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

Abstract

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 E(s) and the energy gain (DeltaE') 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.

Formation of a Four‐Membered Ring: From a Carbonyl‐Conjugated Alkene

“…An attack to the double bond occurred exclusively from the face opposite to the cyclopropane [54]. An attack to the double bond occurred exclusively from the face opposite to the cyclopropane [54].…”

Photochemistry of Cyclobutanes: Synthesis and Reactivity

“…Radical trapping experiments were carried out to justify the involvement of such species. The results obtained and the yields of products are shown in Scheme 11 232,233 . Quinones also undergo addition to alkenes as demonstrated by the addition of 1,1-diarylethenes 377 to p-chloranil.…”

Versatile Domino Rearrangement of Diphenylhomobenzoquinone Epoxides Induced by CF3SO3H

“…[2] The designed tricyclic diones containing both a cyclopropane and a cyclobutene ring were synthesized from the [2+2] photocycloaddition of alkynes to diphenylhomobenzoquinones. [2] The designed tricyclic diones containing both a cyclopropane and a cyclobutene ring were synthesized from the [2+2] photocycloaddition of alkynes to diphenylhomobenzoquinones.…”

“…In this connection, we have previously synthesized highly strained tricyclic diones as molecular building blocks for the construction of various polycyclic compounds in acidcatalyzed systems. [2] The designed tricyclic diones containing both a cyclopropane and a cyclobutene ring were synthesized from the [2+2] photocycloaddition of alkynes to diphenylhomobenzoquinones. [2] The BF 3 -catalyzed reactions were found to give several bicyclic and tricyclic compounds through domino rearrangements involving sequential opening of the cyclobutene and cyclopropane rings.…”

“…[2] The designed tricyclic diones containing both a cyclopropane and a cyclobutene ring were synthesized from the [2+2] photocycloaddition of alkynes to diphenylhomobenzoquinones. [2] The BF 3 -catalyzed reactions were found to give several bicyclic and tricyclic compounds through domino rearrangements involving sequential opening of the cyclobutene and cyclopropane rings. [3] Very recently, we found that the BF 3 -catalyzed reaction of newly prepared diphenylhomobenzoquinone epoxide 1a (R = Me) undergoes π-aryl participated epoxide ring opening to provide quantitatively tricyclic diketo alcohol 2a 3916 tion of various key steps involving (i) transannular cyclization of the endo-phenyl group, (ii) epoxide and cyclopropane ring opening, (iii) ring contraction of the original quinone frame, (iv) dehydration and intramolecular S E 2/S N 2-Ar cyclization, as well as (v) possible pseudopericyclic cheletropic decarbonylation.…”