Reactions of carbonyl compounds in basic solutions. Part 26.1 The mechanisms of the base-catalysed cyclisation of 1,2-diacetylbenzene, 1,8-diacetylnaphthalene, 4,5-diacetylphenanthrene and 2,2′-diacetylbiphenyl

Abstract

Rate coefficients have been measured for the base-catalysed cyclisation of 1,2-diacetylbenzene 1, 1,8-diacetylnaphthalene 2, 4,5-diacetylphenanthrene 3 and 2,2′-diacetylbiphenyl 4 in a range of aqueous dimethyl sulfoxide (DMSO) compositions at 30.0 °C, as well as at 45.0 and 60.0 °C at certain concentrations, containing tetramethylammonium hydroxide. For three of the substrates, 1, 2 and 4, the di-[²H₃]acetyl compounds were also studied. Studies of the relative rates, activation parameters, kinetic isotope and solvent isotope effects and correlation of the rates with an acidity function were made to give detailed mechanistic pathways. Thus, 2, 3 and 4 react with rate-determining base-catalysed ionisation to give the enolate anion, followed by relatively rapid intramolecular nucleophilic attack and dehydration to give the corresponding cyclic enones; whereas, 1 reacts by the same pathway but with rate-determining intramolecular nucleophilic attack. An intermolecular base-catalysed aldol reaction was also detected for the product of the intramolecular reaction of 2 at high DMSO contents.