Addition and redox processes in the reaction of Grignard reagents with 1,4-dinitrobenzene. Factors affecting product distribution

Abstract

1,4-Dinitrobenzene (1) reacts smoothly and irreversibly with alkyl-magnesium or -lithium reagents to give at first the nitroarene radical anion (3)(redox product) and 6-alkyl-2-nitro-5-acinitrocyclohexa-1,2-diene (4)(addition product). Intermediate (4) undergoes an immediate addition to the nitro function by a second mole of RMgX or RLi to give trans-4,5-dialkyl-3,6-di-aci-nitrocyclohexene (5), which can be converted into the corresponding trans-5,6-dialkyl-1,4-dinitrocyclohexa-1,3-diene (6) by oxidation with sodium hypochlorite or dichlorodicyanobenzoquinone. The addition process is favoured by lower temperatures and weakly polar and highly. Viscous solvents, while steric hindrance in the magnesium reagent enhances radical anion (3) formation. These findings are interpreted in terms of a single electron-transfer mechanism in which all factors delaying a geminate recombination of the radical pair [originating from one electron donated from the reagent RM to (1)] favour the redox process to the detriment of addition. The almost absolute stereoselectivity of the double alkylation process is attributed to steric control on the direction of attack of RM to the ene-nitro function of (4) exerted by the axial alkyl group. A detailed e.s.r. study of 1,4-dinitrobenzene radical anion is also reported.