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The effects of a beta-nitrogen substituent on the keto-enol/enolate equilibria in the N-acetylamino-p-methylacetophenone system are investigated.

Research into the kinetics and reaction mechanisms of the quinone methide hydration reactions that form the basis of biological activity in living systems where water is the ubiquitous medium is presented. It was found that decay of the quinone methide is speeded up by acetic acid buffers through both acid- and base-catalyzed pathways.

Research is presented describing the use of flash photolysis on 4-diazoisothiochroman-3-one in aqueous solution to produce 1,3-dihydrobenzo[c]thiophen-1-ylideneketene in a photo-Wolff reaction.

A study on keto-enol tautomerism of beta-ketoesters investigated the enol of 4,4,4-trifluoroactoacetic acid prepared by flash photolytic retro-(2+4)-cycloaddition of a suitable dioxinone and its analogues. Among other findings, trifluoromethyl was found to influence rates of enolization and ketonization. Results also confirm the electron-withdrawing nature of trifluoromethyl that reduces the electron density of the enolic vinyl group and renders it less susceptible to electrophilic attack by acids.

A rapid rise in absorbance of around lambda = 250 nm to 300 nm results from flash photolysis of aqueous solutions of 2-diazocyclohexane-1,3-dione. This is followed by a much slower decay. There are strong similarities between these absorbance changes and those seen in the production of acetoacetic acid by flash photolysis of 2,2,6-trimethyl-4H-1,3-dioxin-4-one. In aqueous solution, 2-ketocyclopentyl-ideneketene is produced from flash photolysis of 2-diazocyclohexane-1,3-dione. This hydrates to the enol of 2-oxocyclopentanecarboxylic acid, with the enol then isomerizing to the keto form of the acid.

Examination of the flash photolysis of phenyldiozoacetic acid and the methy, n-butyl and isobutyl esters of benzoylformic acid in aqueous solution shows that mandelic acid enol is generated by hydration of phenylcaboxycarbene. Isotopic trace studies and transient absorbance effects indicate that reaction paths are supported and described by acid-base catalysis and solvent isotope effects. Results indicate that the racemization of mandelic acid in neutral solution is catalyzed by the enzyme mandelate racemase.

Flash photolysis of 10-diazo-9(10H)-phenanthrenone in aqueous solution provided two successively formed transient species, producing fluorene-9-carboxylic acid. The transients were identified as fluorenylideneketene, produced from a photo-Wolff reaction. Bromine scavenging determined the rate of enolization of fluorene-0-carboxylic acid.

Rates of amide hydrolysis are measured for the substrate succinyl-Ala-Ala-Pro-Phe-p-nitroanilide catalyzed by subtilisin and its N155G mutant. These allow the solvent isotope effects, which show no differences between the wild-type and mutant enzymes, to be derived. The isotope effects also show that the curvature in the proton inventory of a reaction catalyzed by methyl chymotrypsin is not due to oxyanion hole interactions but can be accounted for by a medium effect. The presence of strong medium effects in subtilisin and its mutant is also observed.

Photodecarbonylation of phenylaminocyclopropenones give primary, secondary and tertiary ynamines in aqueous solution. In acid-catalyzed decay of the ynamines, protonation of the beta-carbon atoms leads to the keteniminium ions from which nitrogen-linked protons are eliminated to produce ketenimines. The ketenimines react with water to give phenylacetamides. Tertiary ynamine derived keteniminium ions lack nitrogen-linked protons and so are hydrated to form amide enols which ketonize to yield phenylacetamides. Only primary and secondary ynamines undergo base-catalyzed decay.

Arylynolate and arylketenes are formed as short lived intermediates during the synthesis of arylacetic acid from arylhydroxycyclopropenones. The flash photolysis of kinetics of these arylylnolate and arylketenes shows that the related arylynols are strong acids having a pK(sub a) value of < 3. The rate of ketene hydration reactions reveal large steric effects. The photocarbonylation of hydroxycyclopropenones can be achieved in liquid solution at ambient temperature.

Flash photolytic production of phenylcyanoketene by a photo-Wolff reaction of alpha-cyano-alpha-diazoacetophenone after hydration of the ketene provided the enol of alpha-cyano-alpha-phenylatic acid as a short-lined transient species. Estimations of the rates of ketonisation of the enol in dilute hydrochloric acid solutions, acetic acid, biphosphate ion, tris(hydroxymethyl)methylamine and ammonia buffers provided a rate profile with turning points. The first and second acid ionization constants of the enol could be establish from this profile.

Two methods were used to determine the keto-enol equilibrium constants for the pyruvic acid system in both acidic and basic aqueous solution. The first method used the Meyer halogen titration while the second used the ratio of enolization to ketonization. The two methods produced consistent results but slight differences between values for acidic and basic solution were observed. Such differences can be explained by the fact that the pyruvic acid is an un-ionized carboxylic acid in acid solutions but becomes a pyruvate ion in basic solutions.

