Photochemical Reactions. Part 60 [1]. The photorearrangement of a 2, 5‐diene‐1, 7‐dione

TLDR: The 2, 5-diene-1, 7-dione 12 rearranges photochemically to the cyclopentenonyl-cyclopropyl ketone 13 as discussed by the authors.

Abstract: The 2, 5-diene-1, 7-dione 12 rearranges photochemically to the cyclopentenonyl-cyclopropyl ketone 13. Dienone 12 does not emit phosphorescence at 78K. Sensitization and quenching experiments indicate that the photorearrangement occurs in an excited triplet state with ET between ca. 61–65 kcal/mol according to sensitization data, and between ca. 54–58 kcal/mol according to quenching results. By comparison, the 2, 5-dienone 1 behaves similarly with respect to sensitization and quenching of its photorearrangement [3] (indicating ET 61 kcal/mol for its photorearrangement, respectively. It is concluded that the triplet energy of the reactive state of compound 12 extends either over the two enone groups involving strong interaction or is localized on the ring A enone only. Photoproduct 13 rearranges further to the stereoisomer 14 on direct excitation and sensitization with acetophenone. Thioxanthone as triplet sensitizer and naphthalene as triplet quencher are ineffective. The reverse isomerization process, i.e.14 13, is not observed.