908005-01-6Relevant academic research and scientific papers
Dual reactivity of hydroxy- and methoxy- substituted o-Quinone methides in aqueous solutions: Hydration versus tautomerization.
Arumugam, Selvanathan,Popik, Vladimir V.
scheme or table, p. 7338 - 7346 (2011/01/12)
4-Hydroxy-6-methylene-2,4-cyclohexadien-1-one (1) and 4-methoxy-6- methylene-2,4-cyclohexadien-1-one (2) were generated by efficient (Φ = 0.3) photodehydration of 2-(hydroxymethyl)benzene-1,4-diol (3a) and 2-(hydroxymethyl)-4-methoxyphenol (4a), respectively. o-Quinone methides 1 and 2 can be quantitatively trapped as Diels-Alder adducts with ethyl vinyl ether or intercepted by good nucleophiles, such as azide ion (kN3(1) = 3.15 × 104 M-1 s-1 and kN3(2) = 3.30 × 104 M-1 s-1). In aqueous solution, o-quinone methide 2 rapidly adds water to regenerate starting material (τH2O(2) = 7.8 ms at 25 °C). This reaction is catalyzed by specific acid (kH+(2) = 8.37 × 10 3 s-1 M-1) and specific base (k OH-(2) = 1.08 × 104 s-1 M -1) but shows no significant general acid/base catalysis. In sharp contrast, o-quinone methide 1 decays (τH2O(1) = 3.3 ms at 25 °C) via two competing pathways: nucleophilic hydration to form starting material 3a and tautomerization to produce methyl-p-benzoquinone. The disappearance of 1 shows not only specific acid (kH+(1) = 3.30 × 104 s-1 M-1) and specific base catalysis (kOH-(1) = 3.51 × 104 s -1 M-1) but pronounced catalysis by general acids and bases as well. The o-quinone methides 1 and 2 were also generated by the photolysis of 2-(ethoxymethyl)benzene-1,4-diol (3b) and 2-(ethoxymethyl)-4- methoxyphenol (4b), as well as from (2,5-dihydroxy-1-phenyl)methyl- (3c) and (2-hydroxy-5-methoxy-1-phenyl)methyltrimethylammonium iodides (4c). Short-lived (τ25°C ≈ 20 μs) precursors of o-quinone methides 1 and 2 were detected in the laser flash photolysis of 3a,b and 4a,b. On the basis of their reactivity, benzoxete structures have been assigned to these intermediates.
Substituents on quinone methides strongly modulate formation and stability of their nucleophilic adducts
Weinert, Emily E.,Dondi, Ruggero,Colloredo-Melz, Stefano,Frankenfield, Kristen N.,Mitchell, Charles H.,Freccero, Mauro,Rokita, Steven E.
, p. 11940 - 11947 (2007/10/03)
Electronic perturbation of quinone methides (QM) greatly influences their stability and in turn alters the kinetics and product profile of QM reaction with deoxynucleosides. Consistent with the electron-deficient nature of this reactive intermediate, electron-donating substituents are stabilizing and electron-withdrawing substituents are destabilizing. For example, a dC N3-QM adduct is made stable over the course of observation (7 days) by the presence of an electron-withdrawing ester group that inhibits QM regeneration. Conversely, a related adduct with an electron-donating methyl group is very labile and regenerates its QM with a half-life of approximately 5 h. The generality of these effects is demonstrated with a series of alternative quinone methide precursors (QMP) containing a variety of substituents attached at different positions with respect to the exocyclic methylene. The rates of nucleophilic addition to substituted QMs measured by laser flash photolysis similarly span 5 orders of magnitude with electron-rich species reacting most slowly and electron-deficient species reacting most quickly. The reversibility of QM reaction can now be predictably adjusted for any desired application.
