Visible Light Driven and Copper-Catalyzed C(sp3)-H Functionalization of O-Pentafluorobenzoyl Ketone Oximes

Synthesis of Δ1-Pyrrolines via Formal (3 + 2)-Cycloaddition of 2H-Azirines with Enones Promoted by Visible Light under Continuous Flow

This work reports the first synthesis of Δ1-pyrrolines promoted by visible light under continuous flow, achieved through the formal (3 + 2)-cycloaddition of 2H-azirines to enones. A total of 22 examples of trisubstituted Δ1-pyrrolines were prepared in only 30 min of residence time, with 41-93% yield and diastereomeric ratios up to 7:3. Furthermore, continuous flow conditions were also effective when chalcones were used as starting materials, leading to the formation of 7 tetrasubstituted pyrroles with an overall yield ranging from 12 to 47%, via a photocatalyzed cycloaddition-oxidation sequence.

Visible Light-Promoted Radical Cascade Bicyclization to Access Partially Saturated Pyrrolo[2,3-b]Pyridines

A visible light-promoted cascade [2+2+1] bicyclization reaction of N-cyanamide alkenes and activated alkyl bromides has been successfully established. This reaction exhibits mild conditions, metal-free characteristics, and excellent atom- and step-economy, along with environmental friendliness. It demonstrates broad substrate tolerance and scalability to gram-scale synthesis, underscoring its potential for practical applications. Preliminary mechanistic studies suggest that the reaction proceeds through a photoinduced single-electron transfer (SET) process and a 1,5-hydrogen atom transfer (HAT) process mediated by an iminyl radical.

Microwave-Promoted Synthesis of 1-Tetralones via Iminyl Radical-Mediated 1,5-Hydrogen Atom Transfer

Microwave irradiation of O-phenyloximes produces 1-tetralones via N-O homolysis, 1,5-hydrogen atom transfer (HAT), and cyclization of a radical intermediate onto an aromatic ring. The HAT step is facilitated by coordination of Lewis acid InCl3·H2O to an iminyl radical. The reaction is rapid and exhibits a broad substrate scope. A larger scale transformation can be performed using conventional heating instead of microwave irradiation.

Modular Synthesis of Dehydroprolines by an Energy-Transfer Enabled Cloke-Wilson Rearrangement

A one-pot photocatalytic method is reported for the generation of dehydroprolines, valuable precursors to 5-aryl prolines. Imines, obtained via simple condensation of aminocyclopropane carboxylates (ACPC) with a broad range of aldehydes, were employed in this transformation without purification. We demonstrate this energy-transfer (EnT) enabled Cloke-Wilson-type rearrangement affords both the rare 1,2-dehydroprolines or the more thermodynamically favored 1,5-isomers with up to >20 : 1 selectivity in both directions, using an identical catalytic system from the same starting material. Syntheses of intermediates of bioactive molecules in high yields and selectivity highlight this enabling transformation. Mechanistic studies support the proposed triplet-triplet EnT mode of activation, distinct from the previously developed singlet excited states accessed via UV excitation.

Chiral pyrrolidines via an enantioselective Hofmann-Löffler-Freytag reaction

Radical C-H aminations enable rapid access to the most common heterocycles in medicines (e.g. pyrrolidines), yet stereocontrol of these powerful transformations remains a challenge. Here, we report the discovery of the first enantio- and regio- selective C-H imination, which readily converts ketones to enantioenriched pyrrolidines. This enantioselective Hofmann-Löffler-Freytag reaction mechanism entails iminyl radical generation from an oxime by a chiral Cu catalyst that facilitates 1,5-H-atom transfer (HAT) to form a remote C-radical, regioselectively. The selective capture of this alkyl radical as an organocopper(III) complex then mediates highly stereoselective reductive elimination to unprotected pyrrolines. The broad steric and electronic scope of this remote C-H amination has been probed systematically, along with key mechanistic aspects of enantiodetermination, radical intermediacy, and atypical Cu(III) ligands that enable this uniquely selective C-N coupling. Importantly, either (1) reductions or (2) nucleophilic additions to these enantioenriched pyrrolines provide the most rapid syntheses of chiral pyrrolidines to date.

