Practical Direct α‐Arylation of Cyclopentanones by Palladium/Enamine Cooperative Catalysis

Direct arylation of cyclopentanones has been a long‐standing challenge because of competitive self‐aldol condensation and multiple arylations. Reported herein is a direct mono‐α‐C?H arylation of cyclopentanones with aryl bromides which is enabled by palladium/amine cooperative catalysis. This method is scalable and chemoselective with broad functional‐group tolerance. Application to controlled sequential arylation of cyclopentanones has been also demonstrated.     

Gold‐Catalyzed Formal C−C Bond Insertion Reaction of 2‐Aryl‐2‐diazoesters with 1,3‐Diketones

The transition‐metal‐catalyzed formal C−C bond insertion reaction of diazo compounds with monocarbonyl compounds is well established, but the related reaction of 1,3‐diketones instead gives C−H bond insertion products. Herein, we report a protocol for a gold‐catalyzed formal C−C bond insertion reaction of 2‐aryl‐2‐diazoesters with 1,3‐diketones, which provides efficient access to polycarbonyl compounds with an all‐carbon quaternary center. The aryl ester moiety plays a crucial role in the unusual chemoselectivity, and the addition of a Brønsted acid to the reaction mixture improves the yield of the C−C bond insertion product. A reaction mechanism involving cyclopropanation of a gold carbenoid with an enolate and ring‐opening of the resulting donor–acceptor‐type cyclopropane intermediate is proposed. This mechanism differs from that of the traditional Lewis‐acid‐catalyzed C−C bond insertion reaction of diazo compounds with monocarbonyl compounds, which involves a rearrangement of a zwitterion intermediate as a key step.     

Asymmetric Iron‐Catalyzed C−H Alkylation Enabled by Remote Ligand meta‐Substitution

Highly enantioselective iron‐catalyzed C−H alkylations by inner‐sphere C−H activation were accomplished with ample scope. High levels of enantiocontrol proved viable through a novel ligand design that exploits a remote meta‐substitution on N‐heterocyclic carbenes within a facile ligand‐to‐ligand H‐transfer C−H cleavage.     

Recent Advances in Ring‐Opening Functionalization of Cycloalkanols by C–C σ‐Bond Cleavage

Cycloalkanols prove to be privileged precursors for the synthesis of distally substituted alkyl ketones and polycyclic aromatic hydrocarbons (PAHs) by virtue of cleavage of their cyclic C−C bonds. Direct functionalization of cyclobutanols to build up other chemical bonds (e. g., C−F, C−Cl, C−Br, C−N, C−S, C−Se, C−C, etc.) has been achieved by using the ring‐opening strategy. Mechanistically, the C−C cleavage of cyclobutanols can be involved in two pathways: (a) transition‐metal catalyzed β‐carbon elimination; (b) radical‐mediated ‘radical clock’‐type ring opening. The recent advances of our group for the ring‐opening functionalization of tertiary cycloalkanols are described in this account.     

Organoselenium‐Catalyzed Regioselective C−H Pyridination of 1,3‐Dienes and Alkenes

An efficient approach for organoselenium‐catalyzed regioselective C−H pyridination of 1,3‐dienes to form pyridinium salts has been developed. This method was also successfully applied to direct C−H pyridination of alkenes. Fluoropyridinium reagents, or initially loaded pyridine derivatives, acted as pyridine sources in the pyridination reactions. The obtained pyridinium salts could be further converted under different conditions. This work is the first example of catalytic C‐2 direct C−H functionalization of 1,3‐dienes and the first case of organoselenium‐catalyzed C−H pyridination.     

Copper‐Mediated Late‐Stage Functionalization of Heterocycle‐Containing Molecules

One long‐standing issue in directed C−H functionalization is that either nitrogen or sulfur atoms present in heterocyclic substrates may bind preferentially to a transition‐metal catalyst rather than to the desired directing group. This competitive binding has largely hindered the application of C−H functionalization in late‐stage heterocycle drug discovery. Reported here is the use of an oxazoline‐based directing group capable of overriding the poisoning effect of a wide range of heterocycle substrates. The potential use of this directing group in pharmaceutical drug discovery is illustrated by diversification of Telmisartan (an antagonist for the angiotensin II receptor) through copper‐mediated C−H amination, hydroxylation, thiolation, arylation, and trifluoromethylation.     

Iridium‐Catalyzed,Silyl‐Directed,peri‐Borylation of C−H Bonds in Fused Polycyclic Arenes and Heteroarenes

peri‐Disubstituted naphthalenes exhibit interesting physical properties and unique chemical reactivity, due to the parallel arrangement of the bonds to the two peri‐disposed substituents. Regioselective installation of a functional group at the position peri to 1‐substituted naphthalenes is challenging due to the steric interaction between the existing substituent and the position at which the second one would be installed. We report an iridium‐catalyzed borylation of the C−H bond peri to a silyl group in naphthalenes and analogous polyaromatic hydrocarbons. The reaction occurs under mild conditions with wide functional group tolerance. The silyl group and the boryl group in the resulting products are precursors to a range of functional groups bound to the naphthalene ring through C−C, C−O, C−N, C−Br and C−Cl bonds.     

