Dynamic Libraries From Transacetalation Formal Metathesis Reaction

3 Dynamic Libraries From Transacetalation (Formal Metathesis) Reaction 

Inspired by Kawakami s studya wide exploitation of the formal metath- 

The full reversibility of the reaction was initially demonstrated by a series of experiments. For instance, the equilibrium mixtures obtained from transacetalation of 5 mM cycUc dimer and 3.33 mM cyclic trimer were practically indistinguishable, so were the equilibrates obtained from 25 mM cychc dimer and 16.7 mM cyclic trimer. Thus, as expected for a truly reversible system, the composition at equilibrium is the same, no matter what ohgomer is used as feedstock, on a condition that the equivalent concentration expressed in monomer units is the same. Another important characteristic of dynamic systems is the ability to readjust the product distribution by changing the factors that mle the equilibrium, even once the system has reached the equilibrium composition dictated by the initial conditions. In a further experiment, an equilibrated reaction mixture in which 8.33 mM cyclic trimer was the starting material was perturbed by adding an amount of solid cyclic trimer such as to double the equivalent monomer concentration (50 mM). Comparison of the distribution of the species analysed immediately after complete dissolution of the solid cyclic trimer with that found after re-equilibration showed that excess cyclic trimer was digested and mosdy transformed into h h molecular weight materials, as the total monomer concentration in the ordinal solution was not far from saturation conditions (close to critical concentration CC). In this case, indeed, equilibrium concentration plots of cyclic dimer, trimer and tetramer 

Conversely, when the similar system based on the mefei-phenylene spacer was studied,more typical saturation profiles were obtained for cyclic dimer, trimer and tetramer with EM2 = 13.4 mM, EM3 = 11 mM and EM4 = 6 mM, respectively. Strain energies of 4.2, 1.2 and 0 kJ moP respectively, are consequently estimated. The meto-phenylene spacer makes the ring closure more bearable by the confbrmationally demanding COCOC chain. 

While the full reversibility of the reaction was also confirmed in the case of the diphenylmethane spacer (EM2 = 4.4 and EM3 = 1.5 mM from saturation profiles and related strain energies of 5.0 and 4.61 moC were estimated), when the naphthalene and calixarene spacers depicted in Chart 2 were introduced in the formal structure, the transacetalation was no longer suitable for the generation of DLs, giving rise to irreversible 

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