to promote macrocyclization. The beneficial effect on macrocyclization rates by complex-ation of this type has precedent in the work of Mandolini and Illuminati [44]. 

Ciampolini et al very recently reported the synthesis of the first crown-ether-type phosphorus-containing macrocycle 4,7,13,16-tetra-... 

Scheme 30.26 Graphical representation of the formation of polycatenated macrocycles using ring-closing metathesis to interlock crown ether-type units around ammonium sites along the backbone of a cyclic polymer. Reproduced with permission from Ref. [181] 2010, American Chemical Society.

Ionic supramolecular self-assembly will be discussed in Chapter 6, but some supramolecular systems based upon ionic interactions will be discussed earlier, e.g. the organocyclosiloxanolates, which form sandwich compounds by intercalating transition metal ions between two macrocyclic rings (held together by ionic interactions) and acting as endo receptors which concurrently have crown-ether-type complexing properties (as exo receptors) (see Section 2.1.2). 

This type of coordination stabilizes a crown conformation of the cyclohexa-siloxane ring. There are, in addition, two sodium cations attached at the top and the bottom of the sandwich in a crown ether-type complexation mode, with each sodium ion bonded to six oxygens of the rings, 9, and further solvated by alcohol molecules (not shown). The macrocyclic siloxane acts as both endo and exo receptors and the formation of the supramolecular architecture involves both ionic self-assembly and crown-ether-type ion recognition. 

Some other types of macrocycle compounds have been synthesized using adamantane and its derivatives. Recently, a new class of cyclobisamides has been synthesized using adamantane derivatives, which shows the general profiles of amino acid (serine or cystine)-ether composites. They were shown to be efficient ion transporters (especially for Na+ ions) in the model membranes [159]. Another interesting family of compounds to which adamantane derivatives have been introduced in order to obtain cyclic frameworks is crown ethers [160]. The outstanding feature of these adamantane-bearing crown ethers (which are also called diamond crowns ) is that ot-amino acids can be incorporated into the adamantano-crown backbone [160]. This family of... 

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