Competing solvolytic elimination and substitution reactions via very short-lived ion-pair intermediates

Abstract

The solvolysis of 9-methyl-9-(2-X-2-propyl)fluorene (X = Cl, Br, or OOCCF₃) (1-X) in aqueous acetonitrile or aqueous 1,1,1-trifluoroethanol (TFE) yields the alkene, 9-methyl-9-(propen-2-yl)fluorene (2), and the alcohol, 9-methyl-9-(2-hydroxy-2-propyl)fluorene (1-OH), accompanied by a small amount of substitution product from the reaction with the organic component of the solvent mixture. The fraction of elimination product increases with decreasing fraction of water in the solvent mixture as well as by addition of general bases, which can be expressed by a Brønsted parameter of β = 0.07 for the reaction of 1-Cl, measured with substituted acetate anions. Also, the addition of chloride ions increases the fraction of alkene. The kinetic deuterium isotope effects on the reactions of the hexadeuterated substrates vary with solvent composition in a way which is not consistent with a common carbocation intermediate which has time to choose between dehydronation and addition of a solvent water molecule. A stepwise preassociation mechanism is proposed for the elimination reaction.