Cationic nitrogen, enhancing

Mobile-phase additives can also influence the relative abundance of the various adduct ions. Karlsson [105] performed post-column addition of alkali cations to enhance ESI-MS of carbohydrates and other compormds without nitrogen atoms. For most analytes, the adduct formation increased with the size of the cation. Optimum concentration of the cation in the solution was ca. 5x10 mol/1. Alkali-metal affituties and alkali-metal influence on fragmentation in MS-MS have been studied by others as well [106-107]. 

As shown in Fig. 7, the first step involved in the decarboxylation of an alpha-keto acid by thiamine pyrophosphate is the addition of the carbonyl by the carbanion nucleophile, followed by decarboxylation of the acid. As reviewed by Jencks (1975), investigations of the role of thiamine pyrophosphate analogs in the pyruvate dehydrogenase-catalyzed decarboxylation of pyruvate have provided evidence for rate enhancement by a desolvation effect. This effect comes about because of the removal of both the carboxylate group of the substrate and the cationic nitrogen of the coenzyme from an aqueous environment to the hydrophobic active site of the enzyme. The bulky and apparently chemically nonfunctional pyrophosphate and pyrimidine moieties of the... 

Cobalt trifluoride fluorination corresponds to the electron-transfer mechanism via a radical cation. RF groups attached to the ring enhance the stability of intermediate dienes and monoenes. Perfluoroalkyl pyridines, pyrazines, and pyrimidines were successfully fluorinated but pyridazines eliminated nitrogen. The lack of certain dienes was attributed to the difference in stability of FC=C and RFC=C and steric effects [81JCS(P1)2059]. 

A group with a more powerful (electron-withdrawing) inductive effect, e.g. NOa, is found to have rather more influence. Electron-withdrawal is intensified when the nitro group is in the o- or p-position, for the interaction of the unshared pair of the amino nitrogen with the delocalised it orbital system of the benzene nucleus is then enhanced. The neutral molecule is thus stabilised even further with respect to the cation, resulting in further weakening as a base. Thus the nitro-anilines are found to have related p a values ... 

Only Cram (36) has published a rationale for the very high (99%) enantiomeric excess achieved in the reaction of methyl vinyl ketone and the hydrindanone in the presence of the chiral crown ether. This mechanism envisions a bimolecular complex comprising the potassium cation and chiral host as one entity and the enolate anion of the hydrindanone as the counterion. Methyl vinyl ketone lies outside this complex. The quinine-catalyzed reaction appears to have a termo-lecular character, since the hydroxyl of the alkaloid probably hydrogen bonds with the methyl vinyl ketone—enhancing its acceptor properties—while the quin-uclidine nitrogen functions as the base forming the hydrindanone—alkaloid ion pair. 

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