Cobalt(III)‐Catalyzed Intramolecular Cross‐Dehydrogenative C−H/X−H Coupling: Efficient Synthesis of Indoles and Benzofurans

An efficient cobalt(III)‐catalyzed intramolecular cross‐dehydrogenative C?H/N?H coupling of ortho‐alkenylanilines has been developed utilizing O₂ as a terminal oxidant. The developed reaction tolerates various reactive functional groups and allows the synthesis of diverse indole derivatives in good to excellent yields. The method was successfully extended to the synthesis of benzofurans through the intramolecular cross‐dehydrogenative C?H/O?H coupling of ortho‐alkenylphenols.     

Synthesis of trans‐Disubstituted Alkenes by Cobalt‐Catalyzed Reductive Coupling of Terminal Alkynes with Activated Alkenes

A cobalt‐catalyzed reductive coupling of terminal alkynes, RC?CH, with activated alkenes, R′CH?CH₂, in the presence of zinc and water to give functionalized trans‐disubstituted alkenes, RCH?CHCH₂CH₂R′, is described. A variety of aromatic terminal alkynes underwent reductive coupling with activated alkenes including enones, acrylates, acrylonitrile, and vinyl sulfones in the presence of a CoCl₂/P(OMe)₃/Zn catalyst system to afford 1,2‐trans‐disubstituted alkenes with high regio‐ and stereoselectivity. Similarly, aliphatic terminal alkynes also efficiently participated in the coupling reaction with acrylates, enones, and vinyl sulfone, in the presence of the CoCl₂/P(OPh)₃/Zn system providing a mixture of 1,2‐trans‐ and 1,1‐disubstituted functionalized terminal alkene products in high yields. The scope of the reaction was also extended by the coupling of 1,3‐enynes and acetylene gas with alkenes. Furthermore, a phosphine‐free cobalt‐catalyzed reductive coupling of terminal alkynes with enones, affording 1,2‐trans‐disubstituted alkenes as the major products in a high regioisomeric ratio, is demonstrated. In the reactions, less expensive and air‐stable cobalt complexes, a mild reducing agent (Zn) and a simple hydrogen source (water) were used. A possible reaction mechanism involving a cobaltacyclopentene as the key intermediate is proposed.     

Cobalt Nanoparticles-Catalyzed Widely Applicable Successive C−C Bond Cleavage in Alcohols to Access Esters

Selective cleavage and functionalization of C−C bonds have important applications in organic synthesis and biomass utilization. However, functionalization of C−C bonds by controlled cleavage remains difficult and challenging because they are inert. Herein, we describe an unprecedented efficient protocol for the breaking of successive C−C bonds in alcohols to form esters with one or multiple carbon atoms less using heterogeneous cobalt nanoparticles as catalyst with dioxygen as the oxidant. A wide range of alcohols including inactive long-chain alkyl aryl alcohols undergo smoothly successive cleavage of adjacent −(C−C)_n− bonds to afford the corresponding esters. The catalyst was used for seven times without any decrease in activity. Characterization and control experiments disclose that cobalt nanoparticles are responsible for the successive cleavage of C−C bonds to achieve excellent catalytic activity, while the presence of Co-N_x has just the opposite effect. Preliminary mechanistic studies reveal that a tandem sequence reaction is involved in this process.     

Pd/Light‐Accelerated Atom‐Transfer Carbonylation of Alkyl Iodides: Applications in Multicomponent Coupling Processes Leading to Functionalized Carboxylic Acid Derivatives

The atom‐transfer carbonylation reaction of various alkyl iodides thereby leading to carboxylic acid esters was effectively accelerated by the addition of transition‐metal catalysts under photoirradiation conditions. By using a combined Pd/hν reaction system, vicinal C‐functionalization of alkenes was attained in which α‐substituted iodoalkanes, alkenes, carbon monoxide, and alcohols were coupled to give functionalized esters. When alkenyl alcohols were used as acceptor alkenes, three‐component coupling reactions, which were accompanied by intramolecular esterification, proceeded to give lactones. Pd‐dimer complex [Pd₂(CNMe)₆][PF₆]₂, which is known to undergo homolysis under photoirradiation conditions, worked quite well as a catalyst in these three‐ or four‐component coupling reactions. In this metal/radical hybrid system, both Pd radicals and acyl radicals are key players and a stereochemical study confirmed the carbonylation step proceeded through a radical carbonylation mechanism.     

