Visible light induced azidation of aldehydic C–H with carbon tetrabromide and sodium azide

Photocatalyst-free visible light driven synthesis of gem-dihaloenones from alkynes, tetrahalomethanes and water

Photocatalytic reactions, in particular, processes without photosensitisers, have attracted increased attention due to their green aspect and high economic value and are considered valuable tools in organic synthesis. A new practical photocatalytic system was investigated in this study, and it can efficiently produce gem-dihaloenones by combining terminal alkynes with tetrahalomethanes (BrCCl₃ and CBr₄) and water without a photocatalyst, and the yield can reach up to 87%. The catalytic system is straightforward, the raw materials are inexpensive and easy to obtain, and the operation is simple.

Divergent functionalization of aldehydes photocatalyzed by neutral eosin Y with sulfone reagents

While aldehydes represent a classic class of electrophilic synthons, the corresponding acyl radicals are inherently nucleophilic, which exhibits umpolung reactivity. Generation of acyl radicals typically requires noble metal catalysts or excess oxidants to be added. Herein, we report a convenient and green approach to access acyl radicals, capitalizing on neutral eosin Y-enabled hydrogen atom transfer (HAT) photocatalysis with aldehydes. The generated acyl radicals underwent SOMOphilic substitutions with various functionalized sulfones (X-SO2R') to deliver value-added acyl products. The merger of eosin Y photocatalysis and sulfone-based SOMOphiles provides a versatile platform for a wide array of aldehydic C-H functionalizations, including fluoromethylthiolation, arylthiolation, alkynylation, alkenylation and azidation. The present protocol features green characteristics, such as being free of metals, harmful oxidants and additives; step-economic; redox-neutral; and amenable to scale-up assisted by continuous-flow technology.

Catalyst-Free Visible-Light-Promoted Cyclization of Aldehydes: Access to 2,5-Disubstituted 1,3,4-Oxadiazole Derivatives

An efficient synthesis of a variety of 2,5-disubstituted 1,3,4-oxadiazole derivatives via a cyclization reaction by photoredox catalysis between aldehydes and hypervalent iodine(III) reagents is described. The reaction proceeds under mild conditions and affords various target compounds in excellent yields. The commercially available aldehydes without preactivation and a simple visible-light-promoted procedure without any catalysts make this strategy an alternative to the conventional methods.

Tracking down the brominated single electron oxidants in recent organic red-ox transformations: photolysis and photocatalysis

A wide range of organic and inorganic brominated compounds including molecular bromine have been extensively used as oxidants in many organic photo-redox transformations in recent years, an area of ever growing interest because of greener and milder approaches. The oxidation power of these compounds is utilized through both mechanistic pathways (by hydrogen atom transfer or HAT in the absence of a photocatalyst and a combination of single electron transfer or SET and/or HAT in the presence of a photocatalyst). Not only as terminal oxidants for regeneration of photocatalysts, but brominated reactants have also contributed to the oxidation of the reaction intermediate(s) to carry on the radical chain process in several reactions. Here in this review mainly the non-brominative oxidative product formations are discussed, carried out since the last two decades, skipping the instances where they acted as terminal oxidants only to regenerate photocatalysts. The reactions are used to generate natural products, pharmaceuticals and beyond.

Methods for direct C(sp2)–H bonds azidation

Direct functionalization of C–H bonds has attracted great attention in recent years from the perspectives of atom and step economy.

Hydrogen Atom Transfer (HAT): A Versatile Strategy for Substrate Activation in Photocatalyzed Organic Synthesis

The adoption of hydrogen atom transfer (HAT) in a photocatalytic approach, in which an excited catalyst is responsible for substrate activation, offers unique opportunities in organic synthesis, enabling the straightforward activation of R-H (R = C, Si, S) bonds in desired reagents. Either a direct strategy, based on the intrinsic reactivity of a limited number of photocatalysts in the excited state, or an indirect one, in which a photocatalytic cycle is used for the generation of a thermal hydrogen abstractor, can be exploited. This microreview summarizes the most recent advances (mainly from the last two years) in this rapidly developing area of research, collecting the selected examples according to the nature of the species promoting the HAT process. From the synthetic point of view, this area has led to the development of a plethora of strategies for C-C, C-Si, C-N, C-S, and C-halogen (particularly, fluorine) bond formation, as well as for oxidation reactions.