Premium
Role of exciplex and ion pair in the photosensitized oxygenation of 1,4‐diphenyl‐1,3‐butadiene
Author(s) -
Takahashi Y.,
Wakamatsu K.,
Kikuchi K.,
Miyashi T.
Publication year - 1990
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610030805
Subject(s) - chemistry , photooxygenation , photochemistry , acetonitrile , electron transfer , quenching (fluorescence) , singlet oxygen , ion , excimer , reactivity (psychology) , solvent , oxygen , organic chemistry , fluorescence , medicine , physics , alternative medicine , pathology , quantum mechanics
9,10‐Dicyanoanthracene (DCA) sensitized the photooxygenation of 1,4‐diphenyl‐1,3‐butadiene(1a) and its 4,4′‐dimethoxy derivative (1b) to afford the corresponding endoperoxides 2 and the other oxidized products such as aldehydes and epoxides. The mechanism of the DCA‐sensitized photooxygenation of 1 was diversified by solvent polarity. In non‐polar solvents the reaction involves an exciplex intermediate, which leads to formation of triplet DCA ( 3 DCA*) with an efficiency of 0·64 in the case of 1a. The resulting 3 DCA* acts as a sensitizer for the generation of singlet oxygen ( 1 O 2 ) and thus 1 O 2 reaction occurs exclusively to give 2. On the other hand, single electron‐transfer quenching of 1 DCA* by 1 is feasible in polar acetonitrile and a primary ion pair can be formed. Competing with fast back electron‐transfer deactivation, the primary ion pair produces free ions in limited yield to furnish electron transfer oxygenation. In the case of the DCA–1a system, free ions were produced with an efficiency of ca 4%. Thus, in acetonitrile, electron‐transfer oxygenation partly took place to give the other oxidized products, whereas the 1 O 2 pathway was still valid and responsible for the formation of 2. The effect of solvent polarity was apparently less pronounced in the case of 1b because the reactivity of 1b toward 1 O 2 is about five times higher than tht of 1a. For such 1 O 2 ‐reactive substrates, the electron‐transfer pathway would become dominant only when the substrate concentration is impractically high.