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首页> 外文期刊>European journal of organic chemistry >Dediazoniation of 4-nitrobenzenediazonium ions in acidic MeOH/H2O mixtures: Role of acidity and MeOH concentration on the formation of transient diazo ethers that initiate homolytic dediazoniation
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Dediazoniation of 4-nitrobenzenediazonium ions in acidic MeOH/H2O mixtures: Role of acidity and MeOH concentration on the formation of transient diazo ethers that initiate homolytic dediazoniation

机译：酸性MeOH / H2O混合物中4-硝基苯重氮离子的去重氮反应：酸度和MeOH浓度对引发均质去重氮反应的瞬时重氮醚形成的作用

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We have investigated the dediazoniation of 4-nitrobenzene-diazonium (4NBD) ions in MeOH/H2O mixtures under acidic conditions at 50 degrees C employing a combination of spectrophotometric and chromatographic techniques, The kinetic behaviour is quite complex; in the absence of MeOH, the dediazoniations follow first-order kinetics with a half-life t(1/2) = 1383 min, but the addition of small concentrations of MeOH lead to more rapid but non-first-order kinetics, suggestive of a radical mechanism, with t(1/2) approximate to 6 min at 10% MeOH. Further increases in the MeOH concentration slow down the rate of dediazoniation and reactions progressively revert to first-order behaviour, with clean first-order kinetics at percentages of MeOH higher than 90 % (t(1/2) approximate to 77 min). The analyses of the reaction mixtures by HPLC indicate up to four dediazoniation products depending on the particular experimental conditions. These are 4-nitrophenol (ArOH), 4-nitro-anisole (ArOMe), nitrobenzene (ArH), and 4,4'-dinitrobiphenyl (DNB), this last product being detected only at MeOH percentages in the range 0.5-15%. In the absence of MeOH, ArOH is the only product and formed in quantitative yield; however, at only 20 % MeOH, ArOH is down to less than 10% and the reduction product, ArH, is obtained in more than 90 %. Upon increasing the MeOH content further, the formation of ArOMe becomes competitive and, at 99 % MeOH, both ArH (60 %) and ArOMe (40 %) are obtained. The observed rate constants, k(obsd.), or half-lives, t(1/2), are the same regardless of whether they are determined by monitoring product formation (by a well-established product derivatization protocol followed by HPLC measurements) or reactant decomposition (obtained UV spectrophotometrically). At any given MeOH content, the plot of k(obsd.) or t(1/2) values against the acidity (defined as -log [HCl]) is S-shaped, the inflexion point depending upon the MeOH concentration, The complex behaviour can be accounted for by two competitive mechanisms, and whichever is dominant depends upon the acidity and the composition of the aqueous methanol solvent. Furthermore, the acid-dependence of the switch between the homolytic and heterolytic mechanisms suggests the homolytic dediazoniation proceeds via transient diazo ethers. Thus, the MeOH molecules react with ArN2+ to yield an O-coupling adduct in a highly unstable Z configuration, i.e. a (Z)-diazo ether. Subsequently, this undergoes homolytic fragmentation which initiates a radical process. In the polar alternative, the methanol simply solvates the ArN2+ ions, allowing them to undergo thermal heterolysis, i.e. a DN + A(N) mechanism via the aryl cation.
机译：我们在50°C的酸性条件下，采用分光光度法和色谱技术相结合，研究了MeOH / H2O混合物中4-硝基苯重氮（4NBD）离子的去重氮反应，动力学行为十分复杂。在没有MeOH的情况下，脱氮反应遵循一级动力学，半衰期为t（1/2）= 1383分钟，但是添加少量的MeOH会导致更快但非一级动力学，这表明自由基机理，在10％MeOH下，t（1/2）大约为6分钟。 MeOH浓度的进一步增加会减慢脱氮反应的速度，并且反应会逐渐还原为一级行为，在MeOH百分比高于90％时，干净的一级动力学（t（1/2）约为77分钟）。通过HPLC对反应混合物的分析表明，取决于特定的实验条件，多达四个脱重氮产物。它们是4-硝基苯酚（ArOH），4-硝基苯甲醚（ArOMe），硝基苯（ArH）和4,4'-二硝基联苯（DNB），仅以MeOH百分比在0.5％至15％范围内检测到该最后产物。在没有MeOH的情况下，ArOH是唯一的产物，并以定量收率形成。但是，只有20％的MeOH，ArOH降至低于10％，而还原产物ArH的含量超过90％。随着进一步增加MeOH含量，形成ArOMe变得更具竞争力，并且在MeOH为99％时，同时获得了ArH（60％）和ArOMe（40％）。无论观察速率常数k（obsd。）或半衰期t（1/2）是否相同，无论它们是通过监测产物的形成（通过完善的产物衍生化方案，然后进行HPLC测量）来确定的或反应物分解（通过紫外分光光度法获得）。在任何给定的MeOH含量下，k（obsd。）或t（1/2）值相对于酸度（定义为-log [HCl]）的曲线均为S形，拐点取决于MeOH浓度，行为可以通过两种竞争机制来解释，而哪个占主导地位取决于酸度和含水甲醇溶剂的组成。此外，在均质和异质机理之间转换的酸依赖性表明均质的去重氮基通过瞬时重氮醚进行。因此，MeOH分子与ArN2 +反应生成高度不稳定的Z构型的O偶联加合物，即（Z）重氮醚。随后，其经历均质裂解，从而引发自由基过程。在极性替代方案中，甲醇仅将ArN2 +离子溶剂化，使它们进行热杂解，即通过芳基阳离子进行DN + A（N）机理。

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