Corands

Semirigid phosphonamide ligands have been synthesized from the corand precursors by the reaction of 1,3-propanediol ditosylate or 1,2-dichloroethane <96JOC8904>. 

Cross-fertilisation between the crown ethers (or, more generally, corands), cryptands, spherands and podands has produced an enormous range of hybrid hosts such as cryptaspherands and hemispherands, many exhibiting all the useful features of the parent materials (Figure 3.16). 

Figure 3.18 Podands 3.11 and 3.40, corands 3.41-3.43 and cryptands 3.23 related to [18] crown-6. Compounds 3.41 and 3.42 are members of a hybrid corand-podand family termed lariat ethers. Lariat ethers with two podand arms such as 3.42 are termed BiBLE s (bi-bracchial lariat ethers).

Figure 3.37 Comparison of the K+ binding constant for cryptand and corand hosts (MeOH, 25°C).

Table 3.17 shows the complexation free energies for various ammonium ions with three corand receptors. It is immediately apparent that NH4+ is bound more strongly than either of its alkylated analogues, and methylammonium is bound more strongly than f-butylammonium, with a particularly large difference between NH4+ and CH3NH3+ complexes of 3.95 and 3.96 (10 kj mol-1). 

Table 3.17 Complexation free energies for corand receptors with ammonium ions.

This technique is now used extensively to assess enantiomeric excesses in organic reactions and separate small quantities of enantiomers. Closely related chiral corands are particularly useful in assessing the optical purity of amino acids, although modern chiral columns for HPLC may cost in excess of US 2000. 

Compare and contrast the properties of the following classes of host molecule. Include in your answer information on selectivity, solubility and binding kinetics (a) naturally occurring ionophores (b) podands (c) corands (e) lariat ethers (f) cryptands (g) calixarenes and (h) spherands. You may find it helpful to present the information in the form of a comparative table. 

We have already seen (Section 3.12.6) how strong -stacking interactions occur with the aryl corands 3.116 and 3.117 and the herbicide paraquat (3.113) (e.g. 3.116 + 3.113, Ku = 730 M-1 in Me2CO), resulting in solid-state and solution incorporation of the electron-deficient guest within the corand ring. This inclusion of paraquat within the macrocycle is effectively the formation of a [2] pseudorotaxane, and this... 

The yield of 10.103 is improved further by adopting a stepwise approach in which the preformed corand 10.104 is reacted with [Cu(MeCN)4]+ and one equivalent of the deprotected 10.102 to give an intermediate pseudorotaxane. Cyclisation as before gives the catenate 10.105 in an impressive 42 % yield. 

In the same way as the molecular or supra molecular juxtaposition of a binding site and a chromophore can result in luminescent or colorimetric sensing of target substrates, so incorporation of a redox-active centre may allow electrochemical detection of binding. In Section 4.7.2, we examined a range of hosts for anions based on the Co(III)/Co(II) redox couple in cobaltocinium based podands, corands... 

Not only does 4.12 bind strongly to ATP despite being a flexible corand macrocycle, but it is also capable of accelerating the rate of its hydrolysis by a factor of about 100 over a wide pH range (2.5-8.5). The hydrolysis may be followed very conveniently by NMR spectroscopy using the 31P nucleus, which has a reasonably high receptivity and has a nuclear spin quantum number of V2 (Figure 12.2), which... 

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