Deprotonation of Ion Pairs in Aqueous Solvent. Competing Bimolecular and Solvolytic Elimination Reactions

Article abstract of DOI:10.1021/ja00241a026

Solvolysis of 9-(2-chloro-2-propyl)fluorene (h-1-Cl) or 9-(2-bromo-2-propyl)fluorene (h-1-Br) in 25 vol percent acetonitrile in water at 25 deg C 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 Broensted parameter for deprotonation of the carbocationic intermediate with substituted acetate anions is small, β = 0.05.The deuteriated analogue 9-(1,1,1,3,3,3-2H6)-9-(2-chloro-2-propyl)fluorence (d₆-1-Cl) reacts slower than h-1-Cl.Thus, the overall kinetic isotope effect was measured without base as (k₁₂^H + k₁₃^H + k₁₄^H)/(k₁₂^d6 + k₁₃^d6 + k₁₄^d6) = 2.2 +/- 0.1, which is composed of the isotope effect k₁₂^H/k₁₂^d6 = 1.4 +/- 0.1 for formation of the substitution product 2 and k₁₄^H/k₁₄^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 indicted by the observation that the fraction of elimination increases with rising basicity of the leaving group.The intermediate shows very small discrimination between the nucleophiles azide anion, mathanol, and water.The presence of strong base in the reaction solution opens up a parallel bimolecular elimination route (E1cB or E2) for 1-Cl.Thus 0.16M sodium hydroxide yields mainly 3 but the rates of formation of the solvolysis products 2 and 4 are decreased only slightly.The (9-2H) analogue d-1-Cl, which solvolyzes 3percent slower than h-1-Cl, undergoes hydroxide-promoted elimination with an isotope effect k₁₃^H/k₁₃^D of 8.1.