Formation and Decomposition of Diazo Ethers under Acidic Conditions. Effects of Ethanol Concentration, Acidity, and Temperature on the Ethanolysis of 4‐Methylbenzenediazonium Ions

Abstract

We investigated the effects of solvent composition, acidity, and temperature on the dediazoniation of 4-methylbenzenediazonium (4MBD) ions in EtOH/H 2 O mixtures by employing a combination of spectrometric and chromatographic techniques. First-order behavior is found in all solvent composition ranges. HPLC Analyses of the reaction mixtures indicate that three main dediazoniation products are formed depending on the particular experimental conditions. These are 4-cresol (ArOH), 4-phenetole (ArOEt), and toluene (ArH). At acidities (defined as À log [HCl]) < 2, the main dediazoniation products are the substitution products ArOH and ArOEt but upon decreasing the acidity, the reduction product ArH becomes predominant at the expense of ArOH and ArOEt, indicating that a turnover in the reaction mechanism takes place under acidic conditions. At any given EtOH content, the plot of k obs values against acidity is S-shaped, the inflexion point depending upon the EtOH concentration and the temperature. Similar S-shaped variations are found when plotting the dediazoniation -product distribution against the acidity. The acid dependence of the switch between the homolytic and heterolytic mechanisms suggests that the homolytic dediazoniation proceeds via transient diazo ethers, and this complex kinetic behavior can be rationalized by assuming two competitive mechanisms: i) the spontaneous heterolytic dediazoniation of 4MBD, and ii) an O-coupling mechanism in which the EtOH molecules capture ArN þ 2 to yield a highly unstable (Z)-adduct which undergoes homolytic fragmentation initiating a radical process (Scheme). Analyses of the effects of temperature on the equilibrium constant for the formation of the diazo ether and on the rate of splitting of the diazo ether allowed the estimation of relevant thermodynamic parameters for the formation of diazo ethers derived from methylbenzenediazonium ions under acidic conditions.

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.…”

Radical Arylation of Anilines and Pyrroles via Aryldiazotates

“…Radical adducts 6* , 9* , 4 a* , and 14 a* derived from the benzene‐, anisole‐, aniline‐, and 1‐methylpyrrole‐substituted biaryl compounds 6 , 9 , 4 a , and 14 a , respectively, were proposed as intermediates for two possible reaction mechanisms (Scheme ) . In the first, the general radical adduct 19 is deprotonated to give the radical anion 19 .− , which is then oxidized to give the product 20 .…”

“…[41] Geometries wereo ptimizedw ithouts ymmetry constraints, and all structures reported were confirmed to be local minimab yc alculating their normal vibrations within the harmonic approximation. [37] In the first, the general radicala dduct 19 is deprotonated to give the radicala nion 19C À ,w hich is then oxidized to give the product 20.T he second mechanism reverses the order of the deprotonationa nd oxidation steps to first give the adduct cation 19 + + ,w hich is then deprotonated to give the biaryl 20. [42] Radical adducts 6*, 9*, 4a*,a nd 14 a* derived fromt he benzene-, anisole-,a niline-, and 1-methylpyrrole-substituted biaryl compounds 6, 9, 4a,a nd 14 a,r espectively,w ere proposed as intermediates for two possible reaction mechanisms (Scheme 5).…”

“…The positive effect of tert ‐butanol (Table , entries 11–17) on the arylation of 3 l can be rationalized by the mechanism depicted below (Scheme ), in which the generation of aryl radical 18 is facilitated through the occurrence of diazo ether 17 as a more effective radical source than diazohydroxide 16 . The significant differences observed between the ptGB conditions and the diazotate method (Table ) are also indirectly in agreement with the tert ‐butanol effect and can be explained as described below.…”

Solvolyses of 4‐methylbenzenediazonium ions in trifluoroethanol/water mixtures: Effects of [TFE], acidity and temperature on the kinetics and mechanism of the reaction

“…Only heterolytic dediazoniation products are obtained in significant yields with increasing %TFE, temperature and acidity, and reduction products like ArH are in low yield. In contrast, large amounts of the reduction product ArH were obtained upon decreasing the acidity in solvolytic reactions carried out in MeOH or EtOH; for instance, stoichiometric amounts of ArH are found in the ethanolysis of 4 MBD at −log[HCl] = 5, meanwhile negligible amounts are found at −log[HCl] = 2 15, 26. Therefore, the absence of significant amounts of reduction products like ArH or biaryls, Ar–Ar, indicates that dediazoniation is heterolytic mechanism (Scheme ).…”