A Visible‐Light‐Driven Iminyl Radical‐Mediated C−C Single Bond Cleavage/Radical Addition Cascade of Oxime Esters

Remote C-H Pyridylation of Hydroxamates through Direct Photoexcitation of O-Aryl Oxime Pyridinium Intermediates

Herein, we report an efficient strategy for the remote C-H pyridylation of hydroxamates with excellent ortho-selectivity by designing a new class of photon-absorbing O-aryl oxime pyridinium salts generated in situ from the corresponding pyridines and hydroxamates. When irradiated by visible light, the photoexcitation of oxime pyridinium intermediates generates iminyl radicals via the photolytic N-O bond cleavage, which does not require an external photocatalyst. The efficiency of light absorption and N-O bond cleavage of the oxime pyridinium salts can be modulated through the electronic effect of substitution on the O-aryl ring. The resultant iminyl radicals enable the installation of pyridyl rings at the γ-CN position, which yields synthetically valuable C2-substituted pyridyl derivatives. This novel synthetic approach provides significant advantages in terms of both efficiency and simplicity and exhibits broad functional group tolerance in complex settings under mild and metal-free conditions.

Alkene Synthesis by Photo-Wolff-Kischner Reaction of Sulfur Ylides and N-Tosylhydrazones

A visible-light-driven and room temperature photo-Wolff-Kischner reaction of sulfur ylides and N-tosylhydrazones has been developed for the first time to provide modular access to alkene synthesis. The high functional group tolerance and broad substrate scope were demonstrated by more than 60 examples. Both E- and Z-olefinic stereochemistry in the products could be controlled with excellent stereoselectivity. A series of mechanistic studies support that the reaction should proceed through a radical-carbanion crossover pathway, specifically involving addition of photo-generated sulfur ylide radical cations to N-tosylhydrazones to form carbanions and subsequent Wolff-Kischner process.

Truce-Smiles Rearrangements by Strain Release: Harnessing Primary Alkyl Radicals for Metal-Free Arylation

The ring-opening of 3-aminocyclobutanone oximes enables easy generation of primary alkyl radicals, capable of undergoing an unprecedented strain-release, desulfonylative radical Truce-Smiles rearrangement, providing divergent access to valuable 1,3 diamines and unnatural β-amino acids. Characterized by mild conditions and wide scope of migrating species, this protocol allows the modular assembly of sp³ -aryls under transition metal-free, room-temperature conditions.

Visible-Light-Induced Palladium-Catalyzed Dehydrogenative Carbonylation of Amines to Oxalamides

The palladium-catalyzed oxidative carbonylation of amines toward the synthesis of oxalamides has been established around 30 years ago and it usually needs the presence of (over)stoichiometric amounts of oxidant. In this work, the first transformation of this type in which the oxidant was replaced by visible light is described. The new approach uses a simple robust Pd complex, which can even be partially recycled. A mechanistic reason is provided and supported by control experiments and EPR studies, showing that Pd^I was formed and Pd⁰ was the active species. Both nitrogen- and the intermediate acyl radical can be detected. Moreover, the formation of hydrogen was confirmed by gas GC.

Deacylative Carbon-Carbon Bond Cleavage of Ketone Equivalents: Applications to Radical Carbon-Carbon Bond Formation Reactions

This article describes the synthetic application of ketone-derived oxaziridines as alkyl radical precursors in copper-catalyzed Carbon-Carbon bond formation reactions. Experimental and computational studies indicate a free radical mechanism, where alkyl radicals are efficiently generated via cleavage of a Carbon-Carbon bond of oxaziridines. Acyclic and unstrained cyclic oxaziridines are applicable to the present radical process, allowing for the generation of various alkyl radicals with good functional group compatibility.

Photocatalytic Aerobic Oxidative Ring Expansion of Cyclic Ketones to Macrolactones by Cerium and Cyanoanthracene Catalysis

We describe a cerium-catalyzed aerobic oxidative ring expansion for the expedient construction of synthetically challenging macrolactones under visible-light conditions. Cyanoanthracene has been employed as co-catalyst to accelerate the turnover of the cerium cycle leading to a fast conversion within 20 min of irradiation. Taking advantage of the high efficiency and operationally simple conditions, a collection of over 100 macrolactones equipped with ring systems ranging from 9- to 19-membered macrocycles have been prepared from simple building blocks. Moreover, the enabling potential of this strategy to simplify the generation of molecular complexity has been demonstrated through the concise synthesis of sonnerlactone.

