Ketonization of the remarkably strongly acidic elongated enol generated by flash photolytic decarboxylation of p-benzoylphenylacetic acid in aqueous solution.

Photodecarboxylation of p-benzoylphenylacetic acid in aqueous solution produces the elongated enol 5, whose strength as an oxygen acid (pQ(E/a)= 7.67) makes it more acidic than simple enol analogs by several orders of magnitude.

Kinetics and mechanism of hydration of o-thioquinone methide in aqueous solution. Rate-determining protonation of sulfur.

o-Thioquinone methide, 2, was generated in aqueous solution by flash photolysis of benzothiete, 1, and rates of hydration of this quinone methide to o-mercaptobenzyl alcohol, 3, were measured in perchloric acid solutions, using H2O and D2O as the solvent, and also in acetic acid and tris(hydroxymethyl)methylammonium ion buffers, using H2O as the solvent. The rate profiles constructed from these data show hydronium-ion-catalyzed and uncatalyzed hydration reaction regions, just like the rate profiles based on literature data for hydration of the oxygen analogue, o-quinone methide, of the presently examined substrate. Solvent isotope effects on hydronium-ion catalysis of hydration for the two substrates, however, are quite different: k(H)/k(D) = 0.42 for the oxygen quinone methide, whereas k(H)/k(D) = 1.66 for the sulfur substrate. The inverse nature (k(H)/k(D) < 1) of the isotope effect in the oxygen system indicates that this reaction occurs by a preequilibrium proton-transfer reaction mechanism, with protonation of the substrate on its oxygen atom being fast and reversible and capture of the benzyl-type carbocationic intermediate so formed being rate-determining. The normal direction (k(H)/k(D) > 1) of the isotope effect in the sulfur system, on the other hand, suggests that protonation of the substrate on its sulfur atom is in this case rate-determining, with carbocation capture a fast following step. A semiquantitative argument supporting this hypothesis is presented.

Hydration of phenylketene revisited: a counter-intuitive result.

In previous work (Can. J. Chem. 1987, 65, 1719-1723 and J. Am. Chem. Soc. 1995, 117, 9165-9171), flash photolysis of diazoacetophenone or phenylhydroxycyclopropenone in aqueous solution was found to produce phenylketene as a short-lived transient species with absorbance at lambda congruent with 260 nm, which decayed with single-exponential kinetics. It has now been discovered that, in the acidity region [H(+)] = 0.000 01 to 0.06 M, this decay is preceded by a faster absorbance rise, and that the overall change conforms well to a double exponential rate law. Analysis of the new data produces rate profiles whose general shapes, as well as the numerical values of their constituent rate constants, plus the form of buffer catalysis, indicate that this newly discovered absorbance rise represents ketonization of phenylacetic acid enol, and that the subsequent absorbance decay represents addition of water to phenylketene. The chemistry of the system, however, requires ketene hydration to precede enol ketonization in a time sequence opposite from that of the absorbance changes. This seemingly counter-intuitive result is nevertheless consistent with the rate law that governs the time evolution of the central species in a two-step rise and decay, such as that observed here.

Alkylation of guanosine and 2'-deoxyguanosine by o-quinone alpha-(p-anisyl)methide in aqueous solution.

Rates of alkylation of guanosine and 2'-deoxyguanosine with o-quinone alpha-(p-anisyl)methide were measured by flash photolysis in a series of aqueous sodium hydroxide solutions and bicarbonate ion, t-butylhydrogenphosphonate ion, and biphosphate ion buffers. The data so obtained provide rate profiles for these nucleoside plus quinone methide reactions over the range pH = 7-14, which furnish guanosine and deoxyguanosine acidity constants consistent with literature information. These profiles also provide rate constants that show the reaction of o-quinone alpha-(p-anisyl)methide with guanosine and deoxyguanosine to be fairly fast processes, considerably faster than the biologically wasteful reaction of the quinone methide with water, which is the ubiquitous medium in biological systems; that makes the quinone methide a potent guonosine and deoxyguanosine alkylator.

Photoreactions of 3-diazo-3H-benzofuran-2-one; dimerization and hydrolysis of its primary photoproduct, a quinonoid cumulenone: a study by time-resolved optical and infrared spectroscopy.

Light-induced deazotization of 3-diazo-3H-benzofuran-2-one (1) in solution is accompanied by facile (CO)-O bond cleavage yielding 6-(oxoethenylidene)-2,4-cyclohexadien-1-one (3), which appears with a rise time of 28 ps. The expected Wolff-rearrangement product, 7-oxabicyclo[4.2.0]octa-1,3,5-trien-8-ylidenemethanone (4), is not formed. The efficient light-induced formation of the quinonoid cumulenone 3 opens the way to determine the reactivity of a cumulenone in solution. The reaction kinetics of 3 were monitored by nanosecond flash photolysis with optical (lambda(max) approximately 460 nm) as well as Raman (1526 cm(-1)) and IR detection (2050 cm(-)(1)). Remarkably, the reactivity of 3 is that expected from its valence isomer, the cyclic carbene 3H-benzofuran-2-one-3-ylidene, 2. In aqueous solution, acid-catalyzed addition of water forms the lactone 3-hydroxy-3H-benzofuran-2-one (5). The reaction is initiated by protonation of the cumulenone on its beta-carbon atom. In hexane, cumulenone 3 dimerizes to isoxindigo ((E)-[3,3']bibenzofuranylidene-2,2'-dione, 7), coumestan (6H-benzofuro[3,2-c][1]benzopyran-6-one, 8), and a small amount of dibenzonaphthyrone ([1]benzopyrano[4,3-][1]benzopyran-5,11-dione, 9) at a nearly diffusion-controlled rate. Ab initio calculations (G3) are consistent with the observed data. Carbene 2 is predicted to have a singlet ground state, which undergoes very facile, strongly exothermic (irreversible) ring opening to the cumulenone 3. The calculated barrier to formation of 4 (Wolff-rearrangement) is prohibitive. DFT calculations indicate that protonation of 3 on the beta-carbon is accompanied by cyclization to the protonated carbene 2H(+), and that dimerization of 3 to 7 and 9 takes place in a single step with negligible activation energy.

Flash photolytic generation and study of 5-methoxy-o-quinone alpha-phenylethidet in aqueous solution: comparison of cis and trans isomer reactivity.

Cis and trans isomeric 5-methoxy-o-quinone alpha-phenylethides were generated by flash photolysis of o-hydroxybenzyl alcohol and o-hydroxystyrene precursors and their rates of decay were measured in aqueous solutions of perchloric acid, sodium hydroxide, and biphosphate (dihydrogen phosphate) ion and carbonate ion buffers. The data so obtained gave two parallel rate profiles displaced from one another by about an order of magnitude, the faster of which could be assigned to the cis isomer and the slower to the trans isomer. Both rate profiles showed acid catalyzed, uncatalyzed, and hydroxide ion catalyzed portions, with acid catalysis becoming saturated at the high acidity end of the range investigated. This saturation implies a pre-equilibrium mechanism involving quinone ethide protonation on carbonyl oxygen for the acid catalyzed process, and analysis of the data shows the trans isomer to be a slightly stronger base than the cis isomer. The fact that two separate rate profiles are observed implies that the cationic intermediates formed by substrate protonation in these acid catalyzed processes do not interconvert on the time scale of the quinone ethide decay reactions, and that in turn indicates that the former ethide bonds of the cationic intermediates have substantial double-bond character; a lower limit of 10 kcal mol(-1) may be estimated for the barrier to rotation about this bond.