Direct Mechanism of the First Carbon–Carbon Bond Formation in the Methanol‐to‐Hydrocarbons Process

In the past two decades, the reaction mechanism of C−C bond formation from either methanol or dimethyl ether (DME) in the methanol‐to‐hydrocarbons (MTH) process has been a highly controversial issue. Described here is the first observation of a surface methyleneoxy analogue, originating from the surface‐activated DME, by in situ solid‐state NMR spectroscopy, a species crucial to the first C−C bond formation in the MTH process. New insights into the first C−C bond formation were provided, thus suggesting DME/methanol activation and direct C−C bond formation by an interesting synergetic mechanism, involving C−H bond breakage and C−C bond coupling during the initial methanol reaction within the chemical environment of the zeolite catalyst.     

Ligand‐Promoted Rhodium(III)‐Catalyzed ortho‐C−H Amination with Free Amines

Ligand development for rhodium(III)‐catalyzed C−H activation reactions has largely been limited to cyclopentadienyl (Cp) based scaffolds. 2‐Methylquinoline has now been identified as a feasible ligand that can coordinate to the metal center of Cp*RhCl to accelerate the cleavage of the C−H bond of N ‐pentafluorophenylbenzamides, providing a new structural lead for ligand design. The compatibility of this reaction with secondary free amines and anilines also overcomes the limitations of palladium(II)‐catalyzed C−H amination reactions.     

Electrocatalytic Oxidant‐Free Dehydrogenative C−H/S−H Cross‐Coupling

An environmentally friendly electrocatalytic protocol has been developed for dehydrogenative C−H/S−H cross‐coupling. This method enabled C−S bond formation under catalyst‐ and oxidant‐free conditions. Under undivided electrolysis conditions, various aryl/heteroaryl thiols and electron‐rich arenes afforded the C−S bond‐formation products in 24–99 % yield. A preliminary mechanistic study indicated that the generation of aryl radical cation intermediates is key to the success of this transformation.     

Single and Twofold Metal‐ and Reagent‐Free Anodic C−C Cross‐Coupling of Phenols with Thiophenes

The first electrochemical dehydrogenative C−C cross‐coupling of thiophenes with phenols has been realized. This sustainable and very simple to perform anodic coupling reaction enables access to two classes of compounds of significant interest. The scope for electrochemical C−H‐activating cross‐coupling reactions was expanded to sulfur heterocycles. Previously, only various benzoid aromatic systems could be converted, while the application of heterocycles was not successful in the electrochemical C−H‐activating cross‐coupling reaction. Here, reagent‐ and metal‐free reaction conditions offer a sustainable electrochemical pathway that provides an attractive synthetic method to a broad variety of bi‐ and terarylic products based on thiophenes and phenols. This method is easy to conduct in an undivided cell, is scalable, and is inherently safe. The resulting products offer applications in electronic materials or as [OSO]²⁻ pincer‐type ligands.     

Gold‐Catalyzed [3+2]/Retro‐[3+2]/[3+2] Cycloaddition Cascade Reaction of N‐Alkoxyazomethine Ylides

A novel cascade reaction has been developed for the synthesis of 2,6‐methanopyrrolo[1,2‐b]isoxazoles based on the gold‐catalyzed generation of an N‐allyloxyazomethine ylide. This reaction involves sequential [3+2]/retro‐[3+2]/[3+2] cycloaddition reactions, thus providing facile access to fused and bridged heterocycles which would be otherwise difficult to prepare using existing synthetic methods. Notably, this reaction allows the efficient construction of three C−C bonds, one C−O bond, one C−N bond and one C−H bond, as well as the cleavage of one C−C bond, one C−O bond and one C−H bond in a single operation. The intermolecular cycloaddition of an N‐allyloxyazomethine ylide and the subsequent application of the product to the synthesis of tropenol is also described.     

Ruthenium‐Catalyzed para‐Selective C−H Alkylation of Aniline Derivatives

The para ‐selective C−H alkylation of aniline derivatives furnished with a pyrimidine auxiliary is herein reported. This reaction is proposed to take place via an N−H‐activated cyclometalate formed in situ. Experimental and DFT mechanistic studies elucidate a dual role of the ruthenium catalyst. Here the ruthenium catalyst can undergo cyclometalation by N−H metalation (as opposed to C−H metalation in meta ‐selective processes) and form a redox active ruthenium species, to enable site‐selective radical addition at the para position.     

