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.