Revised Role of Selectfluor in Homogeneous Au‐Catalyzed Oxidative CO Bond Formations

The pairing of transition metal catalysis with the reagent Selectfluor (F‐TEDA–BF₄) has attracted considerable attention due to its utility in myriad C?C and C?heteroatom bond‐forming reactions. However, little mechanistic information is available for Selectfluor‐mediated transition metal‐catalyzed reactions and controversy surrounds the precise role of Selectfluor in these processes. We present herein a systematic investigation of homogeneous Au‐catalyzed oxidative C?O bond‐forming reactions using density functional theory calculations. Currently, Selectfluor is thought to serve as an external oxidant in Au^I/Au^III catalysis. However, our investigations suggest that these reactions follow a newly proposed mechanism in which Selectfluor functions as an electrophilic fluorinating reagent involved in a fluorination/defluorination cycle. We have also explored Selectfluor‐mediated gold‐catalyzed homocoupling reactions, which, when cyclopropyl propargylbenzoate is used as a substrate, lead to an unexpected byproduct.     

Unusual Nitrile–Nitrile and Nitrile–Alkyne Coupling of FcCN and FcCCCN

The reactions of the Group 4 metallocene alkyne complexes, [Cp*₂M(η²‐Me₃SiC₂SiMe₃)] ( 1 a : M=Ti, 1 b : M=Zr, Cp*=η⁵‐pentamethylcyclopentadienyl), with the ferrocenyl nitriles, Fc?C?N and Fc?C?C?C?N (Fc=Fe(η⁵‐C₅H₅)(η⁵‐C₅H₄)), is described. In case of Fc?C?N an unusual nitrile–nitrile C?C homocoupling was observed and 1‐metalla‐2,5‐diaza‐cyclopenta‐2,4‐dienes ( 3 a , b ) were obtained. As the first step of the reaction with 1 b , the nitrile was coordinated to give [Cp*₂Zr(η²‐Me₃SiC₂SiMe₃)(N?C‐Fc)] ( 2 b ). The reactions with the 3‐ferrocenyl‐2‐propyne‐nitrile Fc?C?C?C?N lead to an alkyne–nitrile C?C coupling of two substrates and the formation of 1‐metalla‐2‐aza‐cyclopenta‐2,4‐dienes ( 4 a , b ). For M=Zr, the compound is stabilized by dimerization as evidenced by single‐crystal X‐ray structure analysis. The electrochemical behavior of 3 a , b and 4 a , b was investigated, showing decomposition after oxidation, leading to different redox‐active products.     

Nickel‐ and Photoredox‐Catalyzed Cross‐Coupling Reactions of Aryl Halides with 4‐Alkyl‐1,4‐dihydropyridines as Formal Nucleophilic Alkylation Reagents

A combination of nickel and photoredox catalysts promoted novel cross‐coupling reactions of aryl halides with 4‐alkyl‐1,4‐dihydropyridines. 4‐Alkyl‐1,4‐dihydropyridines act as formal nucleophilic alkylation reagents through a photoredox‐catalyzed carbon–carbon (C?C) bond‐cleavage process. The present strategy provides an alternative to classical carbon‐centered nucleophiles, such as organometallic reagents.     

Nickel‐Catalyzed Allylic Alkylation with Diarylmethane Pronucleophiles: Reaction Development and Mechanistic Insights

Palladium‐catalyzed allylic substitution reactions are among the most efficient methods to construct C?C bonds between sp³‐hybridized carbon atoms. In contrast, much less work has been done with nickel catalysts, perhaps because of the different mechanisms of the allylic substitution reactions. Palladium catalysts generally undergo substitution by a “soft”‐nucleophile pathway, wherein the nucleophile attacks the allyl group externally. Nickel catalysts are usually paired with “hard” nucleophiles, which attack the metal before C?C bond formation. Introduced herein is a rare nickel‐based catalyst which promotes substitution with diarylmethane pronucleophiles by the soft‐nucleophile pathway. Preliminary studies on the asymmetric allylic alkylation are promising.     

