Photochemical [2+2] Cycloaddition Reaction of Carbonyl Compounds with Danishefsky-Kitahara Diene

概要

1.1 Electronically excited states in organic reactions
The field of organic photochemistry has been started in 1900 when Ciamician and
Silber exposed a solution of benzophenone in isopropanol to sunlight.1 They described the
product as a condensation of benzophenone with acetaldehyde. This condition was happened
because of the inaccurate combustion analysis and a lack of chemical degradation studies. Then,
Ciamician has tried to recalling his own previous work on photoreduction-oxidation reactions
and repeated this study. He found the structure of the irradiation product as benzopinacol.
Although Ciamician and Silber did not utilize the photoreaction as a useful and widely applied
photochemical reaction, they are generally given the credit because they discovered the
structure of the photodimer.2
The mechanism of benzophenone transformation3–5 (Scheme 1) involves the formation
of the singlet excited state of benzophenone, followed by the intersystem crossing to the triplet
excited state of benzophenone. The triplet species, which has a radical characteristic, abstracts
a hydrogen atom from the isopropyl alcohol to generate the radical. A coupling of the radical
gives the benzopinacol photoproduct. ...

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