The interaction of alkali metals with unsaturated heterocyclic compounds : I. 2,3-diphenylquinoxaline and its cyclodehydrogenation to dibenzo [ a,c] phenazine

Abstract

Sodium reduces 2,3-diphenylquinoxaline (I) to a dianion which was characterized by reaction with ethyl chloroformate, methyl iodide and 1,3-diiodopropane. The first reagent produced 1,4-bis(ethoxycarbonyl)-2,3-diphenyl-l,4-dihydroquinoxaline(III), the second a mixture of 1,2- and 1,4-dimethyl-2,3-diphenyl-1,4-dihydroquinoxalines [(V) and (VI)] and the third, 3 a,4-diphenyl-1,2,3,3 a-tetrahydropyrrolo[1,2- a] quinoxaline (VIII). Treatment of (I) with lithium effected reduction and cyclodehydrogenation with the formation of the dibenzo[ a,c]phenazine ring system. The lithium derivative so generated was compared in its chemical reactions to the lithium and sodium derivatives formed by reduction of dibenzo[ a,c]phenazine itself by the alkali metal. With methyl iodide, the lithium and sodium derivatives gave 9,14-dimethyl-9,14-dihydrodibenzo[ a,c]phenazine (XI). With ethyl chloroformate, the sodium derivative formed 9,14-bis(ethoxycarbonyl)-9,14-dihydrodibenzo[ a,c]phenazine (XII) while the lithium derivative produced (XII) and 10,13-bis(ethoxycarbonyl)-9,14-dihydrodibenzo[ a,c]phenazine (XIV). Additional products were isolated when excess ethyl chloroformate was added to the lithium derivative formed via the cyclodehydrogenation of (I). The formation of these by-products is traced to the presence of lithium hydride, a co-product of the cyclodehydrogenation reaction.