Synthesis of 2H-imidazoles via copper-catalyzed homo/cross-coupling of oxime acetates

A facile and practical protocol to construct 2H-imidazoles by applying an oxime acetate block as the sole component via oxidative homo/cross-coupling catalyzed by Cu(I) was developed. This strategy provides a straightforward method to produce a series of substituted 2H-imidazoles in moderate to excellent yields. The transformation process is straightforward to operate and is considered as a readily available catalytic system exhibiting good substrate compatibility, eliminating the necessity for pre-functionalization of azides or the use of additives.

Radical reactions enabled by polyfluoroaryl fragments: photocatalysis and beyond

Polyfluoroarenes have been known for a long time, but they are most often used as fluorinated building blocks for the synthesis of aromatic compounds. At the same time, due to peculiar fluorine effect, they have unique properties that provide applications in various fields ranging from synthesis to materials science. This review summarizes advances in the radical chemistry of polyfluoroarenes, which have become possible mainly with the advent of photocatalysis. Transformations of the fluorinated ring via the C-F bond activation, as well as use of fluoroaryl fragments as activating groups and hydrogen atom transfer agents are discussed. The ability of fluoroarenes to serve as catalysts is also considred.

Visible light as a sole requirement for alkylation of α-C(sp3)-H of N-aryltetrahydroisoquinolines with alkylboronic acids

An alkylation of α-C(sp3)-H at N-aryltetrahydroisoquinolines with alkylboronic acids was developed under visible-light irradiation in the absence of additional photocatalyst. The reaction proceeded well, tolerating a variety of functional groups, and featured low-cost and mild reaction conditions. A preliminary mechanistic study indicated that an electron donor-acceptor (EDA) complex between an electron-rich N-aryltetrahydroisoquinoline and an electron-poor alkylboronic acid was involved in the reaction.

Formal γ-C(sp3)-H Activation of Ketones via Microwave-Promoted and Iminyl-Radical-Mediated 1,5-Hydrogen Atom Transfer

Microwave irradiation of O-phenyloximes triggers N-O homolysis and 1,5-hydrogen atom transfer (HAT), resulting in formal γ-C-H functionalization of ketones after trapping of the radical intermediate and in situ imine hydrolysis. The Lewis acid InCl3·H2O facilitated HAT, enabling functionalization of benzylic and nonbenzylic secondary carbon atoms. Functionalization of primary carbons was feasible but afforded low yields, requiring ClCH2CO2H instead of InCl3·H2O as an additive. C-O and C-C bond formation could both be accomplished by this method.

Photoactive Copper Complexes: Properties and Applications

Photocatalyzed and photosensitized chemical processes have seen growing interest recently and have become among the most active areas of chemical research, notably due to their applications in fields such as medicine, chemical synthesis, material science or environmental chemistry. Among all homogeneous catalytic systems reported to date, photoactive copper(I) complexes have been shown to be especially attractive, not only as alternative to noble metal complexes, and have been extensively studied and utilized recently. They are at the core of this review article which is divided into two main sections. The first one focuses on an exhaustive and comprehensive overview of the structural, photophysical and electrochemical properties of mononuclear copper(I) complexes, typical examples highlighting the most critical structural parameters and their impact on the properties being presented to enlighten future design of photoactive copper(I) complexes. The second section is devoted to their main areas of application (photoredox catalysis of organic reactions and polymerization, hydrogen production, photoreduction of carbon dioxide and dye-sensitized solar cells), illustrating their progression from early systems to the current state-of-the-art and showcasing how some limitations of photoactive copper(I) complexes can be overcome with their high versatility.

Chemodivergent photocatalytic access to 1-pyrrolines and 1-tetralones involving switchable C(sp3)–H functionalization

A chemodivergent photocatalytic approach to 1-pyrrolines and 1-tetralones from alkyl bromides and vinyl azides has been developed through chemoselectively controllable intermolecular [3 + 2] and [4 + 2] cyclization. This photoredox-neutral two-component protocol involves intermolecular radical addition and switchable distal C(sp³)–H functionalization enabled by iminyl radical-mediated 1,5-hydrogen atom transfer. Meanwhile, chemoselectivity between C(sp³)–N bond formation and C(sp³)–C(sp²) bond formation is precisely switched by photocatalysts (Ru(bpy)₃(PF₆)₂ vs. fac-Ir(ppy)₃) and additives (base vs. acid).

