Proton-driven diels-alder reaction of azo compounds with cyclic dienes

Abstract

Diels-Alder addition of bicyclic azo compounds to cyclic dienes would produce bis-N,N'-bicyclic hydrazines, but this reaction proves to be endothermic. The greater basicity of the adduct than of the azo compound makes the addition reaction 13.6–14.1 kcal mol-1 less endothermic: (in water) if the protonated azo compound is employed instead of the neutral compound, and the addition reactions proceed well in acetonitrile if HBF4Ėt2O is used to protonate the azo compound before adding the diene. Preparations of the neutral hydrazines derived from 2,3-diazabicyclo[2.2.2]oct-2-ene(5) with 1,3-cyclohexadiene(6) and from 2,3-diazabicyclo [2.2.1] hept-2-ene (10) with 6 and 1,3-cycloheptadiene (7,11, and 13, respectively) as well as the saturated hydrazines derived by catalytic hydrogenation (9,12, and 14, ropectively) are reported. The neutral adduct of 10H+ and cyclopentadiene, 15, proves to be too thermally unstable for isolation, and the saturated form, 16, is prepared by diimide reduction of 15H+. The neutral hydrazines all give radical cations and dications with appreciable solution lifetimes, and 92+ (BF4-)2 is the first isolable hydrazine dication. The thermodynamics of electron loss for these hydrazines are studied by cyclic voltammetry, and prove to be dominated by non-bonded steric interactions between the alkyl groups. Compound 9 is the most easily oxidized hydrazine known, at E°' = -0.54V vs SCE.