Effects of Ascorbic Acid on Arenediazonium Salts Reactivity: Kinetics and Mechanism of the Reaction

Abstract

We have examined the kinetics and mechanism of dediazoniation of o-, m-and p-methylbenzenediazonium (ArN 2 ) tetrafluoroborate in the presence of ascorbic acid (H 2 A) at different pHs by combining spectophotometric (VIS-UV), high performance liquid chromatography (HPLC), and polarographic measurements. Kinetic data show that, at low pH, observed rate constants increase linearly with increasing ascorbic acid concentration, but the saturation kinetics observed at higher pH suggest the formation of a transient diazo-ether complex preceding the slow step of the reaction. Experimental evidence for the formation of such a complex was obtained from a competitive coupling reaction with the Na salt of 2-naphthol-6-sulfonic acid and by titration of ascorbic acid (H 2 A) with the arenediazonium ions (electrochemical measurements). HPLC Analysis of dediazoniation products indicates that, in the absence of H 2 A, only the heterolytic phenol derivative, ArOH, is formed quantitatively, in keeping with the predictions of the D N A N mechanism. In the pH 2 ± 4 range and in the presence of H 2 A, reduction products (ArH) are obtained in addition to heterolytic products (ArOH), corroborating that certain biological reducing agents like ascorbate (HA À ) are capable of inducing reductive fragmentation of ArN 2 into aryl radicals. All evidence is consistent with two competitive reaction pathways, the thermal decomposition of ArN 2 , and a rate-limiting decomposition of the transient diazo ether complex, formed during the reaction of ArN 2 with HA À in a rapid pre-equilibrium step.

C–H Arylation of Heterocyclic N-Oxides Through in Situ Diazotisation Of Anilines without Added Promoters: A Green And Selective Coupling Process

“…In this proposed mechanism, aniline 26 is diazotised in the presence of tert -butyl nitrite and ascorbic acid to form the aryldiazonium ascorbate salt 27 , which reacts to generate the key intermediate 28 , which was believed to generate aryl radicals through spontaneous dediazotisation and generation of a stable ascorbate radical . In order to probe this hypothesis, we elected to prepare the compound 36 according to the method published by Doyle and co-workers and to study its behavior under the reaction conditions (Scheme ).…”

Butanolysis of 4-methylbenzenediazonium ions in binary n-BuOH/H2O mixtures and in n-BuOH/SDS/H2O reverse micelles. Effects of solvent composition, acidity and temperature on the switch between heterolytic and homolytic dediazoniation mechanisms

“…Previous studies on the formation of diazo ethers under alkaline conditions suggest that diazo ethers are initially formed in a highly unstable, kinetically controlled, Z-configuration which then may undergo subsequent isomerization to the thermodynamically stable E-isomers, which can be isolated in some instances, 61 or eventually may give rise to homolytic rupture of the bonds providing the initiation of a radical process. 35,40,46,55,58,62,63 This bond rotating mechanism to transform the Z-into the E-isomer has been described for Sandmeyer hydroxylations and chlorination reactions. 59 In conclusion, we have been able to investigate the formation and decomposition of transient diazo ethers in the course of the butanolysis of 4MBD by employing binary BuOH/H 2 O mixtures and nanostructured systems such as the reverse micelles, which permit one to overcome the solubility problems of BuOH in water.…”

Methanolysis of 4-bromobenzenediazonium ions. Effects of acidity, [MeOH] and temperature on the formation and decomposition of diazo ethers that initiate homolytic dediazoniation

“…Previous studies on the formation of diazo ethers under alkaline conditions suggest that diazo ethers are initially formed in a highly unstable, kinetically controlled, Z-configuration which then may undergo subsequent isomerization to the thermodynamically stable E-isomers, which be in some instances can isolated, 20 or eventually may give rise to homolytic rupture of the bonds providing the initiation of a radical process. 14,19,25,31 This bondrotating mechanism to transform the Z-into the E-isomers has been recently described for Sandmeyer hydroxylations and chlorination reactions. 31 All kinetic and product distribution evidence suggests, therefore, that in the present case there is no conversion of the unstable Z-diazo ether to the much more stable E-isomer, which would eventually undergo acid catalyzed fragmentation, 12,14,32 as was found for the methanolysis of 4NBD.…”

“…Electron-withdrawing substituents (as opposed to methyl, for example) in the aromatic ring make the arenediazonium ions prone to decompose through homolytic pathways. For example, transient O-diazo ether derivatives were detected in dediazoniation of 4NBD ions in the presence of b-cyclodextrin 17,18 or ascorbic acid; 19 for both nucleophiles, the adducts were detected experimentally and, in some instances, isolated. 20 As we shall show, the results obtained for the methanolysis of 4BrBD provide further support for a radical initiation mechanism via formation of a transient diazo ether, Ar-N=N-OMe, which undergoes homolytic fragmentation yielding a bromophenyl radical (Ar ∑ ) and the methoxy radical ( ∑ MeO).…”