Electrochemically Driven C−H Hydrogen Abstraction Processes with the Tetrachloro‐Phthalimido‐N‐Oxyl (Cl4PINO) Catalyst

The radical redox mediator tetrachloro‐phthalimido‐N‐oxyl (Cl₄PINO) is generated at a glassy carbon electrode and investigated for the model oxidation of primary and secondary alcohols with particular attention to reaction rates and mechanism. The two‐electron oxidation reactions of a range of primary, secondary, and cyclic alcohols are dissected into an initial step based on C−H hydrogen abstraction (rate constant k₁, confirmed by kinetic isotope effect) and a fast radical‐radical coupling of the resulting alcohol radical with Cl₄PINO to give a ketal that only slowly releases the aldehyde/ketone and redox mediator precursor back into solution (rate constant k₂). In situ electrochemical EPR reveals Cl₄PINO sensitivity towards moisture. DFT methods are applied to confirm and predict C−H hydrogen abstraction reactivity.     

Reagent‐Free C−H/N−H Cross‐Coupling: Regioselective Synthesis of N‐Heteroaromatics from Biaryl Aldehydes and NH3

An unprecedented synthesis of N‐heteroaromatics from biaryl aldehydes and NH₃ through reagent‐free C−H/N−H cross‐coupling has been developed. The electrosynthesis uses NH₃ as an inexpensive and atom‐economic nitrogen donor, requires no oxidizing agents, and allows efficient and regioselective access to a wide range of phenanthridines and structurally related polycyclic N‐heteroaromatic products.     

Direct C−C Double Bond Cleavage of Alkenes Enabled by Highly Dispersed Cobalt Catalyst and Hydroxylamine

The utilization of a single-atom catalyst to break C−C bonds merges the merits of homogeneous and heterogeneous catalysis and presents an intriguing pathway for obtaining high-value-added products. Herein, a mild, selective, and sustainable oxidative cleavage of alkene to form oxime ether or nitrile was achieved by using atomically dispersed cobalt catalyst and hydroxylamine. Diversified substrate patterns, including symmetrical and unsymmetrical alkenes, di- and tri-substituted alkenes, and late-stage functionalization of complex alkenes were demonstrated. The reaction was successfully scaled up and demonstrated good performance in recycling experiments. The hot filtration test, catalyst poisoning and radical scavenger experiment, time kinetics, and studies on the reaction intermediate collectively pointed to a radical mechanism with cobalt/acid/O₂ promoted C−C bond cleavage as the key step.     

Cobalt‐Catalyzed C(sp2)?H Alkoxylation of Aromatic and Olefinic Carboxamides

The cobalt‐catalyzed alkoxylation of C(sp²) H bonds in aromatic and olefinic carboxamides has been developed. The reaction proceeded under mild conditions in the presence of Co(OAc)₂⋅4H₂O as the catalyst and tolerates a wide range of both alcohols and benzamide substrates, including even olefinic carboxamides. In addition, this reaction is the first example of the direct alkoxylation of alkenes through C H bond activation.     

Cobalt‐Catalyzed C(sp2)H Alkoxylation of Aromatic and Olefinic Carboxamides

The cobalt‐catalyzed alkoxylation of C(sp²)? H bonds in aromatic and olefinic carboxamides has been developed. The reaction proceeded under mild conditions in the presence of Co(OAc)₂?4H₂O as the catalyst and tolerates a wide range of both alcohols and benzamide substrates, including even olefinic carboxamides. In addition, this reaction is the first example of the direct alkoxylation of alkenes through C? H bond activation.     

Direct oxidative C(sp3)─H/C(sp2)─H coupling reaction using recyclable Sr-doped LaCoO3 perovskite catalyst

A strontium-doped lanthanum cobaltite perovskite, La_0.6Sr_0.4CoO₃, was prepared and utilized as a recyclable heterogeneous catalyst for the direct oxidative C(sp³)─H/C(sp²)─H coupling reaction between cyclic ethers and alkenes or coumarins to achieve corresponding α-functionalized ethers. The α-functionalization of cyclic thioethers or amides with alkenes or coumarins was also achieved via this protocol. The La_0.6Sr_0.4CoO₃ catalyst exhibited better performance than a variety of homogeneous and heterogeneous catalysts. Utilizing a recyclable catalyst would offer a greener option for the direct oxidative C(sp³)─H/C(sp²)─H coupling reaction. To our best knowledge, the C(sp³)─H/C(sp²)─H coupling between olefins and ethers to generate α-functionalized ethers using a heterogeneous catalyst has not been previously reported, and the α-functionalization of cyclic thioethers or amides with alkenes or coumarins is new.     