Synthesis of Derivatives of Chlorines Related to Chlorophyll‐a by Vilsmeier Reaction with Methyl Pyropheophorbide‐a

From methyl pyropheophorbide‐a (MPPa, 1 ), the vinyl group was converted into other functional groups including 2‐dimethoxylethyl, 1‐hydroxylalkyl, and alkylcarbonyl groups by addition and oxidization to form chlorin ( 2?5b ). The nickel complexes ( 6a?e ) were prepared by treatment with excess nickel acetate in MeOH by refluxing and were used directly for the next reaction without further separation. The Vilsmeier reactions of nickel chlorins with 3‐(dimethylamino)acrolein/phosphoryl chloride (3‐DMA/POCl₃) are carried out to give meso‐20‐fotmyl‐vinylpyropheophorbide‐a ( 7a?9b ).     

The Advanced Multi‐functional Electrocatalyst Efficiently Built from Multi‐integrated Sites for Overall Water Splitting and Rechargeable Zinc‐air Battery

Exploration of cost‐effective, high‐performance and durable multifunctional electrocatalysts is of significant importance for renewable energy conversion and storage. In this work, a simple strategy is developed to tailor the nickel metal with the collaboration of nitrogen‐doped graphene and single‐walled carbon nanotubes. The resulted nickel catalyst exhibits superior trifunctional activities for oxygen evolution, hydrogen evolution and oxygen reduction reactions in the same electrolyte, even comparable to commercial Pt/C and RuO₂ respectively, which can be attributed to the synergistic advantages between nickel, nitrogen and carbon, mainly including abundant integrated active sites achieved by the irregular charge distribution among C?N and Ni?N coupling centers. Such remarkable effects on trifunctional catalysis elicit the efficient overall water splitting, and endow the assembled zinc‐air battery with a good performance. These highlight the metallic nickel as an advanced multifunctional electrocatalysts with integrated sites developed from the collaboration of two different carbon nanomaterials.     

Nickel‐Catalyzed Esterification of Aliphatic Amides

Recent studies have demonstrated that amides can be used in nickel‐catalyzed reactions that lead to cleavage of the amide C?N bond, with formation of a C?C or C?heteroatom bond. However, the general scope of these methodologies has been restricted to amides where the carbonyl is directly attached to an arene or heteroarene. We now report the nickel‐catalyzed esterification of amides derived from aliphatic carboxylic acids. The transformation requires only a slight excess of the alcohol nucleophile and is tolerant of heterocycles, substrates with epimerizable stereocenters, and sterically congested coupling partners. Moreover, a series of amide competition experiments establish selectivity principles that will aid future synthetic design. These studies overcome a critical limitation of current Ni‐catalyzed amide couplings and are expected to further stimulate the use of amides as synthetic building blocks in C?N bond cleavage processes.     

An alternative CuCl–piperidine‐catalyzed oxidative homocoupling of terminal alkynes affording 1,3‐diynes in air

CuCl with the use of a catalytic amount of piperidine as additive shows high catalytic activity for the oxidative homocoupling reactions of terminal alkynes in toluene at 60 °C in air to afford 1,3‐diynes in high yields. Copyright © 2009 John Wiley & Sons, Ltd.     

Gas‐Phase Dehydrogenation of Alkanes: C−H Activation by a Graphene‐Supported Nickel Single‐Atom Catalyst Model

A gas‐phase anionic nickel(0) fluorenyl complex is shown to effect the dehydrogenation of linear, branched, and cyclic alkanes via C?H activation. It performs dehydrogenations via a C?H insertion followed by β‐hydride elimination. When given energy via collision‐induced dissociation, the system is capable of second and third dehydrogenations to form dienes and aromatics such as benzene. Kinetic isotope effects and DFT calculations completed at the M06/6–311+G** level support the proposed mechanism. The metal complex can act as an experimental model for graphene‐supported nickel single‐atom catalysts and suggests that these catalysts are capable of alkane dehydrogenation via C?H activation.     

