Anionic substrate activation

Dimerization is the characteristic reaction of radical-anions from activated alkenes. The rate constants for dimerization are high and the conjugate acids from such alkene radical-anions in many cases have low pKa values and. The data in Table 3.4 were obtained by following the changes in uv-absorbance after pule-radiolysis of the substrate in an aqueous buffer. Attachment of a solvated electron leads to the radical-anion. Changes in the initial absorbance with pH lead to determination of the pKg value, while the dimerization rate can be determined from changes in absorbance over a longer time scale. Radical-anions from esters and amides are pro-... 

Surfactant Activity in Micellar Systems. The activities or concentrations of individual surfactant monomers in equilibrium with mixed micelles are the most important quantities predicted by micellar thermodynamic models. These variables often dictate practical performance of surfactant solutions. The monomer concentrations in mixed micellar systems have been measured by ultraf i Itration (I.), dialysis (2), a combination of conductivity and specific ion electrode measurements (3), a method using surface tension of mixtures at and above the CMC <4), gel filtration (5), conductivity (6), specific ion electrode measurements (7), NMR <8), chromatograph c separation of surfactants with a hydrophilic substrate (9> and by application of the Bibbs-Duhem equation to CMC data (iO). Surfactant specific electrodes have been used to measure anionic surfactant activities in single surfactant systems (11.12) and might be useful in mixed systems. ... 

Thus it is evident that the modified polyethylenimines provide a matrix for achieving homogeneous catalysis of decarboxylation of activated anionic substrates in an aqueous environment. Clearly, the modified polyethylenimines provide solvation features that stabilize the anionic transition structure in the state with particularly sensitive bonds. Large solvent effects have been observed in kinetic studies of many reactions involving anions.46 47,50,51 Suitable derivatives of polyethylenimine should manifest interesting effects in many of these reactions also. 

The electro-synthetic reactions of activated alkenes involve carbon-carbon bond formation, which, after much controversy, is now believed generally to involve radical-anion/radical anion coupling rather than the alternative radical-anion/substrate reaction. The history of this mechanistic debate is well documented168. 

The most common mechanism of nucleophilic vinylic substitution163 is the two-step process of Equation (48) shown for the reaction of an anionic nucleophile with a vinylic substrate activated by one... 

IM-IM cooperation. It is difficult to establish the IM-IM" cooperation in poly(4(5)-vinylimidazole), since pK2 value is more than 14.5 (74). The IM-IM" cooperation can be observed in hydrolyses of PNPA(5) and 4-acetoxy-3-nitrobenzoic acid [n = 0 in (8)] by poly(5(6)-vinylbenzimidazole), of which pK2 value is estimated to be 12.7 (26). The hydrolyses rates increase sharply as the pH approaches the pK2 value of the polymer. In contrast, such a drastic enhancement in the rate is not observed in the benzimidazole-catalysed hydrolysis, nevertheless, pK2 value of the benzimidazole being 12.7. N-Alkylated polymers such as poly(N-vinylimidazole), poly(NIM), and poly(2-methyl-N-vinylimidazole), poly(2MIM), which have no anionic form in imidazole moieties, show the hydrolytic activities slightly less than that of poly(4(5)-vinylimidazole). In cases of low molecular weight analogues, such as N-methylimidazole and imidazole, the hydrolytic activity of the former is about 75% of the activity of imidazole (75, 76). These phenomena appear to support the IM-IM" cooperation in poly(4(5)-vmylimidazole) indirectly. In the hydrolyses of NABS(l) and NABA(3) by poly(5(6)-vinylbenzimidazole), the activity enhancement is about 50-fold compared with the activity of benzimidazole, in spite of these anionic substrates being used (23, 26). 

Both the coupling of proton uptake to anion binding and the surprising low-temperature structural instability of the active site complex may mimic abstraction of the hydroxylic proton from a coordinated substrate molecule during turnover. Ionization of a coordinated alcohol by a base in the active site would be an important step for substrate activation in the catalytic mechanism. The nonturnover reactions serve as models for this process, giving us clearer insight into the catalytic reaction by mapping out an intrinsic proton transfer coordinate in the active site. 

Arginine, which is a common active site residue in enzymes that act on anionic substrates, like pyruvate kinase, can be chemically modified more or less specifically by dicarbonyl compounds such as butandione (Riordan 1979) or phenylglyoxal (Takahashi 1968). 

The classic SsAi substitution of activated aryl halides by protected cyanohydrin anions provides substituted benzophenones. Another procedure for the arylation of protected cyanohydrin anions involves the use of aromatic substrates activated as their rr-chromium tricarbonyl complexes. - Addition of the anion of (32) to the 1,3-dimethoxybenzene complex, for example, leads principally to the meta-suh-stituted isomer (33 equation 20). Preferential meta regioselectivity is also noted with other -rr-chromium tricarbonyl complexes of arenes. Other arylations of cyanohydrin anions include interesting but synthetically limited additions at the a-position of quinoline N-oxides. In a similar manner, cyanohydrin carbonates of aromatic aldehydes react with A -oxides of quinoline and isoquinoline. ... 

Many structurally refined enzymes have been found to contain more than one metal ion at the active site. These metal ions are often bridged by an anionic substrate. 

In the extreme, the proton affinities of the enolate anion and active site donor could be the same, depending on the identities of the substrate and active site hydrogen bond donor. 

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