Base-catalyzed dehydration of 3-substituted benzene cis-1,2-dihydrodiols : stabilization of a cyclohexadienide anion intermediate by negative aromatic hyperconjugation
Kudavalli, Jaya Satyanarayana ; Rao, S Nagaraja ; Bean, David E ; Sharma, Narain D ; Boyd, Derek R ; Fowler, Patrick W ; Gronert, Scott ; Kamerlin, Shina Caroline Lynn LU
; Keeffe, James R
and More O'Ferrall, Rory A
(2012)
In Journal of the American Chemical Society
134(34).
p.69-14056
- 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,... (More)
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.
(Less)
- author
- Kudavalli, Jaya Satyanarayana
; Rao, S Nagaraja
; Bean, David E
; Sharma, Narain D
; Boyd, Derek R
; Fowler, Patrick W
; Gronert, Scott
; Kamerlin, Shina Caroline Lynn
LU
; Keeffe, James R
and More O'Ferrall, Rory A
- publishing date
- 2012-08-29
- type
- Contribution to journal
- publication status
- published
- keywords
- Anions/chemistry, Benzene/chemistry, Catalysis, Cyclohexenes/chemistry, Kinetics, Phenols/chemistry, Water/chemistry
- in
- Journal of the American Chemical Society
- volume
- 134
- issue
- 34
- pages
- 14 pages
- publisher
- The American Chemical Society (ACS)
- external identifiers
-
- pmid:22830996
- scopus:84865632608
- ISSN
- 1520-5126
- DOI
- 10.1021/ja304366j
- language
- English
- LU publication?
- no
- id
- 88d96463-7306-407b-ab56-9e90b2a3cc7a
- date added to LUP
- 2025-01-11 22:06:51
- date last changed
- 2025-07-14 08:06:05
@article{88d96463-7306-407b-ab56-9e90b2a3cc7a,
abstract = {{<p>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.</p>}},
author = {{Kudavalli, Jaya Satyanarayana and Rao, S Nagaraja and Bean, David E and Sharma, Narain D and Boyd, Derek R and Fowler, Patrick W and Gronert, Scott and Kamerlin, Shina Caroline Lynn and Keeffe, James R and More O'Ferrall, Rory A}},
issn = {{1520-5126}},
keywords = {{Anions/chemistry; Benzene/chemistry; Catalysis; Cyclohexenes/chemistry; Kinetics; Phenols/chemistry; Water/chemistry}},
language = {{eng}},
month = {{08}},
number = {{34}},
pages = {{69--14056}},
publisher = {{The American Chemical Society (ACS)}},
series = {{Journal of the American Chemical Society}},
title = {{Base-catalyzed dehydration of 3-substituted benzene <i>cis</i>-1,2-dihydrodiols : stabilization of a cyclohexadienide anion intermediate by negative aromatic hyperconjugation}},
url = {{http://dx.doi.org/10.1021/ja304366j}},
doi = {{10.1021/ja304366j}},
volume = {{134}},
year = {{2012}},
}