Utilization of CO2 Feedstock for Organic Synthesis by Visible-Light Photoredox Catalysis

CO₂ is a highly abundant, green, and sustainable carbon feedstock. Despite its kinetic inertness and thermodynamic stability, the development of various catalytic techniques has enabled the conversion of CO₂ to value-added products such as carboxylic acids, amino acids, and heterocyclic compounds, where visible-light photocatalysis has emerged to be an efficient promoter of these processes. This Minireview covers the progress in the areas of CO₂ incorporation onto organic matters based on the combined venture of renewable resources of CO₂ and light energy with significant emphasis on the last three years’ developments.     

Vanadium Catalyst for C- and S-Alkylation of Benzhydrols in Water: An Experimental Study and Green Metrics Evaluation

A strategy for the vanadium-catalyzed dehydrative C- and S-alkylation by nucleophilic substitution of benzhydrols with arenes and thiols is reported. The alkylation was achieved with the divanadium oxoperoxo complex [K₃(V⁺⁵)₂(O₂²⁻⁾₄(O²⁻)₂(μ-OH)] in water under air. The newly developed transformation could accommodate a broad substrate scope, including (hetero)arenes and thiols (34 examples). Both the symmetrical and unsymmetrical benzhydrols furnished excellent yields of the alkylated product under mild reaction conditions. The scope of this strategy was further extended to synthesize bis-benzylated arenes (poly-arylated products) in high yields and regioselectivities. The green metrics determination of all the alkylated products suggests the technical and environmental benefits of the present protocol. The longevity experiment reveals the catalytic activity was maintained over seven cycles. To understand the mechanism of the present reaction, spectroscopic and kinetic studies were undertaken. This simple protocol, which affords the desired products with water as the by-product, can be achieved under mild conditions without needing a base or other additives.     

Hantzsch Ester as a Photosensitizer for the Visible‐Light‐Induced Debromination of Vicinal Dibromo Compounds

The debromination of vicinal dibromo compounds to generate alkenes usually requires harsh reaction conditions and the addition of catalysts. Just recently the visible‐light‐induced debromination of vicinal dibromo compounds emerged as a possible alternative to commonly used methods, but the substrate scope of this reaction is limited and a photocatalyst is necessary for the successful conversion of the starting compounds. A catalyst‐free visible‐light‐induced debromination of vicinal dibromo compounds with a base‐activated Hantzsch ester as photosensitizer is reported. The method has a wide substrate scope and a broad functional‐group compatibility.     

Mechanism and Applications of the Photoredox Catalytic Coupling of Benzyl Bromides

The photoredox catalytic coupling of halomethyl arenes to bibenzyl derivatives has been demonstrated. The catalytic protocol employed the Hantzsch ester, potassium phosphate, and a photoactive cyclometalated Ir^III complex catalyst. A photochemical quantum yield as high as 20 % was obtained. The catalytic mechanism was investigated in detail by performing photophysical and electrochemical measurements, as well as by quantum chemical calculations. The results suggest that two‐electron mediation might be responsible for the improved photon economy. The reaction protocol was compatible with halomethyl arenes that contain a variety of functional groups. Finally, the synthetic utility of our protocol was demonstrated by the preparation of a natural dihydrostilbenoid, brittonin A.     

Transition-Metal-Free Coupling of 1,3-Dipoles and Boronic Acids as a Sustainable Approach to C−C Bond Formation

The need for alternative, complementary approaches to enable C−C bond formation within organic chemistry is an on-going challenge in the area. Of particular relevance are transformations that proceed in the absence of transition-metal reagents. In the current study, we report a comprehensive investigation of the coupling of nitrile imines and aryl boronic acids as an approach towards sustainable C−C bond formation. In situ generation of the highly reactive 1,3-dipole facilitates a Petasis–Mannich-type coupling via a nucleophilic boronate complex. The introduction of hydrazonyl chlorides as a complementary nitrile imine source to the 2,5-tetrazoles previously reported by our laboratory further broadens the scope of the approach. Additionally, we exemplify for the first time the extension of this protocol into another 1,3-dipole, through the synthesis of aryl ketone oximes from aryl boronic acids and nitrile N-oxides.     

