Deprotonation of ion pairs in aqueous solvent. Competing bimolecular and solvolytic elimination reactions

Solvolysis of 9-(2-chloro-2-propyl)fluorene (h-1-Cl) or 9-(2-bromo-2-propyl)fluorene (h-1-Br) in 25 vol % acetonitrile in water at 25C produces 9-(2-propenyl)fluorene (h-4), 9-(2-hydroxy-2-propyl)fluorene (h-2), and a trace of 9-isopropylidenefluorene (h-3). The corresponding acetate 9-(2-acetoxy-2-propyl)fluorene (h-1-OAc) yields the same products but comparable amounts of 3 and 4. The presence of base as well as chloride or bromide ions increases the fraction of elimination products but does not increase k12 + k14. The Bronsted parameter for deprotonation of the carbocationic intermediate with substituted acetate anions is small, approx. = 0.05. The deuteriated analogue 9-(1,1,1,3,3,3,-SH6)-9-(2-chloro-2-propyl)fluorene (d6-1-Cl) reacts slower than h-1-Cl. Thus, the overall kinetic isotope effect was measured without base as (k12/sup H/ + k13/sup H/ + k14/sup H/)/(k12/sup d6/ + k13/sup d6/ + k14/sup d6/) = 2.2 +/- 0.1, which is composed of the isotope effect k12/sup H//k12/sup d6/ = 1.4 +/- 0.1 for formation of the substitution product 2 and k14/sup H//k14/sup d6/ = 3.7 +/- 0.3 for production of the olefin 4. The results indicate a branched mechanism involving rate-limiting formation of a common contact ion pair (with a secondary isotope effect of 2.2) which either undergoes nucleophilic attack by water (isotope effect 1.0) or is deprotonated by water (isotope effect 2.8) or other general base (including chloride and bromide anions). Direct elimination from the ion pairs is indicated by the observation that the fraction of elimination increases with rising basicity of the leaving group.