Research was conducted to determine whether alpha-carbonylcarbene intermediates are formed as ketene precursors in the photo-Wolff reaction of alpha-diazocarbonyl compounds. A comparison of pyridine scavenging of transients generated by laser flash photolysis of benzoyldiazomethane and 1-naphthoyldiazomethane with pyridine scavenging of transients formed by flash photolysis of the corresponding arylhydroxycyclopropenones was carried out. Results indicate that the species scavenged by pyridine after flash photolysis of benzoyldiazomethane and 1-naphthoyldiazomethane are ketenes and not alpha-carbonylcarbenes.

The generation of o-quinone alpha-phenylmethide as a short lived transient species in aqueous solution by flash photolysis of o-hydroxy-alpha-phenylbenzyl alcohol, and its rate of decay measured in HClO4 and NaOH solutions as well as in CH3CO2H, H2PO4-, and HCO3- buffers is detailed. It was observed that a preequilibrium exists in the acid-catalyzed reaction.

The enol of 2-hydroxy-2-cyano-N-methylacetamide in aqueous solution was studied using flash photolytic generation.

Kinetic studies of acid dissociation of N-(pentafluorophenyl)-2-phenylethynamine in aqueous solution reveal that the reaction has a pK(a) value of 10.28 and the acidifying effect is evaluated as 10 pK units. Flash photolysis of an aminocyclopropenone produces this ynamine and it undergoes isomerization to ketenimine in aqueous solutions. Kinetic isotopic studies help examine the isomerization reaction and determine rate constants for various steps.

A reexamination of the hydroxide ion-catalyzed enolization of acetone provided deuterium and tritium isotope effects having a Swain-Schaad exponent value of x = 1.49 +/- 0.07 that was consistent with theoretical results. The new procedure involved tritium nuclear magnetic resonance spectroscopy for determining tritium exchange in an acetone-water system. Bromine scavenging of the acetone enolate ion was performed to obtain protium and deuterium hydroxide ion catalytic coefficients which when combined with the experimental rate constants provided highly reliable isotope effects.

Flash photolysis of diazophenylacetamide in aqueous solution produced phenylcarbamoylcarbene, whose hydration generated a transient species that was identified as the enol isomer of mandelamide. This assignment is based on product identification and the shape of the rate profile for decay of the enol transient, through ketonization to its carbonyl isomer, as well as by the form of acid-base catalysis of and solvent isotope effects on the decay reaction. Rates of enolization of mandelamide were also determined, by monitoring hydrogen exchange at its benzylic position, and these, in combination with the ketonization rate measurements, gave the keto-enol equilibrium constant pK[sub E] = 15.88, the acidity constant of the enol ionizing as an oxygen acid, pQ[sup E, sub a] = 8.40, and the acidity constant of the amide ionizing as a carbon acid PQ[sup K, sub a] = 24.29. (These acidity constants are concentration quotients applicable at ionic strength = 0.10 M.) These results show the enol content and carbon acid strength of mandelamide, like those of mandelic acid and methyl mandelate, to be orders of magnitude less than those of simple aldehydes and ketones; this difference can be attributed to resonance stabilization of the keto isomers of mandelic acid and its ester and amide derivatives, through electron delocalization into their carbonyl groups from the oxygen and nitrogen substituents adjacent to these groups. The enol of mandelamide, on the other hand, again like the enols of mandelic acid and methyl mandelate, is a substantially stronger acid than the enols of simple aldehydes and ketones. This difference can be attributed to the electronegative nature of the oxygen and nitrogen substituents geminal to the enol hydroxyl group in the enols of mandetic acid and its derivatives; in support of this, the acidity constants of these enols correlate well with field substituent constants of these geminal groups. [ABSTRACT FROM AUTHOR]

Examines the photolysis of p-quinone methide in aqueous solution. Heterolysis of the carbon-bromide bond in p-hydroxybenzyl bromide; Protonation of the quinone methide in the carbonyl carbon atom; Preequilibrium mechanisms of a p-hydroxybenzyl carbocation.

Focuses on the flash photolytic generation of o-quinone alpha-phenylmethide and o-quinone alpha-(p-Anisyl)methide in aqueous solution. Hydration of the quinone methide; Effects of acid-catalyzed reaction in the solvent isotope; Rate of decay of o-quinine apha-phenylmethide.

Examines the effect of beta-oxygen substituent to keto-enol/enolate equilibria in the isochroman-4-one system. Generation of enol from 1-tetralone; Measurement of ketonization rates in aqueous HClO[sub 4] and NaOH solutions; Hydrolysis of potassium salt and trimethylsilyl ether.

Examines the flash photolysis of cis-enol of N-acetylamino-p-methylacetophenone and the rates of ketonization in aqueous hypochlorite and sodium hydroxide solutions. Rates of enolization of N-acetylamino-p-methylacetophenone; Acid catalytic coefficients for rate-determining proton-transfer reactions.

Reports a study which investigated whether alpha-carbonylcarbene intermediates are formed as ketene precursors in the photo-Wolff reaction of alpha-diazocarbonyl compounds by comparing pyridine scavenging of transients generated by laser flash photolysis of benzoyldiazomethane. Experimental framework of the study; Results; Discussion of the results.