Acid-Catalyzed Dehydration of Naphthalene-<i>cis-</i>1,2-dihydrodiols: Origin of Impaired Resonance Effect of 3-Substituents

Acid-catalyzed dehydrations of substituted naphthalene-<i>cis</i>-1,2-dihydrodiols occur with loss of the 1- or 2-OH group to form 2- and 1-naphthols, respectively. Effects of substituents MeO, Me, H, F, Br, I, and CN at 3-, 6-, and 7-positions of the naphthalene ring are consistent with rate-determining formation of β-hydroxynaphthalenium ion (carbocation) intermediates. For reaction of the 1-hydroxyl group the 3-substituents are correlated by the Yukawa–Tsuno relationship with ρ = −4.7 and <i>r</i> = 0.25 or by σ_p constants with ρ = −4.25; for reaction of the 2-hydroxyl group the 3-substituents are correlated by σ_m constants with ρ = −8.1. The correlations for the 1-hydroxyl imply a surprisingly weak resonance interaction of +M substituents (MeO, Me) with a carbocation reaction center but are consistent with the corresponding correlation for acid-catalyzed dehydration of 3-substituted benzene-<i>cis</i>-1,2-dihydrodiols for which ρ = −6.9 and <i>r</i> = 0.43. Substituents at the 6- and 7-positions of the naphthalene rings by contrast are correlated by σ⁺ with ρ = −3.2 for reaction of the 1-hydroxyl group and ρ = −2.7 for reaction of the 2-hydroxyl group. The unimpaired resonance implied by these substituent effects appears to be inconsistent with a previous explanation of the weak resonance of the 3-substituents in terms of imbalance of charge development and/or nonplanarity of the benzenium ring in the transition state. An alternative possibility is that the adjacent hydroxyl group interferes sterically with conjugation of +M substituents. “Hyperaromaticity” of the arenium ion intermediates does not appear to be a factor influencing this behavior

Base-Catalyzed Dehydration of 3‑Substituted Benzene <i>cis</i>-1,2-Dihydrodiols: Stabilization of a Cyclohexadienide Anion Intermediate by Negative Aromatic Hyperconjugation

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 <i>cis</i>-1,2-dihydrodiol to phenol. Rate constants for 3-substituted benzene <i>cis</i>-dihydrodiols are correlated by σ^– values with ρ = 3.2. Solvent isotope effects for the reactions are <i>k</i>_H₂O/<i>k</i>_D₂O = 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 <i>cis</i>-1,2-dihydrodiol was not found, and thus, the proton transfer step is subject to internal return. A rate constant of ∼10¹¹ 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 10¹¹–10¹² s^–1. From these limiting values and the measured rate constant for hydroxide-catalyzed dehydration, a p<i>K</i>_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

Base-Catalyzed Dehydration of 3-Substituted Benzene cis-1,2-Dihydrodiols : Stabilization of a Cyclohexadienide Anion Intermediate by Negative Aromatic Hyperconjugation

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 values with rho = 3.2. Solvent isotope effects for the reactions are k(H2O)/k(D2O) = 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 similar to 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