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| - Efficient Intramolecular General Acid Catalysis of NucleophilicAttack on a Phosphodiester
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| Abstract
| - The hydrolysis of methyl 8-dimethylamino-1-naphthyl phosphate 4 and its reactions with arepresentative range of nucleophiles are catalyzed by the dimethylammonium group at acidic pH with rateaccelerations of the order of 106. The reaction persists up to pH 7 because the strong intramolecularhydrogen bond, which is the key to efficient general acid catalysis, is present also in the reactant. Thesensitivity to the basicity of the nucleophile (Brønsted βnuc = 0.29) lies between values measured previouslyfor mono- and triesters. The comparisons suggest that general acid catalyzed reactions of phosphate mono-or diesters with strongly basic oxyanion nucleophiles (like those derived from a serine oxygen or a boundwater molecule in an enzyme active site) will be fastest when their negative charges are neutralized byprotonation. Reactions with NH2OH and its N-methylated derivatives show an apparent α-effect, but NH2OMe reacts no faster than a primary amine of similar basicity. It is suggested that the reaction involvingNH2OH as an oxygen nucleophile proceeds through the pre-equilibrium formation of the tautomer H3N+O-as the active nucleophile: ab initio calculations support this idea.
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