Iodine/Visible Light Photocatalysis for Activation of Alkynes for Electrophilic Cyclization Reactions

Abstract

Photocatalytic organic synthesis needs photocatalysts to initiate the reactions and to control the reaction paths. Available photocatalytic systems rely on electron transfer or energy transfer between the photoexcited catalysts and the substrates. We explore a concept based on the photopromoted catalyst coupling to the substrate and the phototriggered catalyst regeneration by elimination from the catalytic cycle. A catalytic amount of elementary I 2 is applied as both a visible light photocatalyst and a π Lewis acid, enabling the direct activation of alkyne CC bonds for electrophilic cyclization reactions, one of the most important reactions of alkynes. Visible light is crucial for both the iodocyclization of the propargyl amide and the deiodination of the intermediate. Singlet oxygen is found to play a key role in the regeneration of I 2 . This system shows good functional group compatibility for the generation of substituted oxazole aldehydes and indole aldehydes. Hence, this study provides a readily accessible alternative catalytic system for the construction of heterocycle aldehyde derivatives by sunlight photocatalysis.

Visible‐Light‐Induced Cleavage of C−S Bonds in Thioacetals and Thioketals with Iodine as a Photocatalyst

“…One of the first reports featuring molecular iodine as a photocatalyst in combination with molecular oxygen under irradiation of visible light was the activation of alkynes for electrophilic cyclizations by Wang and co‐workers (Scheme , a) . In their report, molecular iodine both acted as a photocatalyst for singlet oxygen generation and as a π‐Lewis acid.…”

Metal‐Free Photocatalytic Synthesis of exo‐Iodomethylene 2‐Oxazolidinones: An Alternative Strategy for CO2 Valorization with Solar Energy

“…The Buchwald–Hartwig reaction, Suzuki reaction, and Sonogashira reaction could be successfully implemented by using 2 a as substrate, furnishing the respective products 3 a , 4 a , and 5 a in good‐to‐excellent yields. In addition, 6 a and 7 a could be obtained through the photocatalytic ene reaction and photoreduction of 2 a , respectively. 6 a , featuring an aldehyde group, can be easily transformed into the corresponding alcohol, carboxylic acid, ester, or imidazole .…”

“…[6] The ability to absorb visible light (450-600 nm) [7] makes I 2 as uitable photocatalyst in the Mallory reaction, [8] photo-oxidation, [9] aerobic photo-decarboxylation, [10] and other photoreac-tions. [11] In this regard, I 2 has enormous potentiali nm etal-free photocatalysis.…”

Iodine‐Catalyzed Isomerization of Dimethyl Muconate

“…Further examination revealed that solvent choice has as ubstantial effect on the observed rate of reaction( Figure 5), underscoring the complexityo fh omogeneous iodine catalysis. [52][53][54] Modeling of solvent molecules within the reaction system wasbeyond the scope of the current work. This includes the potentialf or formation of iodine-solvent or solventmuconate interactions, which may or may not affect catalytic activity.…”

“…This includes the potentialf or formation of iodine-solvent or solventmuconate interactions, which may or may not affect catalytic activity. [52][53][54] Modeling of solvent molecules within the reaction system wasbeyond the scope of the current work.…”