Decarboxylation medium effects

These hydrophobic ammonium ions exert a medium effect on spontaneous, unimolecular reactions. Tri-n-octylmethylammonium chloride effectively speeds decarboxylation of 5-nitrobenzisoxazole carboxylate ion (24) (Kunitake et al., 1980), and tri-n-octyl ethylammonium mesylate or bromide... 

Starting with the work of Lienhard and his coworkers there have been several contributions in the literature that indicate that the thiazolium catalyzed decarboxylation is significantly accelerated by the use of a low dielectric medium, leading to the suggestion that a major source of rate acceleration in enzymes that perform this reaction is provided by a medium effect. As estimated recently by Alvarez and coworkers , the enzyme accelerates the overall pyruvate decarboxylation reaction by a factor of 10, over and above the corresponding thiamin catalyzed rate. It was also estimated that the enzyme accelerates the decarboxylation step by a factor of 10. ... 

A second, related aspect of the medium effect is also likely to be important. Decarboxylation converts a polar, highly solvated -C02 group into a nonpolar, nearly unsolvated CO2. For the case of benzisoxazole-3-carboxylic acids, carboxyl desolvation has been demonstrated to be an important component of solvent catalysis of decarboxylation 113). Desolvation of the carboxyl group on binding to an enzyme may likewise be an important factor. Carbon isotope effects are consistent with this possibility (777). Oxygen isotope effects on formate dehydrogenase are also consistent with this mechanism 114). 

Acevedo, O., Jorgensen, W.L. Medium effects on the decarboxylation of a biotin model in pure and mixed solvents from QM/MM simulations. J. Org. Chem. 2006,71(13), 4896-902. 

Medium effects are observed in the oxidation of cinnamic acid by in aqueous acetic acid. The replacement of a hydroxyl hydrogen by an acetyl group makes the vanadium ion a better electrophile, thereby enhancing the rate. No free radicals were detected but the proposed rate-determining step involves the formation of a radical benzylic carbonium ion which immediately decarboxylates and then dimerizes to form the product ... 

T. Furukl, F. Hosokawa, M. Sakurai, Y. Inoue and R. Chujo, Microscopic medium effects on a chemical reaction. A theoretical study of decarboxylation catalyzed by cyclodextrins as an enzyme model, J. Am. Chem. Soc., 115 (1993) 2903. 

Su, Z., Mariano, P.S., Falvey, D.E., Yoon, U.C., and Oh, S.W., Dynamics of aniUnium radical a-heterolytic fragmentation processes. Electrofugal group, substituent and medium effects on desilylation, decarboxylation and retro-aldol cleavage pathways, /. Am. Chem. Soc., 120, 10676, 1998. 

Su, Z., Mariano, P.S., Falvey, D.E., Yoon, U.C,. and Oh, S.W, Dynamics of anihnium radical a-heterolytic fragmentation processes. Electrofugal group, substituent and medium effects on desi-lylation, decarboxylation and retro-Aldol cleavage pathways, /. Am. Chem. Soc., 120, 10676,1998. Hasegawa, E., Xu, W, Mariano, P.S., Yoon, U.C., and Kim, J.U., Electron transfer induced photoadditions of the silylamine EtjNCHjTMS to a,P-unsaturated cyclohexenones. Dual reaction pathways based on ion pair selective cation radical chemistry, /. Am. Chem. Soc., 110, 8099, 1988. Yoon, U.C., Kim, J.U., Hasegawa, E., and Mariano, P.S., Electron transfer photochemistry of a-silylamine cyclohexenone systems. Medium effects on reaction pathways followed, /. Am. Chem. Soc., 109, 4421,1987. 

This interesting reaction is especially useful for the synthesis of medium- and large-ring compounds from dicarboxylic esters, and is effective for ring sizes that cannot be made by the Dieckmann condensation or decarboxylation (Section 18-1 OB). Radical anions formed by addition of sodium to the ester... 

Proteinoids were tested after being stored in the dry state for 5 to 10 years. Acidic proteinoids effective in catalyzing the hydrolysis of p-nitrophenyl acetate showed the same levels of activity as observed 10 years earlier19>. Lysine-rich proteinoids which catalyzed the decarboxylation of oxaloacetic acid were found to be insoluble in assay medium after 5 years of storage in the dry state. Their activity, however, had increased by 32 to 145%, and the activity of the lysine-rich proteinoids was largely associated with the insoluble portion 19). 

More recently, Mariano and coworkers have investigated the effects of substituents and medium on desilylation, decarboxylation, and retro-aldol cleavage reactions... 

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