Electro Organic Synthesis Utilizing Mg Electrodes (II) - Novel Synthesis of Symmetric Azobenzenes from Nitrobenzenes -

Abstract

On the way to study electro organic synthesis (EOS) utilizing alternating Mg electrodes, a novel synthetic route from the nitrobenzenes to symmetric azobenzenes was discovered in our laboratory. The brief review about EOS was introduced by us. The EOS is expected to be an important organic synthetic tool in future because of its diverse application in industry in addition to compatibility with green chemistry. In the previous paper, we reported the optimized reaction conditions for the electro reductive coupling of aromatic halides: Mg rod for both of anode and cathode under a constant current, LiClO4 as a electrolyte, THF or glyme 7 as a solvent. We have tried to extend EOS to other functional groups. Since nitrobenzenes were readily reduced under the various mild reaction conditions, we exposed several nitrobenzenes under EOS reaction conditions. Our reaction conditions were Mg for anode and cathode, LiClO4, THF, room temperature under a constant current (current density = 42 mA/cm). The currents of anode and cathode were alternated at an interval of 30 seconds in order to minimize the consumption of the metal. The reaction mixture was carefully monitored by TLC and then worked up when nitrobenzene 1 was completely disappeared. The new spot was isolated and characterized fully by TLC and spectroscopic methods with an authentic sample. To our surprise, it was azobenzene 2 instead of aniline 3 based on the previously reported paper. The optimized yield of 2 was 87.6 % with no trace of the desired aniline 3. The amount of Mg consumed during the reaction was less then 5.0% for every attempt (eq. 1).