Dual Reactivity of Hydroxy- and Methoxy- Substituted o-Quinone Methides in Aqueous Solutions: Hydration versus Tautomerization.
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), resp. o-Quinone methides 1 and 2 can be quant. trapped as Diels-Alder adducts with Et 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 aq. soln., 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 × 103 s-1 M-1) and specific base (kOH-(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. [on SciFinder(R)]