ABSTRACT Stereochemical reaction courses in thermal and photochemical ring-cleavage reactions of selected diarylcyclobutanes are described in terms of different π-o, π-σ*, or π*-σ* interactions controlled by rotameric conformations of the aryl rings. The cyclobutanes are classified into two different groups, case i and case ii, according to X-ray crystallographic analysis combined with IH NMR analysis. In case i where the two vicinal aryl rings are fixed or sterically free in taking conformations favourable for through-bond coupling between the two π electron systems, breaking of the arylated C-C bond efficiently occurs with high or complete regiospecificity upon thermal or photolytic activation as the consequence of population of the σa* character and upon exciplex or triplex formation with or electron transfer to excited-singlet aromatic nitriles as well as upon complex formation with the cation radical of aromatic hydrocarbons as the consequence of partial or complete population of the positive charge over the π-C(σ)-C(σ)-π orbital array of the HOMO, In case ii where through-bond coupling Is conformationally unfavourable, the ring cleavage slowly occurs In both the symmetric and the asymmetric directions or only in the asymmetric direction upon thermal or photolytic activation but does not at all by exciplex and triplex photosensitisation, by electron-transfer photosensitisation, nor by redox photosensitisation.
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