Photoredox catalysis in nickel-catalyzed C-H functionalization

Synthesis of Indolyl Phenyl Diketones through Visible-Light-Promoted Ni-Catalyzed Intramolecular Cyclization/Oxidation Sequence of Ynones

The combination of visible light catalysis and Ni catalysis has enabled the synthesis of indolyl phenyl diketones through the cyclization/oxidation process of ynones. This reaction proceeded under mild and base-free conditions and showed a broad scope and feasibility for gram-scale synthesis. Several natural products and biologically interesting molecules could be readily postfunctionalized by this method.

Efficient Visible-Light-Driven Carbon Dioxide Reduction using a Bioinspired Nickel Molecular Catalyst

Inspired by natural enzymes, this study presents a nickel-based molecular catalyst, [Ni‖ (N2 S2 )]Cl2 (NiN2 S2 , N2 S2 =2,11-dithia[3,3](2,6)pyridinophane), for the photochemical catalytic reduction of CO2 under visible light. The catalyst was synthesized and characterized using various techniques, including liquid chromatography-high resolution mass spectrometry (LC-HRMS), UV-Visible spectroscopy, and X-ray crystallography. The crystallographic analysis revealed a slightly distorted octahedral coordination geometry with a mononuclear Ni2+ cation, two nitrogen atoms and two sulfur atoms. Photocatalytic CO2 reduction experiments were performed in homogeneous conditions using the catalyst in combination with [Ru(bpy)3 ]Cl2 (bpy=2,2'-bipyridine) as a photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial electron donor. The catalyst achieved a high selectivity of 89 % towards CO and a remarkable turnover number (TON) of 7991 during 8 h of visible light irradiation under CO2 in the presence of phenol as a co-substrate. The turnover frequency (TOF) in the initial 6 h was 1079 h-1 , with an apparent quantum yield (AQY) of 1.08 %. Controlled experiments confirmed the dependency on the catalyst, light, and sacrificial electron donor for the CO2 reduction process. These findings demonstrate this bioinspired nickel molecular catalyst could be effective for fast and efficient photochemical catalytic reduction of CO2 to CO.

Direct C(sp3)-H Acylation by Mechanistically Controlled Ni/Ir Photoredox Catalysis

ConspectusSynthetic chemists have consistently aimed to develop efficient methods for synthesizing ketones, which are essential building blocks in organic chemistry and play significant roles in bioactive molecules. Recent efforts have focused on using photoredox catalysis, which enables previously inaccessible activation modes, to synthesize ketones through the cross-coupling of an acyl electrophile and simple C(sp3)-H bonds. Over the past few years, we have worked on developing effective and versatile approaches for directly acylating activated hydrocarbons to forge ketones.Initially, thioesters were explored as the acyl source to achieve the direct acylation of ethers, but an unexpected thioesterification reaction was observed instead. To gain insights into this reactivity, we conducted the optimization of reaction conditions, substrate scope evaluation, and mechanistic studies. Drawing from our understanding of Ni/Ir photocatalysis obtained in this study, we subsequently developed a method for the direct acylation of simple hydrocarbons. The use of less-reactive amides as the acyl electrophiles was found to be critical for suppressing undesired pathways. This seemingly counterintuitive reactivity was carefully studied, revealing a substrate-assisted reaction mechanism in which the suppressed oxidative addition leads to early-stage nickel oxidation and C-H activation.To address the drawbacks of this method, which primarily arose from decarbonylative and transmetallative side pathways, we employed N-acyllutidiniums as the acyl electrophile. This prevented undesired decomposition pathways, enabling the use of α-chiral acyl substrates with the retention of their stereochemistry, particularly those derived from α-amino acids. The developed versatile methodology allowed us to access a diverse range of α-amino ketones and their homologues.Despite the elegant utility of Ni/photoredox catalysis in developing new synthetic methodologies, the precise behavior of nickel catalysts under redox conditions is incompletely understood. To gain insight into this behavior and develop new chemical reactions, we used a combination of experimental and computational methods. Our investigations revealed that devised adjustments to the reaction conditions in nickel/photoredox catalysis can result in significant differences in the reaction outcomes, providing chemists with opportunities to tailor reactions through carefully designed mechanistic strategies. We believe that continued efforts to study and apply nickel redox modulation will lead to the discovery of additional organic transformations.

