57645-02-0Relevant academic research and scientific papers
Deprotonation of Ion Pairs in Aqueous Solvent. Competing Bimolecular and Solvolytic Elimination Reactions
Thibblin, Alf
, p. 2071 - 2076 (1987)
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.
COMPETING SOLVOLYTIC SUBSTITUTION AND ELIMINATION REACTIONS VIA A COMMON IRREVERSIBLY FORMED ION-MOLECULE PAIR INTERMEDIATE
Sidhu, Harvinder,Thibblin, Alf
, p. 578 - 584 (2007/10/02)
The acid-catalysed solvolysis of 9-(2-phenoxy-2-propyl)fluorene in mixtures of water with acetonitrile or methanol at 25 deg C provides 9-(2-hydroxy-2-propyl)fluorene, 9-(2-propenyl)fluorene, and 9-(2-acetamido-2-propyl)fluorene or 9-(2-methoxy-2-propyl)fluorene, respectively.The overall kinetic deuterium isotope effects for the reactions of the hexadeuterated analogue 9-(1,1,1,3,3,3-2H6)-9-(2-phenoxy-2-propyl)fluorene in 90 vol.percent acetonitrile in water were measured as (kEH+kSH)/(kED6+kSD6)=1.54+/-0.05, which is composed of the isotope effect kSH/kSD6=1.4+/-0.1 for formation of the substitution products and kEH/kED6=4.0+/-0.2 for production of 9-(2-methoxy-2-propyl)fluorene.Similar isotope effects were measured in other solvent mixtures.The results strongly indicate a branched mechanism involving rate-limiting formation of a common carbocation-molecule pair (with a secondary isotope effect of 1.54), which either undergoes nucleophilic attack by a solvent molecule (with a secondary isotope effect of ca 1), or is dehydronated (isotope effect ca 2.8) by the leaving group or by the solvent.The ion-molecule pair shows very low selectivities.Thus, in 50 vol.percent acetonitrile in water, an acetonitrile molecule is as efficient as a water molecule as a nucleophile towards the ion-molecule pair, kMeCN/kHOH>/=1 (ratio of second-order rate constants).The discrimination between methanol and water is anomalously small, kMeOH/kHOH=0.7.
