Orientation in Photochemical Cyclobutane Cleavages: cis‐Effect

Irreversible One‐Way [2+2] Photocycloaddition in Bis‐Styrylbenzo[f]quinoline Dyad: Photoactive and Photoinert Excimers in the Same System

“…Photocycloaddition is generally a reversible reaction, and in photocycloreversion of cyclobutanes, the cis-effect has been earlier discovered: relatively fast cleavage of the cis-disubsti-tuted bond, for example, in the stilbene photodimers. [30] We have discovered a new phenomenon: cyclobutane does not undergo photocycloreversion, because π-stacking between two cis-substituents, vice versa, prevents bond cleavage and ring opening. The possibility of such effects in other substituted cyclobutanes opens a novel way of managing photochemical properties and requires further study.…”

Organic Solid-State Reactions

“…Furthermore, the trans-6,7-diphenyl-bicyclo [3.2.0]heptane photocleaves under the same conditions to give only cyclopentene and trans-stilbene via the less stabilized diradical. However, the exo-6,exo-7-diphenyl-isomer undergoes only the valence isomerization to give the 1,7-diphenyl-1,6-heptatriene [159]. These specificities are clearly predicted by the cis-effect.…”

“…The very long irradiation times must be due to repeated photocycloreversions of the terminal cyclobutane rings (cf. [20,144,159] (the pyridyl substituents serve as light absorbing chromophores). Extensions of this photochemical technique [158] are of course problematic due to the nonabsorption of the filtered broadband light (<300 nm) by inner double bonds when these become isolated from conjugation between cyclobutane units from both sides.…”

“…The former exhibits a lower fluorescence yield in cyclohexane (0.0072) than the latter (0.024), and the photolytic decay into stilben is three times faster with the former at −190 ∘ C in EE glasses (no phosphorescence and no side reactions). Clearly, the cis-disubstituted bonds break again with higher probability than the trans-disubstituted ones [159]. The endo-endo-6,7-diphenyl-bicyclo [3.2.0]heptane (Scheme 21) photocleaves primarily into stilbene and cyclopentene but significantly also into 1,7-diphenyl-1,6-heptatriene (3 : 1-ratio).…”

“…It photocleaves specifically to give 6,7-diphenyl-bicyclo[3.2.0]hept-6-ene and cyclopentene (Scheme 21). The cis-relation of the two phenyl groups in the cyclobutane units is not used for a cleavage of the central bond, but only the cis-relation between phenyl and methylene [159]. An extension of the central bond would increase steric hindrance between phenyl and methylene groups, according to molecular models.…”