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

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• 作者：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鈥橣errall
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：August 29, 2012
• 年：2012
• 卷：134
• 期：34
• 页码：14056-14069
• 全文大小：537K
• 年卷期：v.134,no.34(August 29, 2012)
• ISSN：1520-5126

文摘

Evidence that a 1,2-dihydroxycyclohexadienide anion is stabilized by aromatic 鈥渘egative hyperconjugation鈥?is described. It complements an earlier inference of 鈥減ositive鈥?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 蟽^{鈥?/sup> values with 蟻 = 3.2. Solvent isotope effects for the reactions are k_H2O/k_D2O = 1.2鈥?.8. These measurements are consistent with reaction via a carbanion intermediate or a concerted reaction with a 鈥渃arbanion-like鈥?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 1011 s鈥?, 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鈥?012 s鈥?. From these limiting values and the measured rate constant for hydroxide-catalyzed dehydration, a pK_a of 30.8 卤 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 (鈥渁romatic鈥? ring current in 3,3-difluorocyclohexadienyl anion.}