Studies in heterocyclic chemistry. Part IV. Kinetics and mechanisms for the hydrolysis of benzoxazoles

Abstract

Under acidic conditions simple benzoxazoles hydrolyse principally to the corresponding amidophenols. For benzoxazole and 2-methylbenzoxazole the reactions show acid catalysis for solutions of low acidity but retardation at higher acidities corresponding to a change in rate-determining step from nucleophilic attack on the conjugate acid to fission of the ring C–O bond in the resultant tetrahedral intermediate. A related pathway is observed for 2-trifluoromethylbenzoxazole but the different dependence on pH is attributed to a change as the pH increases from nucleophilic attack on the conjugate acid to attack on the free base and finally to attack by hydroxide ion. The retardation of hydrolysis of 1,2-dimethylbenzoxazolium perchlorate with increasing acidity follows expectation for attack by hydroxide ion and water and confirms that in contrast with other imidates, for benzoxazoles product formation may become rate determining at low pH.