Cyclopropyl Aromatic Chemistry. I. Ultraviolet Spectra of Certain Cyclopropyl Aromatic Systems

Abstract

The ultraviolet spectra of α- and β-cyclopropylnaphthalenes (I and II) resemble those of the corresponding methylnaphthalenes much more closely than those of α- and β-alkenylnaphthalenes. Data are tabulated to support the suggestion that a minimal electron-attracting ability of the naphthalene ó system evokes a minimal conjugative response from the cyclopropyl group. To answer the question of the dependence of cyclopropane conjugation on geometry in arylcyclopropanes, uv spectra were obtained of 3- and 4-nitro-1,1a,6,6a-tetrahydrocycloprop-[α]indenes (β- and βʹ-NO2-IV), 6ʹ- and 5ʹ-nitrospiro[cyclopropane-1,1 ʹindans] (β- andβʹ-N02-V), 5-nitro-1a,2,3,7b-tetrahydro-lH-cyclopropa[α]naphthalene (β′-NO2-VVI), 6ʹ-nitrospiro[cyclopropane-1,1 ʹ-tetralin] (βʹ-N02-VII), and 7- and 6-nitro-1,1a,2,3,4,8b-hexahydrobenzo[α]cyclopropa[]cycloheptenes (β- and βʹ-NO2-VIII). Values for λmaxʹs (1Lα band, 95% EtOH) are: β-nitroindan, 282 mµ β-NO2-IV andβ-NO2-V, 282; β-NO2-VIII, 280 (nitro meta to cyclopropyl inβ series); βʹ-NO2-VIII, 285; βʹ-NO2-IV, 301; βʹ-N02-VI and, βʹ-NO,-VII, 306; βʹ-NO2-V, 313 (nitro para to cyclopropyl in βʹ series). The absence of noticeable differences in the maxima of β-nitroindan,β-N02-IV, and β-N02-V is attributed to an absence of inductive donation by the cyclopropyl group to the nitro-aromatic chromophore. The bathochromic λmax shifts from β-N02-IV to βʹ-N02-IV and from β-N02-V to βʹ-NO2-V are seen as results of cyclopropane conju ative interaction which is possible only in the βʹ-nitro systems. The bathochromic Γmax shifts in the βʹ series fromβʹ-N02-VIII toβʹ-N02-V are related to increasingly favorable cyclopropane geometries for conjugation. It is concluded that cyclopropane conjugation in cyclopropyl aromatic systems is a spectroscopically detectable function of cyclopropane geometry if the interacting chromophore is sufficiently electron attracting. © 1969, American Chemical Society. All rights reserved.