A. J. Kresge

Reactive intermediates. Some chemistry of quinone methides

Quinone methides were produced in aqueous solution by photochemical dehydration of o-hydroxybenzyl alcohols (o-HOC6H4CHROH; R = H, C6H5, 4-CH3OC6H4), 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 (kH+/kD+ < 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 H2O, Br-, or SCN-. With some quinone methides (R = C6H5 and 4-CH3OC6H4) this acid catalysis could be saturated, and analysis of the data obtained in the region of saturation for the example with R = 4-CH3OC6H4 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.

Conjugate addition of water to α‐carbonylcarbenes

α-Carbonylcarbenes (2a–c) generated by UV photolysis of 2-diazophenylacetic acid (1a), its methyl ester (1b) and 4-diazo-3-isochromanone (1c) in aqueous solution undergo conjugate addition of water across the entire carbonylcarbene moiety to give enols (3a–c) of the corresponding α-hydroxycarbonyl compounds. These carbenes are long-lived, with microsecond lifetimes in aqueous solution.

Generation of o-quinone α-carbomethoxymethide by photolysis of methyl 2-hydroxyphenyldiazoacetate in aqueous solutionDedicated to Professor Dr Z. R. Grabowski and Professor Dr J. Wirz on the occasions of their 75th and 60th birthdays.Electronic supplementary information (ESI) available. Tables S1–S4 of rate data. See http://www.rsc.org/suppdata/cp/b2/b211444p/

Flash photolysis of methyl 2-hydroxyphenyldiazoacetate (8) in dilute aqueous perchloric acid solution and acetic acid and biphosphate ion buffers produced a transient species that was identified as o-quinone α-carbomethoxymethide (9). This structural assignment is based upon solvent isotope effects, the form of buffer catalysis, UV absorption maxima, and the identity of decay rate constants with those determined for the transient obtained by flash photolysis of other, more conventional, quinone methide precursors, namely the benzyl alcohol methyl 2-hydroxy mandelate (10) and its acetate, 2′-acetoxy-2-hydroxyphenylacetate (11).