This reaction involves the addition of excited ketone to an olefin forming an oxetane. Only ketones with low - lying π* states react in this reaction.
The radical - like oxygen atom of (n→π*) ketone adds to olefin to from stable biradical intermediate which subsequently form the stable product.
An essential condition of Paterno-Buchi reaction is that the triplet energy of alkene must be either comparable or higher than that of carbonyl compound. If such conditions is not satisfied the product will be a dimer of olefin instead of oxetane. Thus reaction of acetophenone and norbornene yiels dimer of norbornene while reaction between benzophenone and norbornene affords oxetane.
Alkynes used in place of alkene in Paterno-Buchi reaction. The initial product may be oxetane but the final product is a αβ unsaturated ketones.
Photochemical reaction of ketone and alkene with electron withdrawing substiuents proceeds with the formation of oxetane but the reaction path doesn't involve biradical intermediate.
The addition occurs through nucleophilic addition path where (n, π*) act as nucleophile.
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