4‐Amino‐7‐(2‐de­oxy‐2‐fluoro‐β‐d‐arabinofuranos­yl)‐5‐fluoro‐7H‐pyrrolo[2,3‐d]pyrimidine: a bis‐fluorinated analogue of 2′‐deoxy­tubercidin

The title compound, C₁₁H₁₂F₂N₄O₃, exhibits an anti glycosylic bond conformation, with a torsion angle χ = −117.8 (2)°. The sugar pucker is N‐type (C4′‐exo, between ³T₄ and E₄, with P = 45.3° and τ_m = 41.3°). The conformation around the exocyclic C—C bond is −ap (trans), with a torsion angle γ = −177.46 (15)°. The nucleobases are stacked head‐to‐head. The crystal structure is characterized by a three‐dimensional hydrogen‐bond network involving N—H⋯O, O—H⋯O and O—H⋯N hydrogen bonds.     

[IrCl2Cp*(NHC)] Complexes as Highly Versatile Efficient Catalysts for the Cross‐Coupling of Alcohols and Amines

A comparative study on the catalytic activity of a series of [IrCl₂Cp*(NHC)] complexes in several C–O and C–N coupling processes implying hydrogen‐borrowing mechanisms has been performed. The compound [IrCl₂Cp*(I^nBu)] (Cp*=pentamethyl cyclopentadiene; I^nBu=1,3‐di‐n‐butylimidazolylidene) showed to be highly effective in the cross‐coupling reactions of amines and alcohols, providing high yields in the production of unsymmetrical ethers and N‐alkylated amines. A remarkable feature is that the processes were carried out in the absence of base, phosphine, or any other external additive. A comparative study with other known catalysts, such as Shvo's catalyst, is also reported.     

meta‐C−H Bromination on Purine Bases by Heterogeneous Ruthenium Catalysis

Methods for positionally selective remote C−H functionalizations are in high demand. Herein, we disclose the first heterogeneous ruthenium catalyst for meta ‐selective C−H functionalizations, which enabled remote halogenations with excellent site selectivity and ample scope. The versatile heterogeneous Ru@SiO₂ catalyst was broadly applicable and could be easily recovered and reused, which set the stage for the direct fluorescent labeling of purines. In contrast to palladium, rhodium, iridium, or cobalt complexes, solely the ruthenium catalysis manifold provided access to meta ‐halogenated purine derivatives, illustrating the unique power of ruthenium C−H activation catalysis.     

A luminescent bis(pyridyl)‐substituted benzimidazole platinum(II) complex exhibiting an intermolecular anagostic interaction

The photophysical properties of transition metal complexes of the 5,6‐dimethyl‐2‐(pyridin‐2‐yl)‐1‐(pyridin‐2‐ylmethyl)‐1H‐benzimidazole ligand are of interest. Dichlorido[5,6‐dimethyl‐2‐(pyridin‐2‐yl)‐1‐(pyridin‐2‐ylmethyl)‐1H‐benzimidazole‐κ²N ²,N ³]platinum(II), [PtCl₂(C₂₀H₁₈N₄)], is luminescent in the solid state at room temperature. The compound displays a distorted square‐planar coordination geometry. The Pt—N(imidazole) bond length is shorter than the Pt—N(pyridine) bond length. The extended structure reveals that symmetry‐related molecules display weak C—H…N, C—H…Cl, and C—H…Pt hydrogen‐bonding interactions that are clearly discernable in the Hirshfeld surface and fingerprint plots. The intermolecular C—H…Pt and C—H…N interactions have been explored using density functional theory. The result of an analysis of the distance dependence of C—H…Pt yields a value consistent with that observed in the solid‐state structure. The energy of interaction for the C—H…Pt interaction is found to be about −11 kJ mol⁻¹.     

Chelation versus Non‐Chelation Control in the Stereoselective Alkenyl sp2 C−H Bond Functionalization Reaction

A hydroxy group chelation‐assisted stereospecific oxidative cross‐coupling reaction between alkenes was developed under mild reaction conditions. In the presence of palladium catalyst, the alkenes tethered with hydroxy functionality can couple efficiently with electron‐deficient alkenes to form the corresponding multi‐substituted olefin products. The hydroxy group on the substrate could play dual roles in reaction, acting as the directing group for alkenyl C−H bond activation and controlling the stereoselectivity of the products.     

Manganese‐Catalyzed Multicomponent Synthesis of Pyrimidines from Alcohols and Amidines

The development of catalytic reactions for synthesizing different compounds from alcohols to save fossil carbon feedstock and reduce CO₂ emissions is of high importance. Replacing rare noble metals with abundantly available 3d metals is equally important. We report a manganese‐complex‐catalyzed multicomponent synthesis of pyrimidines from amidines and up to three alcohols. Our reaction proceeds through condensation and dehydrogenation steps, permitting selective C−C and C−N bond formations. β‐Alkylation reactions are used to multiply alkylate secondary alcohols with two different primary alcohols to synthesize fully substituted pyrimidines in a one‐pot process. Our PN₅P‐Mn‐pincer complexes efficiently catalyze this multicomponent process. A comparison of our manganese catalysts with related cobalt catalysts indicates that manganese shows a reactivity similar to that of iridium but not cobalt. This analogy could be used to develop further (de)hydrogenation reactions with manganese complexes.     

