Open-chain nitrogen compounds. Part XV. A kinetic study of the hydrolysis of 1-aryl-3-aryloxymethyl-3-methyltriazenes and related triazenes

Abstract

The kinetics of hydyrolysis of a series of 1-aryl-3-aryloxymethyl-3-methyltriazenes Ar-N=N-NMe-CH20Ar1, was studied over the pH range 2-7.5. Reactions were followed by the change in UV absorbance spectra of the triazenes. The aryloxymethyltriazenes decompose more slowly at pH 7.5 than the hydroxymethyltriazenes, Ar-N=NMe-CH20H; the hydrolysis is favoured by the presence of an electron-withdrawing group in Ar'. A mixed isopropanol/buffer system was developed in order to improve solubility of the aryloxymethyl triazenes. Lowering the pH caused an increase in the rate of hydrolysis and under strongly acidic conditions an electron-withdrawing group in Ar' actually slows down the reaction. A Hammett plot of the pseudo-first-order rate constant, k,,,, is curved, indicating that two or more mechanisms operate simultaneously and that the contribution of each mechanism is substituent-dependent. A plot of k,,, vs.[buffer] is linear; the slope of the plot affords the rate constant, kb, for the buffer-catalyzed reaction for each substituent. A Hammett plot of kb vs. a is linear with p = +0.55, suggesting that the buffer-catalyzed reaction involves nucleophilic displacement of the phenoxy group by the buffer anion. Further analysis afforded the specific acid-catalyzed rate constants, k,+, for each substituent; this component of the reaction has a negative p, consistent with a mechanism involving protonation at the ether oxygen. The postulation that specific acid catalysis is a component of the reaction mechanism was confirmed by the observation of a solvent deuterium isotope effect, 2.28 > kH/kD > 1.60. Only the p-NO2 and p-CN phenyloxymethyltriazenes showed any spontaneous decomposition. On a determine la cinetique de l'hydrolyse d'une sene de 1-aryl-3-aryloxym~thyl-3-mCthyltri&nes, Ar-N=N-NMeCH20Ar1, a des pH allant de 2 a 7,5. On a suivi les reactions par le changement du spectre d'absorbance UV des triazknes.A un pH de 7,5, les aryloxymCthyltriazbnes se decomposent plus lentement que les hydroxymCthyltriaz&nes, Ar-N=N-NMe-CH20H; l'hydrolyse est favorisee par la presence d'un groupe Clectro-attracteur dans la portion Ar'. On a develop@ un systkme mixte isopropanol/tampon pour ameliorer la solubilite des aryloxymCthyltriazbnes. Un abaissement du pH provoque une augmentation de la vitesse d'hydrolyse et, dans des conditions fortement acides, un groupe Clectro-attracteur dans la portion Ar' inhibe completement la reaction. La courbe de Hammett de la constante de vitesse du pseudopremier ordre, k,,,, n'est pas rectiligne; ceci indique que deux ou plusieurs mecanismes operent simultanCment et que la contribution de chacun depend du substituant. Une courbe de k,,, vs.[tampon] est lineaire; la pente de la courbe fournit la constante de vitesse, kb, pour la reaction catalysee par le tampon pour chaque substituant. Une courbe de Hammett de kb vs. o est linkaire et p = +0,55, suggerant que la reaction catalysee par le tampon implique une substitution nucleophile du groupe phknoxyle par l'anion du tampon. Une analyse plus approfondi...

The Mechanism of Hydrolysis of Aryl Ether Derivatives of3‐Hydroxymethyltriazenes

“…These values, together with the Hammett ρ values, Brønsted α values and solvent isotope effects, are consistent [23] with the A-S E 2 mechanism shown in Scheme 2b, in which the expulsion of the phenol leaving group, to form a triazenyliminium ion, is aided by the general acid. The nature of the buffer catalysis described here contrasts with the role proposed by Vaughan, [18] who assumed the buffer to be acting as a nucleophile.…”

“…[22] For 5e the reaction was studied by using formic acid buffers, and the corresponding isotope effect was calculated to be 1.4. Vaughan and coworkers reported [18] a value of 1.6 Ͻ k H /k D Ͻ 2.28, but, as mentioned earlier, this includes components from both the proton and general acid pathways.…”

“…[17] Indeed, this is a reaction that has been used to release dialkylamines protected as a triazene functionality. [1] However, an investigation by Vaughan and coworkers [18] showed that aryloxymethyltriazenes behaved more like acetals than 3,3-dialkyltriazenes. Hydrolysis of these compounds was found to be specific acid and buffer catalysed and that there is also a spontaneous reaction for some compounds.…”