Electrostatic catalysis repulsion

Irradiation with UV light isomerized the azobenzene units from the trans to the cis form, while the reverse isomerization occurred thermally in the dark. The cis to trans conversion is catalyzed by both protons and hydroxyl ions. Hence, the catalyzed dark process for tethered azobenzene is greatly modified in comparison with that for free azobenzene. For the tethered azobenzene, beginning at pH 6, the cis to trans return rate sharply decreased with increasing pH up to 10, whereas the rate for free azobenzene rapidly increased in the same pH range owing to OH- catalysis. These observations can be explained by the electrostatic repulsion which lowers the local OH concentration on the polyion surface below that in the bulk aqueous phase. 

Phosphorylation of an enzyme can affect catalysis in another way by altering substrate-binding affinity. For example, when isocitrate dehydrogenase (an enzyme of the citric acid cycle Chapter 16) is phospho-rylated, electrostatic repulsion by the phosphoryl group inhibits the binding of citrate (a tricarboxylic acid) at the active site. 

The foUowing condusions may be obtained from these results (1) The catalytic efficiency increases with pH, because of the increaang involvement of the benzimidazole anion. Lower pK j value of the benzimidazole group is effective in this reject However, the benzimidazole anion is not a good nudeophile toward the anionic substrate NABS because of electrostatic repulsion. (2) Polyvinylbenzimidazole 2 is a better catalyst than benzimidazole. Although this was daimed to be due to the bifunctional catalysis of the pdyn r, substrate binding and microenvironmental effects cannot be neglected. 

On the other hand, phosphorane intermediates are not expected to be involved in the hydrolysis of phosphate monoesters, so the effective observed catalysis by the carboxyl group of salicyl phosphate 3.21 [51] (Scheme 2.26) is presumed to be concerted vith nucleophilic attack. (The hydrolysis reaction involves the less abundant tautomer 3.22 of the dianion 3.21, and the acceleration is >10 -fold relative to the expected rate for the pH-independent hydrolysis of the phosphate monoester dianion of a phenol of pK 8.52.) However, this system differs from the methoxy-methyl acetals discussed above, in that there is a clear distinction between neutral nucleophiles, which react through an extended transition structure similar to 3.16 in Scheme 2.23, and anions, which do not react at a significant rate, presumably because of electrostatic repulsion. This distinction is well-established for the dianions of phosphate monoesters with good leaving groups (p-nitrophenyl [52] and... 

The catalysis of P4VP-Cu complexes during the oxidation of such substrates as 3,4-dioxycinnamic, salicylic and ascorbic acids and substituted phenols has been discussed [93]. The rate of oxidation of the acids depends on the pH of the medium, its maximum being attained at pH 2.5, 3.0 and 4.2, respectively. A maximum is reached at these pH values because at these values, the protonation of uncomplexed nitrogen atoms occurs. Moreover, the electrostatic interaction between the polyelectrolyte and the anionic substrate raises the nitrogen concentration near the active center. In the case of a positively charged substrate such as paraphenylene diamine, however, the oxidation rate sharply drops upon an increase in the solution acidity due to the electrostatic repulsion of the like-charged polyelectrolyte and substrate. 

Phase transfer catalyzed reactions in which ylides are formed from allylic and ben-zylic phosphonium ions on cross-linked polystyrenes in heterogeneous mixtures, such as aqueous NaOH and dichloromethane or solid potassium carbonate and THF, are particularly easy to perform. Ketones fail to react under phase transfer catalysis conditions. A phase transfer catalyst is not needed with soluble phosphonium ion polymers. The cations of the successful catalysts, cetyltrimethylammonium bromide and tetra-n-butylammonium iodide, are excluded from the cross-linked phosphonium ion polymers by electrostatic repulsion. Their catalytic action must involve transfer of hydroxide ion to the polymer surface rather than transport of the anionic base into the polymer. Dicyclohexyl-18-crown-6 ether was used as the catalyst for ylide formation with solid potassium carbonate in refluxing THF. Potassium carbonate is insoluble in THF. Earlier work on other solid-solid-liquid phase transfer catalyzed reactions indicated that a trace of water in the THF is necessary (40). so the active base for ylide formation is likely hydrated, even though no water is included deliberately in the reaction mixture. 

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