Gold‐Palladium Nanocluster Catalysts for Homocoupling: Electronic Structure and Interface Dynamics

The gold‐palladium (Au?Pd) bimetallic nanocluster (NC) catalyst in colloidal phase performs the homocoupling reaction of various aryl chlorides (Ar?Cl) under ambient conditions. We have systematically investigated various aspects of the Au?Pd NC catalysts with respect to this homocoupling reaction by using density functional theory (DFT) calculations, genetic algorithm (GA) approaches, and molecular dynamics (MD) simulations. Our findings include the geometric and electronic structures of the Au?Pd NC, the reactive Pd sites on the NC surface, the electron‐donating effects of surrounding polymer matrix, the reaction mechanism of homocoupling reaction and rate‐determining step, the inverse halogen dependence of the reaction, and the solvation dynamics at interface region between NC and polymer matrix in aqueous solution.     

Computational Insight into Nickel‐Catalyzed Carbon–Carbon versus Carbon–Boron Coupling Reactions of Primary,Secondary, and Tertiary Alkyl Bromides

The nickel‐catalyzed alkyl–alkyl cross‐coupling (C?C bond formation) and borylation (C?B bond formation) of unactivated alkyl halides reported in the literature show completely opposite reactivity orders in the reactions of primary, secondary, and tertiary alkyl bromides. The proposed Ni^I/Ni^III catalytic cycles for these two types of bond‐formation reactions were studied computationally by means of DFT calculations at the B3LYP level. These calculations indicate that the rate‐determining step for alkyl–alkyl cross‐coupling is the reductive elimination step, whereas for borylation the rate is determined mainly by the atom‐transfer step. In borylation reactions, the boryl ligand involved has an empty p orbital, which strongly facilitates the reductive elimination step. The inability of unactivated tertiary alkyl halides to undergo alkyl–alkyl cross‐coupling is mainly due to the moderately high reductive elimination barrier.     

Direct Formation of C−C Triple‐Bonded Structural Motifs by On‐Surface Dehalogenative Homocouplings of Tribromomethyl‐Substituted Arenes

On‐surface synthesis shows significant potential in constructing novel nanostructures/nanomaterials, which has been intensely studied in recent years. The formation of acetylenic scaffolds provides an important route to the fabrication of emerging carbon nanostructures, including carbyne, graphyne, and graphdiyne, which feature chemically vulnerable sp‐hybridized carbon atoms. Herein, we designed and synthesized a tribromomethyl‐substituted compound. By using a combination of high‐resolution scanning tunneling microscopy, non‐contact atomic force microscopy, and density functional theory calculations, we demonstrated that it is feasible to convert these compounds directly into C?C triple‐bonded structural motifs by on‐surface dehalogenative homocoupling reactions. Concurrently, sp³‐hybridized carbon atoms are converted into sp‐hybridized ones, that is, an alkyl group is transformed into an alkynyl moiety. Moreover, we achieved the formation of dimer structures, one‐dimensional molecular wires, and two‐dimensional molecular networks on Au(111) surfaces, which should inspire further studies towards two‐dimensional graphyne structures.     

Nickel‐Catalyzed C−H Chalcogenation of Anilines

The C?H thiolation of aniline derivatives was accomplished with a versatile nickel(II) catalyst under ligand‐free conditions. The robust nature of the nickel catalysis system was reflected by the C?H thiolation with a good functional group tolerance and an ample scope, employing anilines possessing removable directing groups. The widely applicable nickel catalyst also allowed for aniline C?H selenylations, while mechanistic studies provided strong support that the rate‐determining step is the C?H activation.     

Facile synthesis of 1, 2‐benzisothiazole‐3‐one‐1,1 ‐dioxide methylsulfonyl derivatives

An improved and general synthesis of saccharin methylthio and methylsulfone derivatives from chlorosubstituted saccharins is presented. A large‐scale procedure for preparation of chloro‐substituted saccharins was developed. Treatment of the saccharin chlorides with sodium thiomethoxide and t‐BuOK in DMF gave the saccharin methyl sulfides, which upon chromium(VI) oxide catalyzed oxidation with periodic acid afforded the corresponding saccharin methylsulfones in high yields.     

