Photoinduced reactions of chloranil with stilbene derivatives and α,β-unsaturated carbonyl compounds

Abstract

Photolysis of chloranil (CA) with Z- or E-stilbenes 1–5[(Z)- and (E)-XC₆H₄CHCHC₆H₅, X = H, 4-Cl, 3-Cl, 4-Me) in benzene leads to the formation of trans-oxetane products 7–13(80–88% yield) and extensive Z- to E-(when starting from Z-) as well as slight E- to Z-(when starting from E-) isomerizations of the stilbenes. For the monosubstituted stilbenes, two oxetane regioisomers are formed in comparable yields. Similar photolysis in acetonitrile resulted only in the one-way Z- to E-isomerization of the stilbenes without formation of oxetanes. (Z)- and (E)- 4-Methoxystilbenes do not form oxetane products with CA either in benzene or in acetonitrile, and one-way Z- to E-isomerization is the only reaction detected. Diphenylvinylene carbonate 6 gave oxetane product 14 with CA in benzene solution. Electron transfer between ³CA* and the alkenes is involved in the cycloaddition and isomerization. Photolysis of CA with ethyl (E)- or (Z)-cinnamate (E-22 or Z-22), ethyl 3-nitrocinnamate (E-23 or Z-23), benzalacetone (E-24 or Z-24), chalcone (E-25 or Z-25) and (E)-4-fluorochalcone (E-26) in benzene or in acetonitrile gave trans-oxetane products 27–31 respectively in good yield (72–95%) and resulted in slight E-to-Z and considerable Z-to-E isomerizations without SET-process involvement.