Structural effects and transition state acidities in the hydrolysis of 2-aryloxytetrahydrofurans by concerted proton transfer; shortcuts to the Brønsted coefficient

Abstract

Rates of hydrolysis of some substituted 2-aryloxytetrahydrofurans have been measured in dilute aqueous acids. Rate-limiting carbon–oxygen bond cleavage, concerted in some sense with proton transfer to the oxygen atom, is suggested for the mechanism of the reaction. Structural effects of the substituents on the phenoxy group reveal, from the Hammett reaction constant, that protonation is relatively more advanced than carbon–oxygen bond cleavage in the transition state. The substituent constant for para-substituents which tend to withdraw electrons by resonance is suggested to be a linear combination of σ and σ^– values, the combining factor being a Brønsted-like coefficient α. Transition state acidities were calculated using the rate constants for oxonium ion catalysed and uncatalysed reactions. Comparison of this acidity with that of the phenol corresponding to the leaving group in the reaction was suggested to give a Brønsted-like coefficient as well. Both treatments gave similar values for the α coefficient.