Water‐Promoted Ring‐Opening Reactions of N‐Substituted Saccharins and Phthalimides by Amines

The reactions of N‐substituted saccharins and phthalimides with amines were promoted by water. Various o‐sulfamoyl benzamides and N,N′‐disubstituted phthalamides were prepared in moderate to good yields. These reactions have prominent advantages, such as short reaction time, less by‐products and simple isolation of the products. Water can probably stabilize the reaction intermediates and facilitate precipitation of the ring‐opening products. When steric hindrance arose, hydrolytic compounds, either free acid or salts of the acids, were obtained. Possible reason for the formation of amine salts of o‐sulfamoyl benzoic acids was proposed.     

Nickel‐Catalyzed Decarboxylative Acylation of Heteroarenes by sp2 CH Functionalization

Nickel‐catalyzed ligand‐free decarboxylative cross‐coupling of azole derivatives with α‐oxoglyoxylic acids has been developed. This work represents the first example of decarboxylative cross‐coupling reactions, in a C?H bond functionalization manner, through nickel catalysis, and tolerates various functional groups. Additionally, this approach provides an efficient access to azole ketones, an important structural motif in many medicinal compounds with a broad range of biological activities.     

CC Coupling of N‐Heterocycles at the fac‐Re(CO)3 Fragment: Synthesis of Pyridylimidazole and Bipyridine Ligands

A new family of cationic rhenium tricarbonyl complexes with either two N‐alkylimidazole (N‐RIm) and one pyridine (Py) ligand, or two pyridine and one N‐RIm ligand, [Re(CO)₃(N‐RIm)_(3?x)(Py)_x]⁺, has been prepared. The reaction of these complexes with a strong base, followed by an oxidant, selectively afforded 2,2’‐pyridylimidazole complexes as the result of intramolecular dehydrogenative C?C coupling reactions. For tris(pyridine) complexes [Re(CO)₃(Py)₃]⁺ the reaction pattern upon a deprotonation/oxidation sequence is maintained, which allows the generation of complexes with 2,2’‐bipyridine ligands. In the particular combination of two different types of pyridine ligand in the cationic fac‐Re(CO)₃ complexes only the cross‐coupling products with asymmetric 2,2’‐bipyridine ligands were obtained; the homocoupling products were not observed.     

Ammonium Chloride Promoted Palladium-Catalyzed UIImann Coupling of Aryl Bromide

In water, ammonium chloride was found to promote palladium-catalyzed Ullmann coupling reactions of aryl bromides. In the presence of Pd/C, zinc, NH4Cl, and water, coupling of various aryl bromides was carried out smoothly to afford the corresponding homocoupling products in moderate yields.     

Synthesis of Triarylpyridines in Thiopeptide Antibiotics by Using a C−H Arylation/Ring‐Transformation Strategy

We have described a C?H arylation/ring‐transformation strategy for the synthesis of triarylpyridines, which form the core structure of thiopeptide antibiotics. This synthetic method readily gave 2,3,6‐triarylpyridines in a regioselective manner by a two‐phase approach: C?H arylation (a nickel‐catalyzed decarbonylative Suzuki–Miyaura cross‐coupling and decarbonylative C?H coupling for the synthesis of 2,4‐diaryloxazoles) and ring transformation ([4+2] cycloaddition of 2,4‐diaryloxazoles with (hetero)arylacrylic acids). To showcase these methods, we have accomplished the formal synthesis of thiopeptide antibiotics GE2270 s and amythiamicins.     

Highly Chemoselective Iridium Photoredox and Nickel Catalysis for the Cross‐Coupling of Primary Aryl Amines with Aryl Halides

A visible‐light‐promoted iridium photoredox and nickel dual‐catalyzed cross‐coupling procedure for the formation C?N bonds has been developed. With this method, various aryl amines were chemoselectively cross‐coupled with electronically and sterically diverse aryl iodides and bromides to forge the corresponding C?N bonds, which are of high interest to the pharmaceutical industries. Aryl iodides were found to be a more efficient electrophilic coupling partner. The coupling reactions were carried out at room temperature without the rigorous exclusion of molecular oxygen, thus making this newly developed Ir‐photoredox/Ni dual‐catalyzed procedure very mild and operationally simple.