19326-47-7Relevant academic research and scientific papers
Deprotonation of Ion Pairs in Aqueous Solvent. Competing Bimolecular and Solvolytic Elimination Reactions
Thibblin, Alf
, p. 2071 - 2076 (2007/10/02)
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 (k12H + k13H + k14H)/(k12d6 + k13d6 + k14d6) = 2.2 +/- 0.1, which is composed of the isotope effect k12H/k12d6 = 1.4 +/- 0.1 for formation of the substitution product 2 and k14H/k14d6 = 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 k13H/k13D of 8.1.
Studies of the Borderline between Concerted and Stepwise Mechanisms of Elimination : E1cB Elimination of Fluoren-9-ylmethyl Carboxylate Esters
O'Ferrall, Rory A. More,Larkin, Finbar,Walsh, Peter
, p. 1573 - 1580 (2007/10/02)
Rates of β-elimination of carboxylate leaving groups from fluoren-9-ylmethyl carboxylate esters in methanolic sodium methoxide at 25 deg C are reported.An E1cB mechanism with rate-determining formation of a carbanion intermediate is assigned on the basis of near identity of measured elimination rates and rates of carbanion formation predicted from a Taft correlation, and the similarity with elimination of 1-(1-acetoxy-1-methylethyl)indene for which the mechanism has been established by Ahlberg and Thibblin.Values of ρ=0.42 and βlg=0.27 measured for substituted benzoate leaving groups are a little larger than expected (ca. 0.24 and 0.18, respectively) and the discrepancy is tentatively ascribed to conformational enhancement of remote substituent effects, rather than to a contribution of E2 elimination.The effects of alkyl and aryl substitution α to the leaving group are discussed, especially in relation to the borderline between concerted and stepwise mechanisms.The measurements fail to confirm an earlier inference that the borderline shows a discontinuity in transition-state structure at the point of mechanistic change.
