Base-catalyzed dehydration of 3-substituted benzene cis-1,2-dihydrodiols : stabilization of a cyclohexadienide anion intermediate by negative aromatic hyperconjugation

Summary, in English

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.

• Author: Jaya Satyanarayana Kudavalli; S Nagaraja Rao; David E Bean; Narain D Sharma; Derek R Boyd; Patrick W Fowler; Scott Gronert; Shina Caroline Lynn Kamerlin; James R Keeffe; Rory A More O'Ferrall
• Publishing year: 2012-08-29
• Language: English
• Pages: 69-14056
• Publication/Series: Journal of the American Chemical Society
• Volume: 134
• Issue: 34
• Links: http://dx.doi.org/10.1021/ja304366j
• Document type: Journal article
• Publisher: The American Chemical Society (ACS)
• Keywords: Anions/chemistry; Benzene/chemistry; Catalysis; Cyclohexenes/chemistry; Kinetics; Phenols/chemistry; Water/chemistry
• Status: Published
• ISBN/ISSN/Other: ISSN: 1520-5126