Jean Guillerez

Equilibre lactime-lactame des hydroxy-pyridines et des hydroxy-pyrimidines (uraciles) en milieu apolaire : Etude infra-rouge

Abstract In contrast to what observed with mono hydroxy-pyridines and -pyrimidines the lactim-lactam equilibrium of uracils is not found to be markedly influenced by solvent polarity.

Reaction de metathese des olefines induite photochimiquement en presence d'un complexe de metal de transition. IV.- etude cinetique de l'interconversion catalysee cis-trans du butene-2.

Abstract The cis-trans interconversion of olefins in the system W(CO) 6 + CCl 4 + 2-butene is studied, both with initial irradiation of a solution of W(CO) 6 in CCl 4 (photoinduction), and with continuous irradiation of the system, for cis- and trans-butene concentrations between 0.09 and 6.0 M. Analysis of the results of the photoinduction experiments (rate of conversion and kinetic law as a function of the initial concentration of the olefin) allowed us to propose a simple kinetic scheme comprising a cis-trans interconversion of 2-butene and olefin-catalyzed destruction of the catalytic entity (k 2 = (0.62 ± 0.06)x10 −4 M s −1 ). In the continuous irradiation experiments the final distribution of the olefin was independent of the initial butene concentration (cis-2-butene/trans-2-butene 3.0) and the reaction kinetics are of first-order (k obs = (3±1) x10 −4 s −1 . Comparison of the two experiments suggests continuous photochemical regeneration of the catalytically active entity. The first-order reaction kinetics are in agreement with a carbene-metal carbonyl structure of the intermediate

Equilibrium and activation thermodynamic parameters of the tautomerism of 6-methoxy-2-pyridone in water

In neutral water, the interconversion of the two tautomers of the lactim–lactam equilibrium of 6-methoxy-2-pyridone occurs essentially via a pH-independent process. The influence of temperature on the position and on the dynamics of this tautomeric equilibrium has been studied by temperature-jump relaxation kinetics. The equilibrium and activation thermodynamic parameters obtained (ΔH° 26.8 kJ mol–1, ΔS° 58 J K–1 mol–1; lactam → lactim, ΔHc‡ 46 kJ mol–1, ΔSc‡–40 J K–1 mol–1) indicate a high value for the kinetic entropy term. These results indicate that the interconversion mechanism is ionic (rather than concerted) and involves the anionic form of the substrate in a cyclic transition state. In this transition state, at least two solvent molecules would temporarily ensure a hydrogen bond connection between the sites which undergo tautomerism.

Influence of carboxylic acid association upon the lactim–lactam tautomeric equilibrium of 2-hydroxypyridines

I.r. and u.v. absorption spectroscopy in CCl4 at room temperature provides evidence for lactim–acid and lactam–acid heterodimer formation in 6-chloro-2-hydroxypyridine–acetic acid mixtures. Measurements of association constants for two types of 1:1 hydrogen-bonded complexes reveal preferential association with the lactam tautomer, leading to a shift in the apparent tautomeric equilibrium constant. These results suggest that specific interactions are of great importance in understanding solvent effects on protomeric equilibria.

Complexation between 3-methylorotic (3-methyluracil-6-carboxylic) acid dianion and divalent metal cations. The slow acid dissociation of transition metal complexes and its possible relevance to orotic acid phosphoribosylation

3-Methylorotic (3-methyluracil-6-carboxylic) acid dianion gives 1 : 1 complexes with divalent metal ions. Their stability constants were determined by u.v. spectroscopy for Ca2+, Mg2+, Co2+, Ni2+, and Cu2+. The kinetics of formation of these complexes was investigated by T-jump relaxation in the alkaline pH range. The rate constants thus determined for complexation by Ca2+, Mg2+, Co2+, and Cu2+, show that it fits the Eigen mechanism. Complex dissociation was studied by the stopped-flow technique in the acidic pH range. Surprisingly, the observed first-order rate constants for transition metal ions (Co2+, Ni2+, and Cu2+) are found to be dependent on the final acidity. These results indicate a mechanism involving the slow protonation of these complexes (kH+ < 104 l mol–1s–1). A possible explanation of the inhibiting properties of transition-metal ions in the enzyme-catalysed syntheses of 6-carboxyuridine 5′-monophosphate is inferred.