The electrochemical reduction of fluorenone hydrazone, fluorenone fluorenylhydrazone, fluorenone azine and their benzophenone analogs in DMF

Abstract

The electrochemical reductions of fluorenone hydrazone (Fl=NNH 2), fluorenone fluorenylhydrazone (FIHNHN=Fl), fluorenone azine (Fl=N−N=Fl) and benzophenone analogs have been studied at platinum cathodes in DMF−0.1 M(n-Bu) 4NClO 4 in the absence and presence of an added proton donor. Fl=NNH 2 undergoes reduction to give an unstable anion radical which decomposes by an unidentified pathway to afford Fl=NH. The latter species is electroactive at the applied potential and is reduced to the corresponding amine, FlHNH 2. Four electrons per molecule of Fl=NNH 2 are required for this process when electroreduction is effected in the presence of diethyl malonate (DEM), an electroinactive proton donor. Unreacted Fl=NNH 2 serves as the source of protons when electroreduction is conducted in the absence of DEM. Dual reaction channels are observed for the reduction of FlHNHN=Fl. If the corresponding anion radical is not protonated by either an added proton donor or unreacted starting material, decomposition occurs by carbon-nitrogen bond cleavage to give FlH 2 and Fl=NNH 2 as products. Reduction of the latter species occurs at the same potential as FlHNHN=Fl, ultimately affording FlHNH 2. The reaction channel involving carbon-nitrogen bond cleavage is replaced by a pathway involving nitrogen-nitrogen bond cleavage in the presence of an added proton donor. The final reduction product by this route is FlHNH 2. Fl=N−N=Fl is reduced stepwise and reversibly to its dianion in an aprotic medium. In the presence of a relatively strong proton donor such as hexafluoro-2-propanol, reduction gives FlHNHN=Fl. The redox behavior of the benzophenone analogs, Ph 2C=N−N=CPh 2, Ph 2CHNHN=CPh 2 and Ph 2C=NNH 2, parallels that of their counterparts in the fluorene series.