PROTON-DRIVEN DIELS-ALDER REACTION OF AZO COMPOUNDS WITH CYCLIC DIENES

Article abstract of DOI:10.1016/S0040-4020(01)87595-5

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 HBF4*Et2O 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, respectively) are reported.The neutral adduct of 10H(1+) and cyclopentadiene, 15, proves to be too thermally unstable for isolation, and the saturated form, 16, is prepared by diimide reduction of 15H(1+).The neutral hydrazines all give radical cations and dications with appreciable solution lifetimes, and 9(2+) (BF4(1-))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' = -0.54 V vs SCE.