Borderline betwwen E1cB and E2 Mechanisms. Elimination of HCl from Fluorene Derivatives

Article abstract of DOI:10.1021/ja00222a015

The base-promoted elimination of HCl from 9-(2-chloro-2-propyl)fluorene (2-Cl) exhibits a kinetic deuterium isotope effect that varies with base strenth and solvent character, from a maximum of k^H/k^D = 8.1 with HO^- in 25 vol percent acetonitrile in water to ca. 3 in pyridine (neat) at 25 gradC.The Broensted parameter was measured in methanol with substituted quinuclidine bases as β = 0.5.The large variation in isotope effect could be the result of a varying degree of internal return from a tightly hydrogen-bonded carbanion.However, analysis of β as a function of substrate acidity, leaving group,,and α-substituents suggest that the elimination of HCl from the fluorene derivatives is of E2 type.For example, a change in leaving group from AcO^- to Cl^- corresponds to a decrease in β from 0.73 to 0.56 for the 9-(X-methyl)fluorene (3-X) series.It is concluded that the reaction coordinate has a relatively large horizontal component corresponding to proton transfer.