Intramolecular Catalysis of Sulfate Ester Hydrolysis. a Model for Aryl Sulfate Sulfohydrolase

The hydrolysis of 2-(4(5)-imidazolyl)phenyl sulfate (I) was studied as a possible model for aryl sulfate sulfohydrolase. The pH–rate profile may be divided into two regions: (1) at pH 1–3 the compound undergoes conventional A-1 acid-catalyzed hydrolysis, (2) at pH 4–7 the neighboring imidazole moiety catalyzes the reaction. The compound which would result from intramolecular nucleophilic attack by imidazole, 4(5)-(2′-hydroxyphenyl)imidazole N-sulfate, was synthesized and its solvolytic behavior compared with the initial substrate. Although trapping experiments designed to detect the intermediacy of the N-sulfate do not entirely exclude that mode of catalysis, the hydrolysis of I at pH 4–7 appears to involve mainly intramolecular general acid catalysis. Comparison of I with salicyl sulfate and other compounds capable of hydrogen bonding indicates rate accelerations can be rationalized in terms of transition state stability as manifested by ΔpKa for the corresponding phenolic products.