Protodemetalation of (Bipyridyl)Ni(II)-Aryl Complexes Shows Evidence for Five-, Six-, and Seven-Membered Cyclic Pathways

Protonation of C-M bonds and its microscopic reverse, metalation of C-H bonds, are fundamental steps in a variety of metal-catalyzed processes. As such, studies on protonation of C-M bonds can shed light on C-H activation. We present here studies on the rate of protodemetalation (PDM) of a suite of arylnickel(II) complexes with various acids that provide evidence for a concerted, cyclic transition state for the PDM of C-Ni bonds and demonstrate that five-, six-, and seven-membered transition states are particularly favorable. Our data show that while the rate of protodemetalation of arylnickel(II) complexes scales with acidity for many acids, several are faster than predicted by pKa. For example, while acetic acid and acetohydroxamic acid are much less acidic than HCl, they both protodemetalate arylnickel(II) complexes significantly faster than HCl. Our data also show how in the case of acetohydroxamic acid, a seven-membered cyclic transition state (CH3C(O)NHOH) can be more favorable than a six-membered transition state (CH3C(O)NHOH). Similarly, five-membered transition states, such as for pyrazole, are highly favorable as well. Comparison of transition state polarization (from density functional theory) compares these new nickel transition states to better-studied precious-metal systems and demonstrates how the base can change the polarization of the transition state giving rise to opposing electronic preferences. Collectively, these studies suggest several new avenues for study in C-H activation as well as approaches to accelerate or slow protodemetalation in nickel catalysis.

Spiroheterocyclic Photocatalyst for Reducing QHIn-Persistent Pollutants, Dyes, and Transition-Metal Ions Cocatalyzed with Electrolytes

The 7-nitro-2'-phenyl-5',6',7',7a'-tetrahydrospiro[indeno[1,2-b]quinoxaline-11,3'-pyrrolizine]-1',1'(2'H)-dicarbonitrile (SIQPI), 2'-(4-cyanophenyl)-7-nitro-5',6',7',7a'-tetrahydrospiro[indeno[1,2-b]quinoxaline-11,3'-pyrrolizine]-1',1'(2'H) dicarbonitrile (SIQPII), and 2'-(4-methoxyphenyl)-7-nitro-5',6',7',7a'-tetrahydrospiro[indeno[1,2-b]quinoxaline-11,3'-pyrrolizine]-1',1'(2'H)-dicarbonitrile (SIQPIII) were used to photocatalyze quinonoid phenolphthalein (QHIn) in aq-ACN-EtOH (mixed solvent) with NaCl and KCl electrolytes. SIQPI, II, and III spiroindenoquinoxaline pyrrolidines (SIQPs) as spiroheterocyclic photocatalysts alone could not reduce QHIn, but with the addition of electrolytes they are reduced via π cationic interactions (PCI). SIQPI, II, and III with NaCl reduced QHIn in 120, 28, and 50 min, unlike in 138, 58, and 63 min with KCl in mixed solvent. SIQPI, II, and III alone have reduced methylene blue (MB) in 120, 45, and 70 min, unlike in 110, 27, and 55 min with graphene oxide (GO), whereas with NaCl and KCl hey are reduced in 82, 36, and 44 min and 89, 43, and 50 min, respectively. SIQPs with GO had reduced MB in less time than the SIQPs alone, and SIQPs with NaCl had reduced QHIn in a shorter time than KCl. The electrolytes have cocatalyzed a reduction of dyes under sunlight (SL). The electrolytes have reduced a quinonoid structure (QS) and dyes by generating negative and positive (e ^- and h ⁺) holes in a shorter time. SIQPII and magnetic nanoparticles (MNPs) of 58 nm with NaCl photocatalyzed the QHIn in 2880 min. The SIQPs also reduced methyl orange (MO) and brilliant blue R (BBR) at variable temperature (T) and pH range, whereas SIQPs have developed a molecular organic framework (MOF) with transition-metal salts (NiCl₂, CrO₃, KMnO₄, CuSO₄, and MnCl₂) on photocatalysis.

