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The Oxa‐di‐π‐methane Rearrangement of 1‐Phenyl‐3‐acetylcyclopentene: Identification of the Reactive Triplet as the Localized Styrene π, π*‐State. Preliminary Communication
Author(s) -
TegmoLarsson IngaMai,
Gonzenbach HansUlrich,
Schaffner Kurt
Publication year - 1976
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19760590443
Subject(s) - chemistry , phosphorescence , intersystem crossing , triplet state , singlet state , styrene , photochemistry , medicinal chemistry , pentane , ketone , excited state , organic chemistry , molecule , fluorescence , physics , quantum mechanics , nuclear physics , copolymer , polymer
1‐Phenyl‐3‐acetylcyclopentene IUPAC‐Nomenclature for 3 : (3‐phenyl‐cyclopent‐2‐enyl)‐ethanone or methyl (3‐phenylcyclopent‐2‐enyl) ketone. ( 3 ) undergoes triplet‐sensitized oxa‐di‐π‐methane rearrangement to a ca. 3:1 mixture of endo ‐ and exo ‐1‐phenyl‐5‐acetylbicyclo[2.1.0]pentane IUPAC‐Nomenclature for 4 : endo ‐ or exo ‐(4‐phenyl‐bicyclo[2.1.0]pent‐5‐yl)‐ethanone. ( 4 ). The reactive triplet of 3 has been identified as the localized styrene π, π*‐state of E T = 59 kcal/mol by comparison of its phosphorescence at 77 K in a rigid glass with that of 1‐phenylcyclopentene, and by sensitization quantum yields at room temperature in liquid solution. These quantum yields are independent of sensitizer energy in the range of 61–74 kcal/mol and reach zero for product formation with a sensitizer of E T = 59 kcal/mol. Although S 1 → T 1 intersystem crossing is appreciable under phosphorescence conditions (Φ p = 0.05), direct irradiation of 3 at 20° does not lead to triplet rearrangement, and the typical singlet 1,3‐acetyl shift to 3‐acetyl‐3‐phenylcyclopentene IUPAC‐Nomenclature for 8 : (1‐phenyl‐cyclopent‐2‐enyl)‐ethanone. ( 8 ) is observed only.