2′‐De­oxy‐2‐fluoro­tubercidin

In the title compound [systematic name: 7‐(2‐de­oxy‐β‐d ‐erythro‐pentofuranos­yl)‐2‐fluoro‐7H‐pyrrolo[2,3‐d]pyrimidin‐2‐amine], C₁₁H₁₃FN₄O₃, the conformation of the N‐glycosylic bond is between anti and high‐anti [χ = −110.2 (3)°]. The 2′‐deoxy­ribofuranosyl unit adopts the N‐type sugar pucker (₄T³), with P = 40.3° and τ_m = 39.2°. The orientation of the exocyclic C4′—C5′ bond is −ap (trans), with a torsion angle γ = −168.39 (18)°. The nucleobases are arranged head‐to‐head. The crystal structure is stabilized by four inter­molecular hydrogen bonds of types N—H⋯N, N—H⋯O and O—H⋯O.     

Convenient and Mild Epoxidation of Alkenes Using Heterogeneous Cobalt Oxide Catalysts

A general epoxidation of aromatic and aliphatic olefins has been developed under mild conditions using heterogeneous Co_xO_y–N/C (x=1,3; y=1,4) catalysts and tert‐butyl hydroperoxide as the terminal oxidant. Various stilbenes and aliphatic alkenes, including renewable olefins, and vitamin and cholesterol derivatives, were successfully transformed into the corresponding epoxides with high selectivity and often good yields. The cobalt oxide catalyst can be recycled up to five times without significant loss of activity or change in structure. Characterization of the catalyst by XRD, TEM, XPS, and EPR analysis revealed the formation of cobalt oxide nanoparticles with varying size (Co₃O₄ with some CoO) and very few large particles with a metallic Co core and an oxidic shell. During the pyrolysis process the nitrogen ligand forms graphene‐type layers, in which selected carbon atoms are substituted by nitrogen.     

Preparation and characterization of silicone rubber through the reaction between γ‐chloropropyl and amino groups with siloxane polyamidoamine dendrimers as cross‐linkers

A novel heat‐curable silicone rubber (MCSR/Si‐PAMAM) was prepared by using siloxane polyamidoamine (Si‐PAMAM) dendrimers as cross‐linkers and polysiloxane containing γ‐chloropropyl groups as gums. The chemical cross‐linking occurs through the reaction between Si‐PAMAM dendrimers and polysiloxane containing γ‐chloropropyl groups. The effect of various amounts of cross‐linkers on mechanical properties of MCSR/Si‐PAMAM was discussed in this paper. MCSR/Si‐PAMAM exhibits favorable mechanical properties with a tensile strength of 10.06 MPa and a tear strength of 47.9 kN/m when the molar ratio r of [N‐H]/[CH₂CH₂CH₂Cl] is 1:1. These excellent mechanical properties can be attributed to the formation of concentrative cross‐linking from Si‐PAMAM dendrimers in the cross‐linking networks, along with the introduction of Si–O–Si units in the internal structure of dendrimers. The introduction of Si–O–Si units reduces the steric hindrance of molecular structure, which facilitates the N–H bonds in the interior layers of dendrimers to react with γ‐chloropropyl groups. In addition, thermogravimetric analysis results indicate that MCSR/Si‐PAMAM is thermally stable even at high temperatures in a nitrogen atmosphere. Differential scanning calorimetry analysis reveals that the glass transition peak of MCSR/Si‐PAMAM is not identified in the temperature range −150 to −30°C, only a melting endothermic peak at −40°C.     

Defluorinative C(sp3)−P Bond Construction for the Synthesis of Phosphorylation gem‐Difluoroalkenes under Catalyst‐ and Oxidant‐Free Conditions

Defluorinative C(sp³)?P bond formation of α‐trifluoromethyl alkenes with phosphine oxides or phosphonates have been achieved under catalyst‐ and oxidant‐free conditions, giving phosphorylation gem‐difluoroalkenes as products. α‐Trifluoromethyl alkenes bearing various of aryl substituents such as halogen, cyano, ester and heterocyclic groups are available in this transformation. The results of control experiments demonstrated that the mechanism of dehydrogenative/defluorinative cross‐coupling reactions was not a radical route, but might be an S_N2′ process involving phosphine oxide anion.