Kinetics of the enolisation reactions of 3-acetyl-2,5-dimethylfuran and of 2-acetylselenophene

Article abstract of DOI:10.1002/(SICI)1099-0690(199809)1998:9<1867::AID-EJOC1867>3.0.CO;2-M

Rate constants for the enolisation reactions of title compounds have been measured by their rates of halogenation at 25 °C in water, in several buffers, in dilute hydrochloric acid, in dilute sodium hydroxide, and in the presence of some metal ion salts. The results have been compared with those previously obtained from the corresponding reactions of acetophenone and a number of other acetyl heterocycles. Electronegativity of the heteroatoms and the "π-excessive" nature of the heterocyclic rings appear to be the main factors determining the relative reactivities in the acid-catalysed reactions. Bronsted β values and isotope effects, k_H/k_D, point to a more symmetrical transition state for the investigated acetyl heterocycles than that for acetophenone in the general base-catalysed reaction. Metal-activating factors (MAF), i.e. the catalytic constant for metal-ion (Cu²⁺, Zn²⁺, and Ni²⁺) catalysis, k_M, relative to that for proton catalysis, k_H, are discussed as an empirical measure of the "hard or soft" character of the carbonyl groups in acyl heterocycles.