Metal-Ion Oxidative Decarboxylations. 9. Reaction Of Benzilic Acid With Cerium(IV) In Acidic Perchlorate And Sulfate Media

The kinetics of oxidation of benzylic acid (HLH) to benzophenone by cerium (IV) has been studied in several media using the stopped-flow technique. In HClO4-Na2SO4-NaClO4 media, which allow for the variation and control of the concentrations of the various Ce(IV) species at constant [H+] and ionic strength, the rate data suggest CeSO42+ as the kinetically relevant species. In HClO4 the observed first-order rate constants, tetrad, first increase with increasing HLH concentration, then decrease slightly at higher substrate concentrations suggesting the intermediacy of two complexes: a reactive [Ce(IV)HL] and a [Ce(IV)(HL)2] which resists reaction. The rate law which best describes the observed kinetics in HClO4 media is given by [formula ommitted] where K7 ′ and K8′ are H+-dependent equilibrium constants for the formation of the 1:1 and 1:2 complexes, respectively. The kinetic parameters kα and kβ are the rate constants for the intramolecular redox reaction of the 1:1 and 1:2 complexes, respectively. The rates in H2SO4 are much slower than those in HClO4 and do not reflect strong complex formation before electron transfer. Whereas H+ had an inhibiting effect in the above-mentioned media, catalysis by H+ was manifest in HClO4-HOAc media, possibly through decreasing acetato complexation of Ce(IV). © 1977, American Chemical Society. All rights reserved

Evidence For An Iminium Cation Intermediate And Its Non‐Oxidative Decarboxylation In A Cerium (IV)‐Mediated Oxidative Decarboxylation. Preliminary Communication

Evidence is provided for the occurrence of a non‐oxidative decarboxylation in an iminium cation intermediate which is produced in an oxidative decarboxylation induced by cerium (IV). Copyright © 1980 Verlag GmbH & Co. KGaA, Weinhei

The Aminolysis Of N-Hydroxysuccinimide Esters. A Structure-Reactivity Study

Twelve amines, which vary substantially in basicity and in steric environment around N, have been allowed to compete—in anhydrous dioxane solution—in the aminolysis of the N-hydroxysuccinimide esters of unsubstituted, p-OCH3, p-N02, and 3,5-(N02)2 benzoic acids. The amines, which encompass a basicity range of 6.5 pk units, display a 10000-fold variation in reactivity in their reaction with the p-N02ester. for the sterically unhindered amines, a Bronsted-type plot of log kobsdvs. pKa has a slope of ~0.7. The data fit a model (Satterthwait, A. C.; Jencks, W. P. J. Am. Chem. Soc. 1974, 96, 7018–7044) in which reversible formation of a tetrahedral intermediate is followed by rate-determining breakdown to products. Appreciable sensitivity to steric factors, as evidenced from the depressed rates with a-methylbenzylamine and diethylamine, substantiates reversible formation of a crowded tetrahedral intermediate prior to the rate-determining step. The Hammett p values for the competitive acylation of aniline, a-methylbenzylamine, and benzylamine, by substituted N-succinimidyl benzoates, are 1.4,1.2, and 1.1, respectively. These values reflect the selectivity expected for these amines, and the substantial accumulation of charge density at the acyl C in the formation of the tetrahedral intermediate. Individual rate constants for the aminolysis of N-succinimidyl p-methoxybenzoate by n-butylamine and by piperidine, both show First-order and second-order terms in [amine]. The general-base catalysis term is suggestive of a path involving proton transfer in the rate-determining step. © 1987, American Chemical Society. All rights reserved

Metal-Ion Oxidative Decarboxylations. 10. Substituent Effects In The Cerium(IV)-Benzilic Acids Reaction

The rates of oxidation by cerium (IV) of several substituted benzylic acids (unsubstituted, 2,2′-dichloro, 4,4′-dichloro, 4,4′-dimethyl, 4,4′-dimethoxy, and 4,4′-dinitro) to the corresponding benzophenones have been studied spectrophotometrically in aqueous H2SO4, aqueous HClO4-HOAc, and acetonitrile. The rates are much slower in H2SO4 than in the other two media, probably because of heavy sulfato complexation of cerium(IV). The px as of the various acids were determined potentiometrically. The Hammett p values in acetonitrile and HClO4-HOAc media, derived from correlations with several sets of σ constants and with the newly determined pKas, are about -0.7 and -0.35 in the two media, respectively. These values rule out the possibility of positive charge development at the benzhydrilic carbon, at or prior to the rate-determining step. Instead, a mechanism involving free (?) radicals is proposed. © 1977, American Chemical Society. All rights reserved

Kinetics And Mechanisms Of The Aminolysis Of N-Hydroxysuccinimide Esters In Aqueous Buffers

Rate constants for the aminolysis of the N-hydroxysuccinimide (NHS) ester of ρ-methoxybenzoic acid, in aqueous buffer systems (20% dioxane), have been determined under pseudo-first-order conditions. For the amines studied (pKa = 7.60-11.1), the data fit the rate expression kobsd - KOH-⨯ [OH-] = k1[amine]free. This rate equation is in contrast to the two-term rate equation (kobsd = k1[amine] + k2[amine]2) obtained for this reaction in anhydrous dioxane (Cline, G. W.; Hanna, S. B. J. Am. Chem. Soc. 1987,109, 3087) and is suggestive of a disproportionate decrease in the catalyzed vs the uncatalyzed reaction path upon changing from a nonaqueous to an aqueous solvent system. The correlation of amine basicity with the nucleophilic rate constant, k1 yields a slope αnuc = 1.0. The magnitude of αnuc, in terms of a reaction mechanism where a tetrahedral intermediate is formed in a fast preequilibrium followed by rate-determining breakdown to products, reflects the sensitivity to changes in charge accumulation in the formation of the tetrahedral intermediate. The resultant increased rate constants, with increased basicity, are due to the effect of an increased concentration of the tetrahedral intermediate. A qualitative evaluation of the literature and current data concerning the leaving ability of N-hydroxy esters, in comparison to phenyl esters (equivalent acyl groups and nucleophiles), reveals that, with leaving groups of comparable basicity, the nucleophilic rate constants for N-hydroxy esters are about 2 orders of magnitude greater than that for phenyl esters. © 1988, American Chemical Society. All rights reserved

The Oxidative Decarboxylation Of Polyaminocarboxylic Acids - II. A Comparative Kinetic Study Of The Oxidation Of NTA, EDTA, CDTA And Dtpa With Cerium(IV) In Sulfuric Acid Media

The rates of oxidation of four chelating agents, NTA, EDTA, CDTA, and DTPA with Ce(IV), in sulfuric acid media, were determined spectrophotometrically by a stopped-flow technique. The reductive ability is in the order CDTA \u3e EDTA \u3e DTPA \u3e NTA. The influence of varying the acidity of the medium was studied, and in each case a maximum in the rate constant vs. [H+] plot was observed. A possible interpretation of the reactivities and the influence of acidity is advanced. © 1971 Springer-Verlag