A Convenient Heterogeneous Reduction of Knoevenagel Product by Hantzsch Ester and Its Development into Reductive Alkylation of Malononitrile

Poor solubility of Hantzsch ester is used as indicator in the reduction of methylidene malononitrile. The Knoevenagel reaction is integrated to develop a reductive alkylation of malononitrile with aryl and aliphatic aldehyde as the carbonyl substrate.     

Remarkable Differences in Reactivity between Benzothiazoline and Hantzsch Ester as a Hydrogen Donor in Chiral Phosphoric Acid Catalyzed Asymmetric Reductive Amination of Ketones

Described herein are differences in behavior between a Hantzsch ester and a benzothiazoline as hydrogen donors in the chiral phosphoric acid catalyzed asymmetric reductive amination of ketones with p‐anisidine. The asymmetric reductive amination of ketones with a Hantzsch ester as a hydrogen donor provided the corresponding chiral amines exclusively, regardless of the structures of the ketones, whereas a similar transformation with a benzothiazoline provided chiral amines and p‐methoxyphenyl‐protected primary amines in variable yields, depending on the structures of both the ketones and benzothiazolines. Because a benzothiazoline has an N,S‐acetal moiety that is vulnerable to p‐anisidine, the primary amine can be formed through transimination of the benzothiazoline with p‐anisidine followed by reduction of the resulting aldimine with remaining benzothiazoline.     

Towards Visible-Light Photocatalytic Reduction of Hypercoordinated Silicon Species

Nowadays, the quest of new radical precursors based on heteroatom complexes occupies an increasingly prominent position in contemporary research. Herein, we investigated the behavior and the limitations of hexa- or pentacoordinated organochlorosilanes and related pentacoordinated silyliums as new families of complexes for the generation of radicals under photocatalytic reductive conditions. Particularly, treatment of chlorophenylbis[N,S-pyridine-2-thiolato(−)]silicon(IV) or the related silylium derivative with the fac-Ir(ppy)₃ (5 mol-%)/NEt₃ (1.5 equiv.) system under blue LEDs irradiation generates a thiopyridyl radical which can participate in the formation of a carbon−sulfur bond by reaction with an allylsulfone. Computational studies supported this experimental finding, and particularly by showing that homolytic fragmentation of C−Ts bond is favored over the fragmentation of thiopyridyl radical.     

A Reusable CNT-Supported Single-Atom Iron Catalyst for the Highly Efficient Synthesis of C−N Bonds

C−N bond formation is regarded as a very useful and fundamental reaction for the synthesis of nitrogen-containing molecules in both organic and pharmaceutical chemistry. Noble-metal and homogeneous catalysts have frequently been used for C−N bond formation, however, these catalysts have a number of disadvantages, such as high cost, toxicity, and low atom economy. In this work, a low-toxic and cheap iron complex (iron ethylene-1,2-diamine) has been loaded onto carbon nanotubes (CNTs) to prepare a heterogeneous single-atom catalyst (SAC) named Fe-N_x/CNTs. We employed this SAC in the synthesis of C−N bonds for the first time. It was found that Fe-N_x/CNTs is an efficient catalyst for the synthesis of C−N bonds starting from aromatic amines and ketones. Its catalytic performance was excellent, giving yields of up to 96 %, six-fold higher than the yields obtained with noble-metal catalysts, such as AuCl₃/CNTs and RhCl₃/CNTs. The catalyst showed efficacy in the reactions of thirteen aromatic amine substrates, without the need for additives, and seventeen enaminones were obtained. High-angle annular dark-field scanning transmission electron microscopy in combination with X-ray absorption spectroscopy revealed that the iron species were well dispersed in the Fe-N_x/CNTs catalyst as single atoms and that Fe-N_x might be the catalytic active species. This Fe-N_x/CNTs catalyst has potential industrial applications as it could be cycled seven times without any significant loss of activity.     

The Cyclohexa-2,5-dienyl Group as a Placeholder for Hydrogen: Organocatalytic Michael Addition of an Acetaldehyde Surrogate

An aldehyde with a cyclohexa-2,5-dienyl group in the α-position is introduced as a storable surrogate of highly reactive acetaldehyde. The cyclohexa-2,5-dienyl unit is compatible with an enantioselective Michael addition to nitroalkenes promoted by a Hayashi–Jørgensen catalyst and can be removed by a boron Lewis acid mediated C−C bond cleavage. The robust two-step sequence does not require a large excess of the aldehyde component that is typically needed when directly using acetaldehyde.     

