UNSTABLE CARBANIONS - GENERAL ACID CATALYSIS OF THE CLEAVAGE OF 1-PHENYLCYCLOPROPANOL AND 1-PHENYL-2-ARYLCYCLOPROPANOL ANIONS

Cleavage of the anion of 1-phenylcylopropanol to 1-phenylpropanone in aqueous solution, a reverse aldol-type reaction, is subject to general acid catalysis by protonated tertiary amines with α=0.25, a primary deuterium (discrimination) isotope effect of KH/KD= 1.9 ± 0.2 for quinuclidine-H+ and water as proton donors to carbon, and a kinetic isotope effect of kOH/kOD= 1.22± 0.05. 1-Phenyl-2-arylcyclopropanols cleave 105 times faster and show general acid catalysis with a and an inverse kinetic isotope effect of KOH/KOD= 0.7±⪕ 0.1 that indicate less proton transfer in the transition state. The Hammett p values for a small series of the cis and trans compounds are 5.0 and 4.0, respectively. It is concluded that these SE2 displacements and some related reactions proceed through preassociation and concerted mechanisms that are determined by the short lifetimes of carbanion “intermediates”. © 1979, American Chemical Society. All rights reserved.