Mechanistic studies in strongly basic media. Part IX. Patterns of reactivity of sodium salts of 9-substituted fluorenes with molecular oxygen and with acrylic esters

Abstract

The kinetics of autoxidation and of Michael addition to methyl acrylate, methacrylate, and crotonate of sodium salts of three 9-substituted fluorenes, FIHX (X = CN, CO₂Me, or SO₂Ph) in t-butyl alcohol solution have been studied. The effect on the rate of changing the concentration of free sodium ions in the solution, by dilution and by addition of the common-ion salts sodium perchlorate and sodium tetraphenylborate, has been examined in an attempt separately to evaluate the reactivities of the free and associated carbanions. In autoxidation reactions, the kinetic effect is small and the common-ion effect cannot be distinguished with certainty from the medium effect of the added salt. Much larger effects arise in t-butyl alcohol containing dimethyl sulphoxide and these suggest that, for sodium 9-phenylsulphonylfluorenide, the free carbanion is more reactive than the ion pair, while, for sodium 9-cyanofluorenide, the reverse is the case. In Michael additions in which methyl acrylate is the acceptor or in which 9-cyanofluorene is the addend, no common-ion effect is detectable, suggesting that free and paired anions are equally reactive in these systems. In all other cases studied, the free ion was 10–100 times more reactive than the ion-pair. Some of the more important factors relevant to the interpretation of this complex pattern of behaviour are indicated.