The Photochemistry of 2‐Cyclopentenyl Methyl Ketones

Abstract

Abstract 2‐Cyclopentenyl and 3‐phenyl‐2‐cyclopentenyl methyl ketones (15–18, 30, 31) undergo a 1,3‐acetyl shift on direct irradiation, and the oxa‐di‐π‐methane rearrangement to photochemically non‐interconverting endo and exo bicyclo‐[2.1.0]pentyl methyl ketones on triplet sensitization. Exceptions include the 2‐methyl‐3‐phenyl‐2‐cyclopentenyl methyl ketone 32 and the 1‐phenyl‐2‐cyclo‐pentenyl methyl ketone 44 which are unreactive on direct irradiation and on triplet sensitization, respectively, and the 2‐phenyl‐2‐cyclopentenyl methyl ketones 42 and 43 which do not react under either condition. The reactive triplet of the 3‐phenyl‐2‐cyclopentenyl methyl ketone 30 has been identified as the localized styrene π,π*‐state of E T =59 kcal/mol by comparison of its phosphorescence at 77K in rigid glasses with that of 1‐phenyl‐cyclopentene, and by the independence of the quantum yield on sensitizer energy in the range of 61–74 kcal/mol.