Photochemistry of β, γ-unsaturated ketones-V : The direct irradiation of some γ-phenyl β, γ-enones

Abstract

The photochemistry of some members of the two series of γ-phenyl substituted acyclic β, γ-unsaturated ketones 1 and 2 upon direct irradiation with γ 310nm has been investigated, viz 1c–1h and 2b+2c.The alkyl substituted ( E)-5-phenyl-4-penten-2-ones 1c–1h yield the corresponding 1,3-acyl shift products and ( Z)-isomers, and 1g and 1h in addition two decarbonylated products. 2b only yields the ( Z)-isomer and some benzaldehyde, but 2c yields the 1,3-acyl shift product, the ODPM product, three hydrocarbons formed by disproportionation of the allyl radical, and some benzaldehyde. The β-phenyl β, γ-UK 3a proved to be photostable. The 1,3-acyl shift products of 1c–1h result mainly from the singlet excited state in a cage radical process. The exclusive formation of the ( E)-configuration of the 1,3-acyl shift product is explained in terms of conformational preference of the intermediate allyl radical. It is proposed that the formation of the ( Z)-isomer proceeds from 1T(π -π *) which is populated according to ▪. Evidence is presented which supports the proposed mechanism. The β,γ-UK 2b containing a benzoyl moiety leads to a higher degree of ( E)-( Z) isomerization than the corresponding 1d which has an acetyl moiety. The triplet energies of ( E)- and ( Z)- 1h are 56 and ca 70 kcal/mol respectively.