Homogeneous, Supramolecular and Bio-Inspired Catalysis, van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.
Molecules. 2018 May 1;23(5):1052. doi: 10.3390/molecules23051052.
Azobenzenes are versatile compounds with a range of applications, including dyes and pigments, food additives, indicators, radical reaction initiators, molecular switches, etc. In this context, we report a general method for synthesizing -aminoazobenzenes using the commercially available cobalt(II) tetraphenyl porphyrin . The net reaction is a formal dimerization of two phenyl azides with concomitant loss of two molecules of dinitrogen. The most commonly used methodology to synthesize azobenzenes is based on the initial diazotization of an aromatic primary amine at low temperatures, which then reacts with an electron rich aromatic nucleophile. As such, this limits the synthesis of azobenzenes with an amine functionality. In contrast, the method we report here relies heavily on the -amine moiety and retains it in the product. The reaction is metal catalyzed and proceeds through a porphyrin Co(III)-nitrene radical intermediate, which is known to form on activation of organic azides at the cobalt center. The synthesized -aminoazobenzenes are bathochromatically shifted, as compared to azobenzenes without amine substituents. Based on the crystal structure of one of the products, strong H-bonding between the N-atom of the azo functionality and the H of the NH₂ substituent is shown to stabilize the isomeric form of the product. The NH₂ substituents offers possibilities for further functionalization of the synthesized azo compounds.
偶氮苯是一类用途广泛的化合物，包括染料和颜料、食品添加剂、指示剂、自由基反应引发剂、分子开关等。在这种情况下，我们报告了一种使用商业上可获得的钴 (II) 四苯基卟啉合成 - 氨基偶氮苯的通用方法。净反应是两个苯叠氮化物的正式二聚化，同时失去两个分子的氮气。合成偶氮苯最常用的方法是基于在低温下对芳族伯胺进行初始重氮化，然后与富电子芳族亲核试剂反应。因此，这限制了具有胺官能团的偶氮苯的合成。相比之下，我们在这里报告的方法严重依赖于 - 胺部分，并在产物中保留它。该反应是金属催化的，通过卟啉 Co(III)-氮烯自由基中间体进行，已知该中间体在钴中心激活有机叠氮化物时形成。与没有胺取代基的偶氮苯相比，合成的 - 氨基偶氮苯发生了红移。基于其中一种产物的晶体结构，显示出偶氮官能团的 N 原子和 NH₂取代基的 H 之间的强氢键稳定了产物的异构形式。NH₂取代基为合成的偶氮化合物的进一步官能化提供了可能性。
