Cryptand complex, solvent effect

Based on a suggestion by Odell and Earlam [119] that crown ethers and cryptands can cause proteins to dissolve in methanol, Broos and coworkers [120] investigated the effects of crown ethers on the enzymatic activity of a-chymotrypsin in the transesterification reaction of N-acetyl-L-phenylalanine ethyl ester with n-propanol in organic solvents. They observed a 30-fold rate acceleration when 18-crown-6 was used in octane. At that time, it was proposed that the water- and cation-complexing... 

As has been clearly demonstrated by X-ray studies (Section III,B), complexation of a salt by a cryptand results in cation-anion separation or dissociation. This enhances anionic reactivity and may effect solubilization of the salt in organic solvents. For example the highly hindered ester methyl mesitoate, 13, may be smoothly hydrolyzed by powdered potassium hydroxide in dry benzene in the presence of [2.2.2] (98). Indeed cryptands have proved particularly useful in phase-trans-... 

In contrast to the crown ether 28, the cryptand K211 (33) is very beneficial to the control of MMA polymerization initiated by diphenyhnethyllithium in THF at —78 °C. As a result, PMMA with a very low polydispersity index (1.01) was obtained" . The effect of this ligand on the unimeric model, e.g. MiBLi, was studied by NMR spectroscopy. The equilibrium commonly observed in THF between tetramers and dimers is actually shifted towards a monomeric complexed species" . Both the E/Z molar ratio (90/10 instead of 0/100) and the chain tacticity (Table 5) are affected by the cryptand K211 (33) whatever the solvent used ". ... 

The result with allyllithium 23 a differs only insignificantly from that of an earlier report (10.5 kcal/mol)25>. Complexation of 23 a with TMEDA does not influence the rate of exchange. Hexamethylphosphoric triamide (HMPT), 15-crown-5 ether and [2.1.1]cryptand in tetrahydrofuran (THF) led to rapid decomposition of 23a. Addition of n-butyllithium had essentially no effect on the barrier. Since the 13C NMR chemical shifts of 23 a are independent of the solvent, it is assumed that 23 a exists as a contact ion pair or higher aggregate in the NMR experiments. (The other alkali metals should also form contact ion pairs with the allyl anion because of their well-known tendency to form contact ion pairs even more readily than the lithium cation 26)). 

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