title: Base-catalyzed dehydration of 3-substituted benzene cis-1,2-dihydrodiols: stabilization of a cyclohexadienide anion intermediate by negative aromatic hyperconjugation.
abstract: Evidence that a 1,2-dihydroxycyclohexadienide anion is stabilized by aromatic "negative hyperconjugation" is described. It complements an earlier inference of "positive" hyperconjugative aromaticity for the cyclohexadienyl cation. The anion is a reactive intermediate in the dehydration of benzene cis-1,2-dihydrodiol to phenol. Rate constants for 3-substituted benzene cis-dihydrodiols are correlated by σ(-) values with ρ = 3.2. Solvent isotope effects for the reactions are k(H(2)O)/k(D(2)O) = 1.2-1.8. These measurements are consistent with reaction via a carbanion intermediate or a concerted reaction with a "carbanion-like" transition state. These and other experimental results confirm that the reaction proceeds by a stepwise mechanism, with a change in rate-determining step from proton transfer to the loss of hydroxide ion from the intermediate. Hydrogen isotope exchange accompanying dehydration of the parent benzene cis-1,2-dihydrodiol was not found, and thus, the proton transfer step is subject to internal return. A rate constant of ~10(11) s(-1), corresponding to rotational relaxation of the aqueous solvent, is assigned to loss of hydroxide ion from the intermediate. The rate constant for internal return therefore falls in the range 10(11)-10(12) s(-1). From these limiting values and the measured rate constant for hydroxide-catalyzed dehydration, a pK(a) of 30.8 ± 0.5 was determined for formation of the anion. Although loss of hydroxide ion is hugely exothermic, a concerted reaction is not enforced by the instability of the intermediate. Stabilization by negative hyperconjugation is proposed for 1,2-dihydroxycyclohexadienide and similar anions, and this proposal is supported by additional experimental evidence and by computational results, including evidence for a diatropic ("aromatic") ring current in 3,3-difluorocyclohexadienyl anion.
authors: School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland.
authors: Jaya Satyanarayana
authors: JS
authors: Kudavalli
authors: S Nagaraja
authors: SN
authors: Rao
authors: David E
authors: DE
authors: Bean
authors: Narain D
authors: ND
authors: Sharma
authors: Derek R
authors: DR
authors: Boyd
authors: Patrick W
authors: PW
authors: Fowler
authors: Scott
authors: S
authors: Gronert
authors: Shina Caroline Lynn
authors: SC
authors: Kamerlin
authors: James R
authors: JR
authors: Keeffe
authors: Rory A
authors: RA
authors: More O'Ferrall
mesh_terms: D000838:Anions / Q000737:chemistry*; D001554:Benzene / Q000737:chemistry*; D002384:Catalysis; D053138:Cyclohexenes / Q000737:chemistry*; D007700:Kinetics; D010636:Phenols / Q000737:chemistry; D014867:Water / Q000737:chemistry
chemical_list: D000838:Anions; D053138:Cyclohexenes; D010636:Phenols; D014867:Water; D001554:Benzene