Rapid and Accurate Estimation of Activation Free Energy in Hydrogen Atom Transfer-Based C-H Activation Reactions: From Empirical Model to Artificial Neural Networks

A well-performing machine learning (ML) model is obtained by using proper descriptors and artificial neural network (ANN) algorithms, which can quickly and accurately predict activation free energy in hydrogen atom transfer (HAT)-based sp³ C-H activation. Density functional theory calculations (UωB97X-D) are used to establish the reaction system data sets of methoxyl (CH₃O·), trifluoroethoxyl (CF₃CH₂O·), tert-butoxyl (tBuO·), and cumyloxyl (CumO·) radicals. The simplified Roberts' equation proposed in our recent study works here [R ² = 0.84, mean absolute error (MAE) = 0.85 kcal/mol]. Its performance is comparable with univariate Mulliken-type electronegativity (χ) with the ANN model. The ANN model with bond dissociation free energy, χ, α-unsaturation, and Nolan buried volume (%V _buried) successively improves R ² and MAE to 0.93 and 0.54 kcal/mol, respectively. It reproduces the test sets of trichloroethoxyl (CCl₃CH₂O·) with R ² = 0.87 and MAE = 0.89 kcal/mol and accurately predicts the relative experimental barrier of the HAT reactions with CumO· and the site selectivity of CH₃O·.

Aliphatic C-H Functionalization Using Pyridine N-Oxides as H-Atom Abstraction Agents

The alkylation and heteroarylation of unactivated tertiary, secondary, and primary C(sp3)-H bonds was achieved by employing an acridinium photoredox catalyst along with readily available pyridine Noxides as hydrogen atom transfer (HAT) precursors under visible light. Oxygen-centered radicals, generated by single-electron oxidation of the Noxides, are the proposed key intermediates whose reactivity can be easily modified by structural adjustments. A broad range of aliphatic C-H substrates with electron-donating or -withdrawing groups as well as various olefinic radical acceptors and heteroarenes were well tolerated.

Mesoporous Graphitic Carbon Nitride as a Heterogeneous Organic Photocatalyst in the Dual Catalytic Arylation of Alkyl Bis(catecholato)silicates

Mesoporous graphitic carbon nitride (mpg-CN) is introduced as a heterogeneous photocatalyst to perform dual photoredox- and nickel-catalyzed cross-coupling reactions between alkyl bis(catecholato)silicates as radical precursors and aryl or alkenyl bromides. The synergy between this recyclable photocatalyst and the broadly applied homogeneous nickel complex [Ni(dtbbpy)Br₂] gives access to C(sp²)-C(sp³) cross-coupling products in a sustainable fashion. The recycled mpg-CN photocatalyst was analyzed by time-resolved emission spectroscopy and EPR spectroscopy.

DABCO-promoted photocatalytic C-H functionalization of aldehydes

Herein we present a direct application of DABCO, an inexpensive and broadly accessible organic base, as a hydrogen atom transfer (HAT) abstractor in a photocatalytic strategy for aldehyde C–H activation. The acyl radicals generated in this step were arylated with aryl bromides through a well stablished nickel cross-coupling methodology, leading to a variety of interesting aryl ketones in good yields. We also performed computational calculations to shine light in the HAT step energetics and determined an optimized geometry for the transition state, showing that the hydrogen atom transfer between aldehydes and DABCO is a mildly endergonic, yet sufficiently fast step. The same calculations were performed with quinuclidine, for comparison of both catalysts and the differences are discussed.

Advances in allylic and benzylic C-H bond functionalization enabled by metallaphotoredox catalysis

Metallaphoto-catalysis has been established as a robust platform for efficient construction of a range of chemical bonds. Moreover, transformation of native functionalities such as C(sp³)-H bonds to produce functional molecules represents one of the most attractive strategies in organic synthesis. Merging two powerful methodologies, metallaphoto-catalyzed benzylic and allylic C(sp³)-H bond functionalizations provide a series of general and mild approaches for diversification of alkylbenzenes and alkenes.

Enantioselective β-C(sp3)-H arylation of amides via synergistic nickel and photoredox catalysis

An enantioselective benzylic β-C(sp³)-H arylation of amides via synergistic nickel and photoredox catalysis is reported. The C-H bond is activated by a bromine-radical-mediated C-H cleavage. This mild yet straightforward protocol provides arylation products in up to 96% yield and with up to 95% ee.