Mechanism of reduction of 1,1-diphenyl-2,2-dinitroethylene by 1-benzyl-1,4-dihydronicotinamide: transition state with partial diradical and partial covalent bonding character

Abstract

The mechanism of reduction of 1,1-diphenyl-2,2-dinitroethylene (DPDN) by 1-benzyl-1,4-dihydronicotinamide (BNAH) in acetonitrile has been investigated. Based on product analysis, isotopic tracing and electrochemical analysis, the reaction takes place by a hydride transfer mechanism giving 1,1-diphenyl-2,2-dinitroethane (DPDNH). Single crystal X-ray analysis shows that DPDN conforms to idealized C2 symmetry, but steric repulsions between the bulky substituents result in an appreciable twist about the central bond, such that the phenyl rings make a dihedral angle of 77.7°and the planar C–NO2 fragments make a dihedral angle of 68.5°. A small kinetic H/D isotope effect was obtained, that we propose is due to steric hindrance. Reaction in oxygen-saturated acetonitrile produced DPDNH and benzophenone in the ratio of 59.2∶22.0 as the final products with a total yield of 68.6% by GC. Control experiments were performed, by stirring a solution of DPDN or DPDNH alone in oxygen-saturated acetonitrile, or by stirring a solution of DPDN or DPDNH alone in aqueous acetonitrile containing a small amount of hydrochloric acid or in acetonitrile containing triethylamine. These produced no benzophenone. The results clearly indicate the trapping of a radical species by oxygen in the reaction. A curve-crossing model for the reaction projects that the transition state has partial diradical and partial covalent bonding character. As DPDN has a low-lying π* orbital (LUMO), the radical anion DPDN−· is a stabilized radical. It is known that the reaction of alkyl and benzyl radicals with oxygen is exothermic with a rate close to the diffusion-controlled limit. Thus, with use of More O’Ferrall’s two-dimensional potential energy diagram, the results are rationalized by a mechanistic change induced by steric hindrance so that the transition state collapses in two directions leading to the formation of DPDNH (polar pathway) and benzophenone (ET pathway), respectively.