Polyether ring, templating

A great variety of polyether macrocycles, containing from 3 to 20 oxygen atoms and with ring sizes from 9 to 60, have been prepared by Pedersen (105-108). The synthetic routes used in cyclic polyether formation all require NaOH, but a template role for the Na+ ion has not been established ... 

Thus, cationic polymerization of oxiranes is of little synthetic value, if the preparation of linear polymers is attempted. The high tendency for cyclization may be employed, however, for preparation of macrocyclic polyethers (crown ethers). Polymerization of ethylene oxide in the presence of suitable cations (e.g., Na+, K+, Rb +, Cs + ) leads to crown ethers of a given ring size in relatively high yields, due to the template effect [105], Thus, with Rb+ or Cs+ cations, cyclic fraction contained exclusively 18-crown-6. 

The macrocyclic polyether, tris(l,5-naphtho)-57-crown-15 (Figure 5.12) contains a sufficiently large ring size to permit the simultaneous accommodation of two threaded 4,4 -bipyridinium units, which are then appropriately orientated for further template formation of tetracationic cyclophanes. However, the efficacy of template formation for [3]-catenane formation proved to be less than anticipated. The... 

Cesium carbonate and fluoride are often used as bases. The cesium ion plays a characteristic role as a counter ion. The ionic radius of a cesium ion is ideal for the template effect for polyethers of particular ring size [37]. A typical example is the formation of a 20-membered polyefher. The yield is excellent compared with the other large ring aimulation method (Scheme 2.22). 

One of the appealing aspects of catenanes that are based upon the interactions between 7t-electron-rich crown ethers and 7t-electron-deficient tetracationic cyclophanes is the modularity that can be employed (Fig. 25) in their template-directed syntheses. Virtually any 7t-electron-rich crown ether with identical or different recognition sites can be introduced into this class of catenanes. Also, by exploiting the different ways in which cyclophanes can be clipped around the preformed ring of a macrocyclic polyether, it is possible to incorporate [46] different 7r-electron-deficient units into the cyclophanes. 

A few macrocyclic polyethers containing a ring phosphorus atom are known. As a rule, the synthesis of such substances is not carried out by template processes. However, the assembly of crown ethers possessing ring phosphonyl groups has been achieved on alkali metal ion (lithium, sodium, potassium) matrices [181-184]. The product LI546 is synthesised in an inert atmosphere (Eq. 6.66). 

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