Substituent Effects on the Stereochemistry in the [2 + 2] Photocycloaddition Reaction of Homobenzoquinone Derivative with Variously Substituted Alkenes and Alkynes

Irradiation of a homobenzoquinone derivative with variously substituted alkenes and alkynes
gave the [2 + 2] photocycloadducts, tricyclic diones, almost quantitatively as a mixture of regio- and
stereoisomers. The preferred regioisomer for all reactions is attributed to the more stable 1,4-biradical
intermediate (major addition mode), and the minor isomer is attributed to the less stable biradical (minor
addition mode). A radical trapping experiment using benzeneselenol proved the generation of these two
regioisomeric biradicals, reflecting the regioselectivity in selenol-free photoreaction. Both biradicals tended
to preferentially yield the endo-isomer for the alkenes with smaller substituents such as ethoxy, cyano,
and acetoxy groups, but the exo-isomer for the alkenes with larger substituents such as phenyl, carbazolyl,
and tert-butyl groups. The logarithmic exo/endo ratios were well correlated with a combination of Taft's
steric factor Es and the energy gain (ΔE‘) associated with the orbital interactions between the spin centers
of 1,4-biradicals. These results were interpreted in terms of Griesbeck's SOC mechanism as well as the
possible bond rotation around the armed radical chain. Therefore, it is concluded that a balance of repulsive
steric hindrance and the attractive FMO interaction determines the stereochemical course of the [2 + 2]
photoaddition of homobenzoquinone derivative with variously substituted alkenes.