- Comparing isotope effects and rates for the methanolic sodium methoxide reactions of 9-r-fluorene to those for p-CF3C6H4CHClR (R = CH2Cl, CH2F and CF3)
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Kinetic primary isotope effects and activation parameters associated with the methanolic sodium methoxide-promoted dehydrohalogenation reactions of 9-(chloromethyl)fluorene and 9-(fluoromethyl)fluorene are reported and compared to for p-CF3C6H4CHClCH2Cl and p-CF3C6H4CHClCH2F. The element effect, kHCl/kHF = 4 at 25 °C, for the fluorenyl compounds is only a tenth of the value, kHCl/kHF = 54, obtained for the benzylic compounds. Also reported are the activation parameters for the methanolic sodium methoxide reactions of 9-(trifluoro)methylfluorene for deuterium exchange and dehydrofluorination, and are compared to those for p-CF3C6H4CHClCF3. In both cases the exchange reaction is faster than the elimination; however, in the fluorenyl system there is a large difference between the entropies of activation for exchange, ΔS≠ = -14 eu, and elimination, ΔS≠ = +10 eu; however, the ΔS≠ of 12-13 eu are the same for the benzylic systems. The eliminations for 9-(chloromethyl)fluorene and 9-(fluoromethyl)fluorene are 80 to 1,000 times faster than those for p-CF3C6H4CHClCH2X. The reactions of 9-(trifluoromethyl)fluorene are favored over p-CF3C6H4CHClCF3 by 200,000 for the dehydrofluorination and 1,000 for the exchange at 25 °C; however, due to the large differences in activation entropies, the exchange reaction is favored by 107 at -50 °C.
- Koch, Heinz F.,Pomerantz, William C.,Ruggles, Erik L.,Van Laren, Martijn,Van Roon, Anne-Marie
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- Stepwise and concerted solvolytic elimination and substitution reactions: E1 reaction via a primary carbocation
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Solvolysis of 9-(X-methyl)fluorene (1-X, X = I, Br, OBs) in 25 vol % acetonitrile in water gives the elimination product 9- methylidenefluorene and the substitution products 9- (hydroxymethyl)fluorene (1-OH) and 9-(acetamidomethyl)fluorene (1- NHCOMe). Kinetic studies of the corresponding ring-substituted compounds 2-X and 3-X show that the rate of elimination increases with increasing acidity of the substrate, Bronsted α > 0. The small kinetic deuterium isotope effects measured for the elimination reactions of the brosylates 1-OBs and 3-OBs, k(H)/k(D) = 2.0 ± 0.1 and 2.8 ± 0.1, respectively, suggest significant amounts of E1 reaction. The bimolecular reactions of the brosylates with added bases may be of irreversible E1cB type in contrast to the reactions of the halides which exhibit E2 reaction with added bases as well as with solvent water.
- Meng, Qingshui,Thibblin, Alf
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p. 4834 - 4840
(2007/10/03)
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