Electroréduction de dérivés dinitrés aromatiques. III. Rôle du sulfate de titanyle comme médiateur redox

Abstract

The electrochemical behavior of titanium oxysulfate, used as a redox mediator in the reduction of 4,4′-dinitrodibenzyl, is studied in a 5 N H2SO4/EtOH 50:50 medium. The role played by electrogenerated Ti3+ ions in the chemical reduction of nitro groups is also defined. The results obtained allow the determination of the most suitable conditions leading to selective syntheses. Macroscale electrolyses at a constant potential in the presence of TiOSO4 give 4,4′-diaminodibenzyl with a great purity and a higher yield (93%) than without mediator (75%). The homogeneous chemical reduction of 4,4′-dinitrodibenzyl by first electrogenerating the Ti3+ ions, then adding the organic substrate, is particularly selective. The nature of the reduction products depends on the [Ti3+]/[dinitro derivative] ratio. The 4-amino-4′-nitrodibenzyl is synthesized in a 70% yield for a 6:1 ratio whereas the amount of diamino derivative increases and can constitute the sole reduction product for higher ratios. A comparison is made with 1,5- and 1,8-dinitronaphthalenes and 2,2′-dinitrodiphenyl. Using a 6:1 ratio, 1-amino-8-nitronaphthalene is not obtained while 45% of 1-amino-5-nitronaphthalene and 25% of 2-amino-2′-nitrodiphenyl are formed by reduction of the corresponding dinitro derivatives.