ProPhenol Derived Ligands to Simultaneously Coordinate a Main-Group Metal and a Transition Metal: Application to a Zn−Cu Catalyzed Reaction

A new bifunctional ligand bearing chiral N-heterocyclic carbene (NHC) and prolinol moieties is presented. Utilizing the designed ligand, an in situ formed Cu/Zn hetero-bimetallic complex unlocks the asymmetric allylic alkylation reactions of allyl phosphates with zinc keto-homoenolates, leading to the formation of various γ-vinyl ketones with good regio- and enantio-selectivity. DF sT calculation supports that the chelation of allyl phosphates with catalyst promotes the S_N2’ addition and the ligand-substrate steric interactions account for the stereoselective outcome.     

LiBr as an Efficient Catalyst for One-pot Synthesis of Hantzsch 1,4-Dihydropyridines under Mild Conditions

A simple, inexpensive and efficient one‐pot synthesis of 1,4‐dihydropyridines has been accomplished via lithium bromide‐catalyzed Hantzsch three‐component condensation reaction of an aldehyde, α,β‐ketoester and ammonium acetate in acetonitrile at room temperature in good to excellent yields. The present protocol is applicable to wide range of substrates including aliphatic, aromatic and heterocyclic aldehydes affording 1,4‐dihydropyridines.     

Vinyl Triflate–Aldehyde Reductive Coupling–Redox Isomerization Mediated by Formate: Rhodium-Catalyzed Ketone Synthesis in the Absence of Stoichiometric Metals

Direct conversion of aldehydes to ketones is achieved via rhodium-catalyzed vinyl triflate-aldehyde reductive coupling-redox isomerization mediated by potassium formate. This method circumvents premetalated C-nucleophiles and discrete redox manipulations typically required to form ketones from aldehydes.     

Palladium nanoparticles supported on silica diphenylphosphinite as efficient catalyst for C‐O and C‐S arylation of aryl halides

Silica diphenylphosphinite (SDPP) can be easily obtained through direct phosphorylation of silica gel. Further reaction of SDPP with Pd(II) produces nano Pd(0)/SDPP. This nano Pd(0) catalyst exhibits excellent reactivity for the C‐O and C‐S arylation of different aryl iodides, bromides and chloride with phenols and thiophenols. This heterogeneous catalyst shows excellent recyclability and can be easily recovered and reused for several runs. Copyright © 2013 John Wiley & Sons, Ltd.     

Al/P- and Ga/P-Based Frustrated Lewis Pairs and Electronically Unsaturated Substrates: Ring Cleavage and Ring Closure,C−C and C−N Bond Formation

Al/P- and Ga/P-based frustrated Lewis pairs (FLPs) reacted with an azirine under mild conditions under cleavage of the heterocycle on two different positions. Opening of the C−C bond yielded an unusual nitrile–ylide adduct in which a C−N moiety coordinated to the FLP backbone. Cleavage of a C−N bond afforded the thermodynamically favored enamine adduct with the N atom bound to P and Al or Ga atoms. Ring closure was observed upon treatment of an Al/P FLP with electronically unsaturated substrates (4-(1-cyclohexenyl)-1-aza-but-1-en-3-ynes) and yielded by C−N bond formation hexahydroquinoline derivatives, which coordinated to the FLP through P−C and Al−C bonds. Diphenylcyclopropenone showed a diverse reactivity, which depending on steric shielding and the polarizing effect of Al or Ga atoms afforded different products. An AltBu₂/P FLP yielded an adduct with the C=O group coordinated to P and Al. The dineopentyl derivative gave an equilibrium mixture consisting of a similar product and a simple adduct with O bound to Al and a three-coordinate P atom. Both compounds co-crystallize. The Ga/P FLP only formed the simple adduct with the same substrate. Rearrangement resulted in all cases in C₃-ring cleavage and migration of a mesityl group from P to a former ring C atom by C−C bond formation. Diphenylthiocyclopropenone (evidence for the presence of P=C bonds) and an imine derivative afforded similar products.