Electroreduction in Aqueous Media, Saturation of Polycyclic Aromatics

摘要

The preparative cathodic reduction of anthracene and phenanthrene was investigated aiming at obtaining Birch-type products in aqueous solutions. Following previous reports, reductions were carried out using mercury pool cathodes and tetrabutylammonium electrolytes. Optimum conditions were found to involve tetrabutylammonium hydroxide as the electrolyte and water as the sole solvent. Reactions were carried out in a simple cell, using constant-current and the products were isolated and identified. Anthracene formed initially 9,10-dihydroanthracene which reduced with additional charge transfer to 1,4,5,8,9,10-hexahydroanthracene in high yield. To obtain more saturated anthracene derivatives, a process was developed which involves a ''one pot'' reduction-isomerization-reduction sequence. Anthracene was electrolyzed first to completion, the product was isomerized by reflux in the electrolyte solution (in the cell but without charge transfer) and was then electrolyzed again. Using this process 1,2,3,4,5,6,7,8-octahydroanthracene and 1,2,3,4,5,6,7,8,9,10-decahydroanthracene were formed in significant yields. Reduction of phenanthrene resulted in mixtures of high saturated derivatives without need for isomerization steps. The major products varied with the amount of charge transferred from dihydrophenanthrene (3F/mol) to octahydrophenanthrene (10F/mol) and decahydrophenanthrene (20F/mol). In general it was shown that cathodic hydrogenation of polycyclic aromatics is possible in aqueous solutions. The products resemble those obtained by metal-ammonia or amine reductions. The degree of saturation can be controlled by the amount of charge transferred and can be increased by a combination of alternate reductions and isomerizations. 15 refs., 3 tabs. (ERA citation 11:020586)