Intramolecular [4+2] cycloaddition of a photogenerated o-quinone methide in aqueous solution

Barker, Beverly; Diao, Li; Wan, Peter

doi:10.1016/s1010-6030(96)04535-2

Cited by 28 publications

(19 citation statements)

References 17 publications

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“…Based on our work on the photosolvolysis of methoxy-substituted benzyl alcohols and related compounds, we hypothesized that the corresponding hydroxy-substituted benzyl alcohols (phenols) could react to give QMs that would offer an explanation for their high photosolvolytic reactivity [7]. Indeed, photolysis of all the isomeric phenols 4-6 gave the corresponding QMs, via formal loss of water [7,8]. Photolysis of 4 in the presence of ethyl vinyl ether or dihydropyran gave the corresponding [4+2] chroman adducts (7 and 8, respectively) in high yield consistent with efficient photogeneration of an ortho-QM.…”

Section: Parent Quinone Methidesmentioning

confidence: 99%

Quinone methide intermediates in organic photochemistry

Wan

Brousmiche

Chen

et al. 2001

Pure and Applied Chemistry

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Quinone methides are widely encountered reactive intermediates in the chemistry of phenols and related compounds. This paper summarizes our recent progress in uncovering new and general photochemical methods for forming quinone methides of various structural types in aqueous solution. Their mechanism of formation and subsequent chemistry are also discussed. New examples of excited-state intramolecular proton transfer (ESIPT) have been uncovered in these studies. We have also discovered that appropriately designed biphenyls and terphenyls display photochemistry that is best rationalized by highly polarized and planar excited states of these ring systems, which can efficiently lead to the corresponding extended quinone methides.

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Section: Parent Quinone Methidesmentioning

confidence: 99%

Quinone methide intermediates in organic photochemistry

Wan

Brousmiche

Chen

et al. 2001

Pure and Applied Chemistry

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“…1, produces o-quinone methide by dehydration of o-hydroxybenzyl alcohol [1,3]. Flash photolysis of this alcohol in aqueous solution generated a transient species with the strong absorbance at λ = 400 nm that is characteristic of oquinone methide [4]. Decay of this transient conformed to the first-order rate law well, and analysis of spent reaction mixtures showed only o-hydroxybenzyl alcohol to be present, as expected for hydration of the quinone methide back to starting substrate.…”

Section: O-quinone Methidementioning

confidence: 99%

Reactive intermediates. Some chemistry of quinone methides

Chiang

Kresge

Zhu

2000

Pure and Applied Chemistry

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Quinone methides were produced in aqueous solution by photochemical dehydration of o-hydroxybenzyl alcohols (o-HOC 6 H 4 CHROH; R = H, C 6 H 5 , 4-CH 3 OC 6 H 4 ), and flash photolytic techniques were used to examine their rehydration back to starting substrate as well as their interaction with bromide and thiocyanate ions. These reactions are acid-catalyzed and show inverse isotope effects (k H+ /k D+ < 1), indicating that they occur through preequilibrium protonation of the quinone methide on its carbonyl carbon atom followed by rate-determining capture of the benzyl carbocations so formed by H 2 O, Br -, or SCN -. With some quinone methides (R = C 6 H 5 and 4-CH 3 OC 6 H 4 ) this acid catalysis could be saturated, and analysis of the data obtained in the region of saturation for the example with R = 4-CH 3 OC 6 H 4 produced both the equilibrium constant for the substrate protonation step and the rate constant for the rate-determining step. Energy relationships comparing the quinone methides with their benzyl alcohol precursors are derived.

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“…Conversely, mquinomethane (2b) is a non-Kekule structure, because it is not possible to draw a resonance structure in which all the electrons are paired (Scheme 1). o-Quinomethane, 2a, its substituted analogues, and p-quinomethane, 2c, have closed-shell electronic structures and are well known in condensed-phase synthetic chemistry [7,[20][21][22][23][24][25][26][27][28]. Similarly, 2b has been characterized by UV-vis optical absorption spectroscopy [29] and electron paramagnetic resonance (EPR) spectroscopy [30,31].…”

Section: Introductionmentioning

confidence: 99%