Photochemical Generation and the Reactivity of o-Naphthoquinone Methides in Aqueous Solutions.
Irradn. of 3-hydroxy-2-naphthalenemethanol (3a) and 2-hydroxy-1-naphthalenemethanol (4a) results in efficient (Φ254 = 0.17 and 0.20) dehydration and the formation of isomeric naphthoquinone methides, 2,3-naphthoquinone-3-methide (1) and 1,2-naphthoquinone-1-methide (2), resp. In aq. soln., naphthoquinone methides 1 and 2 undergo rapid hydration to regenerate starting materials (τH2O (1) = 7.4 ms and τH2O (2) = 4.5 ms at 25 °C). The hydration reaction is strongly catalyzed by the hydroxide ion but shows acid catalysis only at pH < 1. Reactive intermediates 1 and 2 can be intercepted by other nucleophiles, such as the azide ion (kN3(1) = 2.0 × 104 M-1 s-1 and kN3(2) = 3.0 × 104 M-1 s-1) or thiol (kSH(1) = 2.2 × 105 M-1 s-1 and kSH(2) = 3.3 × 105 M-1 s-1). Et vinyl ether readily reacts with 1 and 2 (kDA(1) = 4.1 × 104 M-1 s-1 and kDA(2) = 6.0 × 104 M-1 s-1) to produce Diels-Alder adducts in excellent yield. O-Naphthoquinone methides 1 and 2 were also generated by photolysis of 3-ethoxymethyl- (3b) and 1-(ethoxymethyl)-2-naphthols (4b), as well as from (2-hydroxy-3-naphthyl)methyl- (3c) and [(2-hydroxy-1-naphthyl)methyl] trimethylammonium iodides (4c). Laser flash photolysis of 3a,b and 4a,b allows the detection of short-lived (τ25°C ∼ 12 μs) precursors of naphthoquinone methides 1 and 2. From the precursor reactivity and the results of DFT calcns., 2H-naphthoxete structure was assigned to these species. [on SciFinder(R)]