Photocatalytic C-N bond construction toward high-value nitrogenous chemicals

The construction of carbon-nitrogen bonds is vital for producing versatile nitrogenous compounds for the chemical and pharmaceutical industries. Among developed synthetic approaches to nitrogenous chemicals, photocatalysis is particularly prominent and has become one of the emerging fields due to its unique advantages of eco-sustainable characteristics, efficient process integration, no need for high-pressure H2, and tunable synthesis methods for developing advanced photocatalytic materials. Here, the review focuses on potential photocatalytic protocols developed for the construction of robust carbon-nitrogen bonds in discrepant activation environments to produce high-value nitrogenous chemicals. The photocatalytic C-N bond construction strategies and involved reaction mechanisms are elucidated.

Copper-Mediated Radical-Induced Ring-Opening Relay Cascade Carboannulation Reaction of [60]Fullerene with Cyclobutanone Oxime Esters: Access to [60]Fullerene-Fused Cyclopentanes

An unexpected copper-mediated radical-induced ring-opening relay cascade carboannulation reaction of [60]fullerene with cyclobutanone oxime esters is presented for the preparation of various Cl-/Br-incorporated [60]fullerene-fused cyclopentanes. The unique relay cascade transformation uses inexpensive copper salts as promoters and halogen sources and features simple redox-neutral conditions and a broad substrate scope, providing a practical access to a class of novel five-membered carbocycle-fused fullerenes.

Metal-catalysed C-Het (F, O, S, N) and C-C bond arylation

The formation of C-aryl bonds has been the focus of intensive research over the last decades for the construction of complex molecules from simple, readily available feedstocks. Traditionally, these strategies involve the coupling of organohalides (I, Br, Cl) with organometallic reagents (Mg, Zn, B, Si, Sn,…) such as Kumada-Corriu, Negishi, Suzuki-Miyaura, Hiyama and Sonogashira cross-couplings. More recently, alternative methods have provided access to these products by reactions with less reactive C-Het (F, O, S, N) and C-C bonds. Compared to traditional methods, the direct cleavage and arylation of these chemical bonds, the essential link in accessible feedstocks, has become increasingly important from the viewpoint of step-economy and functional-group compatibility. This comprehensive review aims to outline the development and advances of this topic, which was organized into (1) C-F bond arylation, (2) C-O bond arylation, (3) C-S bond arylation, (4) C-N bond arylation, and (5) C-C bond arylation. Substantial attention has been paid to the strategies and mechanistic investigations. We hope that this review can trigger chemists to discover more efficient methodologies to access arylation products by cleavage of these C-Het and C-C bonds.

Visible-Light Photoredox-Catalyzed Ring-Opening Carboxylation of Cyclic Oxime Esters with CO2

The carboxylation of cyclic oxime esters with carbon dioxide via visible-light photoredox catalysis is demonstrated for the first time. A variety of cyclic oxime esters undergo ring-opening C-C bond cleavage and carboxylation to give cyanoalkyl-containing carboxylic acids in moderate to good yields. Moreover, this methodology features mild reaction conditions (room temperature, 1 atm), wide substrate scope, good functional group tolerance as well as facile derivations of products. Mechanistic studies indicate that the benzylic radicals and anions might be the key intermediates.

Cleavage of Carbon-Carbon σ-Bonds of Four-Membered Rings

This article reviews synthetic transformations involving cleavage of a carbon-carbon bond of a four-membered ring, with a particular focus on the examples reported during the period from 2011 to the end of 2019. Most significant is the progress of catalytic reactions involving oxidative addition of carbon-carbon bonds onto transition metals or β-carbon elimination of transition metal alkoxides. When they are looked at from synthetic perspectives, they offer unique and efficient methods to build complex natural products and structures that are difficult to construct by conventional methods. On the other hand, β-scission of radical intermediates has also attracted increasing attention as an alternative elementary step to cleave carbon-carbon bonds. Its site-selectivity is often complementary to that of transition metal-catalyzed reactions. In addition, Lewis acid-mediated and thermally induced ring-opening of cyclobutanone derivatives has garnered renewed attention. On the whole, these examples demonstrate unique synthetic potentials of structurally strained four-membered ring compounds for the construction of organic skeletons.

A Mitochondrion-Localized Two-Photon Photosensitizer Generating Carbon Radicals Against Hypoxic Tumors

The efficacy of photodynamic therapy is typically reliant on the local concentration and diffusion of oxygen. Due to the hypoxic microenvironment found in solid tumors, oxygen-independent photosensitizers are in great demand for cancer therapy. We herein report an iridium(III) anthraquinone complex as a mitochondrion-localized carbon-radical initiator. Its emission is turned on under hypoxic conditions after reduction by reductase. Furthermore, its two-photon excitation properties (λ_ex =730 nm) are highly desirable for imaging. Upon irradiation, the reduced form of the complex generates carbon radicals, leading to a loss of mitochondrial membrane potential and cell death (IC₅₀ ^light =2.1 μm, IC₅₀ ^dark =58.2 μm, PI=27.7). The efficacy of the complex as a PDT agent was also demonstrated under hypoxic conditions in vivo. To the best of our knowledge, it is the first metal-complex-based theranostic agent which can generate carbon radicals for oxygen-independent two-photon photodynamic therapy.

Copper-catalysed Csp3–Csp cross-couplings between cyclobutanone oxime esters and terminal alkynes induced by visible light

A novel transformation for the construction of Csp³–Csp bonds was achieved via a photo-induced copper-catalysed C–C bond cleavage.

Single-Electron-Transfer-Induced C(sp3)-N Couplings via C-C Bond Cleavage of Cycloketoxime Esters

A practical single-electron-transfer-induced selective C(sp³)-N coupling of cycloketoximes with anilines via C-C bond cleavage under copper-catalytic and synergetic photoredox/copper-catalytic reaction systems has been uncovered. These two powerful and simple protocols demonstrated excellent selectivity and good functional group compatibility without any base or ligand control. Preliminary mechanistic experiments indicated that a radical-mediated process was involved in these transformations.

Photoredox-Catalyzed Sulfonylation of O-Acyl Oximes via Iminyl Radicals with the Insertion of Sulfur Dioxide

A multicomponent sulfonylation of O-acyl oximes via iminyl radicals with the insertion of sulfur dioxide under photoredox catalysis is achieved. This multicomponent reaction of O-acyl oximes, potassium metabisulfite, alkenes, and nucleophiles under visible-light irradiation is efficient, giving rise to a range of sulfones in moderate to good yields. A broad reaction scope is presented with good functional group compatibility.

Ring-opening C(sp3)–C coupling of cyclobutanone oxime esters for the preparation of cyanoalkyl containing heterocycles enabled by photocatalysis

A ring-opening C(sp³)–C coupling of cyclobutanone oxime esters for the preparation of cyanoalkyl containing heterocycles was realized under visible-light or sunlight irradiation.

Visible-light-induced radical cascade cyclization of oxime esters and aryl isonitriles: synthesis of cyclopenta[b]quinoxalines

The in situ-formed nitrile produced by oxime esters was taken into a visible-light-induced radical cascade cyclization with aryl isonitriles.