Promoting effect of water on light and phenanthroline-diphosphine Cu(I) complex-initiated iodine atom transfer cyclisation

Water can greatly facilitate the iodine atom transfer cyclisation of 2-allyloxy(or prop-2-yn-1-yloxy)-3-iodo tetrahydropyrans and tetrahydrofurans initiated by phenanthroline-diphosphine Cu(I) complexes under 455 nm light irradiation. Good yields were obtained in a mixture of acetonitrile and water (1 : 4, v/v) or in pure water, whereas no reaction took place in acetonitrile under the otherwise same conditions. The copper complexes are virtually heterogeneous in the water-dominant reaction media, which is believed to be a main reason for the beneficial effect of water.

Directed, Remote Dirhodium C(sp3)-H Functionalization, Desaturative Annulation, and Desaturation

Iminodirhodium reactive intermediates generated in situ from O-tosyloximes using Rh₂(esp)₂ in CH₂Cl₂ at rt were exploited for an agile trichotomy of challenging transformations: (1) remote C-H functionalizations using an exceptionally broad diversity of inorganic and organic nucleophiles including several unconventional examples, for example, ethers and acyl silanes; (2) desaturative annulation, a biomimetic 1,3-methylene C-C ring-closure with an overall loss of two hydrogens; and (3) directed desaturation for the acceptor-less, regioselective creation of γ,δ- or γ,δ,ε,ζ-olefins. Compared with typical iminyl transition-metal-mediated and 1,5-hydrogen atom-transfer (1,5-HAT) processes, iminodirhodium intermediates are largely underexplored, especially with respect to C(sp³)-H centers and, yet, have the potential to be transformative by virtue of their substrate breadth, regiocontrol, and elusive reaction modality. A substrate scope includes benzylic, allylic, propargylic, tertiary, and α-alkyloxy centers.

PdII-Catalyzed γ-C(sp3)-H (Hetero)Arylation of Ketones Enabled by Transient Directing Groups

Pd(II)-catalyzed γ-C(sp3)-H (hetero)arylation of aliphatic ketones is developed using α-amino acid as transient directing groups (TDG). A variety of aliphatic ketones were (hetero)arylated at the γ-position via a 5,6-membered fused cyclopalladation intermediate to afford the remotely arylated products in up to 88% yield. The crucial ligand effect of 2-pyridone is further enhanced by reducing the loading of acid additives. Consequentially, the improved reactivity of this catalytic system has also made possible the cyclic γ-methylene C(sp3)-H arylation of ketones. Mechanistic investigtigation and comparison to the γ-C-H arylation of aldehydes revealed a structural insight for designing site selective TDG.

Photoinduced, copper-catalysed direct perfluoroalkylation of heteroarenes

An efficient and general process is reported for the photoinduced, copper-catalysed direct perfluoroalkylation of C-H bonds in a broad range of heteroarenes with commercially available perfluoroalkyl iodides. This redox neutral process is simply based on the use of [Cu(bcp)DPEPhos]PF₆ as the photoredox catalyst in the presence of potassium acetate and smoothly operates at room temperature.

Cu(C2N4H4)2Br2·2H2O: an antiferromagnetic cyanoguanidine coordination compound and its characterization

Phase-pure copper(II) cyanoguanidine bromide hydrate, Cu(C2N4H4)2Br2·2H2O (1), was precipitated from aqueous solution and its structure was solved and refined from single-crystal X-ray diffraction data at 100 K. 1 crystallizes in space group P21/n with a = 12.09(3) Å, b = 3.925(9) Å, c = 13.79(3) Å, β = 96.62(6)°, Z = 2, and V = 650(2) Å3. The copper(II) cation is coordinated by two cyanoguanidine molecules adopting the cyanoimine shape and four bromide anions in a Jahn–Teller-distorted motif, forming infinite chains of edge-sharing octahedra along the crystallographic b axis. IR spectroscopic and magnetic susceptibility measurements were carried out in addition to density-functional electronic-structure calculations performed to assess both the magnetic ground state and the exchange interactions. Experiment and theory agree as regards antiferromagnetism and weak magnetic exchange.