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.

Deconstructive Oxygenation of Unstrained Cycloalkanamines

A deconstructive oxygenation of unstrained primary cycloalkanamines has been developed for the first time using an auto-oxidative aromatization promoted C(sp³ )-C(sp³ ) bond cleavage strategy. This metal-free method involves the substitution reaction of cycloalkanamines with hydrazonyl chlorides and subsequent auto-oxidative annulation to in situ generate pre-aromatics, followed by N-radical-promoted ring-opening and further oxygenation by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and m-cholorperoxybenzoic acid (mCPBA). Consequently, a series of 1,2,4-triazole-containing acyclic carbonyl compounds were efficiently produced. This protocol features a one-pot operation, mild reaction conditions, high regioselectivity and ring-opening efficiency, broad substrate scope, and is compatible with alkaloids, osamines, and peptides, as well as steroids.

Iron-Catalysed Remote C(sp3 )-H Azidation of O-Acyl Oximes and N-Acyloxy Imidates Enabled by 1,5-Hydrogen Atom Transfer of Iminyl and Imidate Radicals: Synthesis of γ-Azido Ketones and β-Azido Alcohols

In the presence of a catalytic amount of iron(III) acetylacetonate [Fe(acac)₃ ], the reaction of structurally diverse ketoxime esters with trimethylsilyl azide (TMSN₃ ) afforded γ-azido ketones in good to excellent yields. This unprecedented distal γ-C(sp³ )-H bond azidation reaction went through a sequence of reductive generation of an iminyl radical, 1,5-hydrogen atom transfer (1,5-HAT) and iron-mediated redox azido transfer to the translocated carbon radical. TMSN₃ served not only as a nitrogen source to functionalise the unactivated C(sp³ )-H bond, but also as a reductant to generate the catalytically active Fe^II species in situ. Based on the same principle, a novel β-C(sp³ )-H functionalisation of alcohols via N-acyloxy imidates was subsequently realised, leading, after hydrolysis of the resulting ester, to β-azido alcohols, which are important building blocks in organic and medicinal chemistry.

Deaminative (Carbonylative) Alkyl-Heck-type Reactions Enabled by Photocatalytic C-N Bond Activation

The palladium-catalyzed Heck reaction is a well-known, Nobel Prize winning transformation for producing alkenes. Unlike the alkenyl and aryl variants of the Heck reaction, the alkyl-Heck reaction is still underdeveloped owing to the competitive side reactions of alkyl-palladium species. Herein, we describe the development of a deaminative alkyl-Heck-type reaction that proceeds through C-N bond activation by visible-light photoredox catalysis. A variety of aliphatic primary amines were found to be efficient starting materials for this new process, affording the corresponding alkene products in good yields under mild reaction conditions. Moreover, this strategy was successfully applied to deaminative carbonylative alkyl-Heck-type reactions.

Leaving Group Assisted Strategy for Photoinduced Fluoroalkylations Using N-Hydroxybenzimidoyl Chloride Esters

Redox-active esters (RAEs) as alkyl radical precursors have been extensively developed for C-C bond formations. However, the analogous transformations of fluoroalkyl radicals from the corresponding acid or ester precursors remain challenging because of the high oxidation potential of the fluoroalkyl carboxylate anions. The newly developed N-hydroxybenzimidoylchloride (NHBC) ester provides a general leaving group assisted strategy to generate a portfolio of fluoroalkyl radicals, and can be successfully applied in photoinduced decarboxylative hydrofluoroalkylation and heteroarylation of unactivated olefins. In addition, DFT calculations revealed that the NHBC ester proceeds by the fluorocarbon radical pathway, whereas other well-known RAEs proceed by the nitrogen radical pathway.

Photocatalytic Additions of 1-Sulfoximidoyl-1,2-Benziodoxoles to Styrenes

Sulfoximidoyl-containing 1,2-benziodoxoles add to styrenes by a photoredox radical process affording difunctionalized products with high regioselectivity. The solvent plays a significantly role in the reaction path, in which Eosin Y appears to have a dual role rendering the process diastereoselective.