Diazenes. IV. Circular Dichroism of DialKyldiazenes (Azoalkanes)

The long-wavelength absorption band of a 1,2-dialkyldiazene (RN=NR, “azoalkane,” R = 2-cyclo-hexyl-2-butyl) with γ_max 3805 A (e 22) in isooctane is accompanied by a single Gaussian-shape circular dichroism band (γ_max 3790 A, [θ]_max 4450). This observation supports the recent conclusion that the low energy “n→ π* absorption band” of diazenes is a single n₊ → *π transition. The optically active dialkyldiazene with R = 2-phenyl-2-butyl exhibits two overlapping CD bands of opposite signs. Of the two possible explanations for two CD bands, two species or two (or more) vibronic couplings, the latter is preferred. The reversal of sign in the CD absorptions of certain cyclic ketones is reexamined and interpreted in the same way. The opposite signs of the CD curves are explained as a natural consequence of the difference in the polarizability of the highest and lowest vibrational states of the n,π* electronic state. The syntheses of the optically active dialkyldiazenes and 1,2-bis(l-adamantyl)diazene are described. The shift of the n₊ → π* transition to longer wavelengths with (a) increasing substitution on the α carbon and (b) increasing steric hindrance between the alkyl and diazene groups is noted.