Electrochemical reduction of 2,2'-dinitrodiphenylmethane and 2,2'-dinitrobenzophenone at mercury electrodes

The last two compounds in a 5-membered series of aromatic dinitro compounds are reduced in a completely different manner at mercury electrodes. 2,2'-Dinitrodiphenylmethane (I) - in which a conjugation of both symmetrical moieties is ruled out - is electrolytically reduced in an eight-electron step to a bishydroxylamine this being most stable between pH 4.5 and 5.0. In processing the catholyte increase in concentration of this product leads to its intermolecular disproportionation, resulting in the formation of dibenzo[b, e]-1,2-diazepine 5-oxide (IV) and 2,2'-diaminodiphenylmethane (III). 2,2'-Dinitrobenzophenone (II) reduces at more positive potentials. Its preparative electrolysis in acetonitrile (with 0.1M-N(n-C4H9)4PF6 as supporting electrolyte) the application of which was made necessary by the low solubility of II in ethanol, proceeding in an anomalous way. In the most positive cathodic wave a radical anion results, the following cathodic wave corresponds to a 6-electron reduction of the one nitro group to an amino group while the other nitro group splits off as the anion NO-2 (this later giving an anodic wave). Its formation has been proved by standard addition in polarography and by a qualitative analytical test. The product which results through this electrode process and a follow-up chemical reaction is acridone. This in turn, reduces in the third, most negative 4-electron wave to dihydroacridine. The comparison of all substances studied in this series reveals that their reducibility decreases with respect to the link X, viz. in the sequence CO > O > S and CH2 > NH. The electrolytical reduction on mercury cathodes occurs in a similar manner with analogues where X = O, S or CH2. Here, the main intermediate is the bis-hydroxylamine the stability of which predetermines the structure of final products. The other group comprises the substances with X = CO and NH. Here the main intermediate is the 2-nitro-2'-amino-diphenyl-X which is formed in a 6-electron process taking place at one of the nitro groups. The follow-up cyclization reaction leading to seven-membered heterocyclic rings located between two benzene nuclei only occurs with analogues of the type X = CH2, O and S. A partial reduction of dinitro compounds of this series has been observed with the analogue containing the NH link although that with X = CO has generally good preconditions for this mechanism.