Selectivity-complex formation function

The selectivity-complex formation function (81) describes the variation of the selectivity coefficient of the transition metal ion versus a non complex forming reference cation with increasing ligand concentration as... 

The complex formation constants presented in Tables 2—4 were measured in water, methanol or ethanol. There is no guarantee, however, that the selectivity sequences thus found will apply in lipid membranes. This being so, it becomes necessary to investigate the stability of a given complex as a function of the embedding medium. 

In biological systems, a macromolecular chain effectively selects a complementary one to form an intermacromolecular complex. In this way, very specific functionalities become effective. Synthetic polymers can also form intermacromolecular complexes, but the ability of a synthetic polymer to select only one objective polymer as in biological systems has not yet been realized, except for several specific systems of pairs of polymers which include one of the complementary base pairs of nucleic add individually, e.g. po y(A)-poly(U) and poly(I)-poly(C) (see Sect. 3.3). The intermacromolecular complex formation of synthetic polymers is controlled by many factors such as interaction forces, solvent, ionic strength, temperature, pH, etc. Moreover, the cooperative and concerted interactions of each active site play an important role in complex formation. These phenomena suggest that the selective intermacromolecular complexation can be realized under suitable conditions. 

Among the systems with chemical different donor and acceptor molecules, the photocopolymerization between maleic anhydride (MSA), which functions as an acceptor, and electron-rich monomers has been widely investigated. As donor monomers such compounds as styrene (Sty) [19-29], cyclohexene [30], N-vinylcarbazole [31], 2-vinyl naphthalene [32], vinyl acetate [33], 2.4.8.10-tetra-oxaspiro[5.5]undecan [34] and phenyl glycidyl ether (2,3-epoxypropyl phenyl ether, PGE) [35] have been used. In all the above cases, using high concentrations of both monomers, the absorption of the CT has been obtained in various solvents. Thus, with spectroscopic methods the complex formation constant Kct can be calculated (e.g., MSA-cyclohexene Kcl = 0.0681 mol -1 [33], MSA-tetrahydrofuran Kct = 0.331 mol-1 [36]), and a selective excitation of the CT is possible in many cases. 

Alder reactions, in most instances titanium or aluminum Lewis acids afford greater yields and/or selectivity. The stereoselectivity in Lewis acid-promoted Diels-Alder reactions between chiral a,/3-unsaturated A/-acyloxazolidinones results in unexpected selectivity as a function of the nature of the Lewis acid (Table 4) [102]. Optimum selectivity is expected for chelated intermediates, yet both SnCU and TiCU perform poorly relative to Et2AlCl (1.4 equiv.). The formation of the SnCU-A-acyloxazolidi-none chelate has been confirmed by solution NMR studies [103]. These data suggest that other factors such as the steric bulk associated with complexes might contribute to stereoselectivity. 

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