Highly para‐Selective C−H Alkylation of Benzene Derivatives with 2,2,2‐Trifluoroethyl α‐Aryl‐α‐Diazoesters

Compared to the most popular directing‐group‐assisted strategy, the “undirected” strategy for C−H bond functionalization represents a more flexible but more challenging approach. Reported herein is a gold‐catalyzed highly site‐selective C(sp²)−H alkylation of unactivated arenes with 2,2,2‐trifluoroethyl α‐aryl‐α‐diazoesters. This protocol demonstrates that high site‐selective C−H bond functionalization can be achieved without the assistance of a directing group. In this transformation, both the gold catalyst and trifluoroethyl group on the ester of the diazo compound play vital roles for achieving the chemo‐ and regioselectivity.     

Lewis Acid–Base Interaction‐Controlled ortho‐Selective C−H Borylation of Aryl Sulfides

An iridium/bipyridine‐catalyzed ortho ‐selective C−H borylation of aryl sulfides was developed. High ortho ‐selectivity was achieved by a Lewis acid–base interaction between a boryl group of the ligand and a sulfur atom of the substrate. This is the first example of a catalytic and regioselective C−H transformation controlled by a Lewis acid–base interaction between a ligand and a substrate. The C−H borylation reaction could be conducted on a gram scale, and with a bioactive molecule as a substrate, demonstrating its applicability to late‐stage regioselective C−H borylation. A bioactive molecule was synthesized from an ortho ‐borylated product by converting the boryl and methylthio groups of the product.     

Manganese(I)‐Catalyzed Regio‐ and Stereoselective 1,2‐Diheteroarylation of Allenes: Combination of C−H Activation and Smiles Rearrangement

Heteroarenes are important structural motif in functional molecules. A Mn^I‐catalyzed 1,2‐diheteroarylation of allenes via a C−H activation/Smiles rearrangement cascade is presented. The reaction occurred under additive‐free or even solvent‐free conditions, which allowed the creation of two C−C and one C−N bonds in a single operation. A series of structurally diverse bicyclic or tricyclic compounds bearing an exocyclic double bond were constructed in good to excellent efficiency. The decarboxylative ring‐opening of the products led to the facile synthesis of vicinal biheteroaryls. Synthetic applications were demonstrated and preliminary mechanistic studies were conducted.     

Rhodium‐Catalyzed meta‐C−H Functionalization of Arenes

Rhodium‐catalyzed ortho ‐C−H functionalization is well known in the literature. Described herein is the Xphos‐supported rhodium catalysis of meta ‐C−H olefination of benzylsulfonic acid and phenyl acetic acid frameworks with the assistance of a para ‐methoxy‐substituted cyano phenol as the directing group. Complete mono‐selectivity is observed for both scaffolds. A wide range of olefins and functional groups attached to arene are tolerated in this protocol.     

Ligand‐Enabled Auxiliary‐Free meta‐C−H Arylation of Phenylacetic Acids

The meta ‐C−H arylation of free phenylacetic acid was realized using 2‐carbomethoxynorbornene (NBE‐CO₂Me) as a transient mediator. Both the modified norbornene and the mono‐protected 3‐amino‐2‐hydroxypyridine type ligand are crucial for this auxiliary‐free meta ‐C−H arylation reaction. A series of phenylacetic acids, including mandelic acid and phenylglycine, react smoothly with various aryl iodides to provide the meta ‐arylated products in high yields.     

Synthesis of Substituted 2‐Amino‐1,3‐oxazoles via Copper‐Catalyzed Oxidative Cyclization of Enamines and N,N‐Dialkyl Formamides

A facile synthesis of 2‐amino‐1,3‐oxazoles via Cu^I‐catalyzed oxidative cyclization of enamines and N ,N ‐dialkyl formamides has been developed. The reaction proceeds through an oxidative C−N bond formation, followed by an intramolecular C(sp²)−H bond functionalization/C−O cyclization in one pot. This protocol provides direct access to useful 2‐amino‐1,3‐oxazoles and features protecting‐group‐free nitrogen sources, readily available starting materials, a broad substrate scope and mild reaction conditions.     

Divergent Synthesis of Heteroatom‐Centered 4,8,12‐Triazatriangulenes

The increasing attention devoted to triangulenes and their heteroatom derivatives inspired us to explore a divergent synthesis of heteroatom‐centered 4,8,12‐triazatriangulenes, which involved the preparation of a nitrogen‐containing macrocyclic precursor and subsequent central heteroatom introduction by electrophilic C−Li and C−H substitution. The boron‐centered triangulene has a planar structure unlike the bowl‐shaped phosphorus‐ and silicon‐centered triangulenes. The described synthetic procedure can be used to fabricate a broad range of attractive functional materials, for example, for organic light‐emitting diodes, based on heteroatom‐centered triangulenes.