Photo-Induced Cycloaddition Reactions of α-Diketones and Transformations of the Photocycloadducts

Abstract

Photocycloaddition, along with subsequent transformation of the photocycloadducts, provides expeditious ways to construct various structures. The photoinduced reactions of α-diketones have been reported to proceed via different reaction pathways with the involvement of one or two of the carbonyl groups. Photoinduced reactions of cyclic α-diketones including N-acetylisatin, phenanthrenequinone and isoquinolinetrione with different C=C containing compounds could take place via [2 + 2], [4 + 2] or [4 + 4] photocycloaddition pathways. We have investigated the photoreactions of these cyclic α-diketones with different types of alkenes and alkynes, with a focus on the unusual cascade reactions initiated by the photocycloaddition reactions of these cyclic α-diketones and the applications of these photocycloaddition reactions along with the transformation of the photocycloadducts. In this paper, we discuss the diverse photo-cycloaddition pathways found in the photocycloaddition of o-diones leading to various photocycloadducts and the potential applications of these reactions via further transformation reactions of the adducts.

Synthesis of Three‐Membered and Four‐Membered Heterocycles with the Assistance of Photochemical Reactions

“…The reactions of alkenes having high oxidation potential like styrenes, the regioselectivity was rationalized by the frontier molecular orbital interactions of the excited N-acetylisatin with the ground state alkene and the formation of the most stable 1,4-diradical intermediate in the reaction. For instance, oxetanes were synthesized from most stable benzyl 1,4-diradical intermediate in the photoreaction of isatin (IS) with styrene (Scheme 38) [161].…”

“…The bicyclic spiroindoleoxetanes were obtained with high diastereoselectivity and regioselectivity in high yields when N-acetylisatin was photochemically reacted with acyclic or cyclic enol ethers [183]. For instance, head-to-tail product with syn-configuration was reported upon irradiation of C 6 H 6 solution containing benzofuran and N-acetylisatin (λ > 400 nm) (Scheme 49) [161].…”

“…For instance, the [4 + 4]-photocycloadduct was obtained in yields up to 89% from the reaction of 2,4,5trimethyloxazole and 9,10-phenanthrenequinone. Besides, further photocycloaddition of [4 + 4]-photocycloadduct did not occur [161,220].…”

“…Other alicyclic olefins provided not only keto oxetanes but also photoreducts and dihydrodioxines. The character of intermediary biradicals elucidated the differences of chemoselectivity [161].…”

Catalyst-free [2 + 2] photocycloadditions between benzils and olefins under visible light

“…α-Diketones absorb directly visible light, which allows to avoid the use of sensitizers. Different classes of α-diketones, such as isatin, isoquinoline-1,3,4-triones and phenanthrenediones have been investigated as carbonyl surrogates in the PB reaction in a catalyst-free approach [19]. More recently, Dell'Amico demonstrated the use of the PB reaction for the dearomatization of indoles (Scheme 1c) [10,20,21]: herein, a handful of benzils was also tested besides conventional ketones, however traditional reaction partners (i.e.…”