Cobalt-Catalyzed, Room-Temperature Addition of Aromatic Imines to Alkynes via Directed C–H Bond Activation

Cobalt-catalysed site-selective intra- and intermolecular dehydrogenative amination of unactivated sp(3) carbons

Cobalt-catalysed sp² C–H bond functionalization has attracted considerable attention in recent years because of the low cost of cobalt complexes and interesting modes of action in the process. In comparison, much less efforts have been devoted to the sp³ carbons. Here we report the cobalt-catalysed site-selective dehydrogenative cyclization of aliphatic amides via a C–H bond functionalization process on unactivated sp³ carbons with the assistance of a bidentate directing group. This method provides a straightforward synthesis of monocyclic and spiro β- or γ-lactams with good to excellent stereoselectivity and functional group tolerance. In addition, a new procedure has been developed to selectively remove the directing group, which enables the synthesis of free β- or γ-lactam compounds. Furthermore, the first cobalt-catalysed intermolecular dehydrogenative amination of unactivated sp³ carbons is also realized.

Functionalizing unactivated carbon–hydrogen bonds is challenging, especially when using non-precious metals and dealing with sp³ hybridized carbons. Here, the authors report an intramolecular cobalt catalysed amination of C–H bonds of sp³ carbons, giving access to β- and γ-lactams.

C-H activation/functionalization catalyzed by simple, well-defined low-valent cobalt complexes

A facile C-H activation and functionalization of aromatic imines is presented using low-valent cobalt catalysts. Using Co(PMe3)4 as catalyst we have developed an efficient and simple protocol for the C-H/hydroarylation of alkynes with an anti selectivity. Deuterium-labeling experiments, DFT calculations coupled with the use of a well-defined catalyst have for the first time shed light on the elusive black box of cobalt catalyzed C-H functionalization.

Merging C-H activation and alkene difunctionalization at room temperature: a palladium-catalyzed divergent synthesis of indoles and indolines

A palladium-catalyzed 1,2-carboamination through C-H activation at room temperature is reported for the synthesis of 2-arylindoles, and indolines from readily available, inexpensive aryl ureas and vinyl arenes. The reaction initiates with a urea-directed electrophilic ortho palladation, alkene insertion, and β-hydride elimination sequences to provide the Fujiwara-Moritani arylation product. Subsequently, aza-Wacker cyclization, and β-hydride elimination provide the 2-arylindoles in high yields. Intercepting the common σ-alkyl-Pd intermediate, corresponding indolines are also achieved. The indoline formation is attributed to the generation of stabilized, cationic π-benzyl-Pd species to suppress β-hydride elimination.

Cobalt(III)-catalyzed synthesis of indazoles and furans by C-H bond functionalization/addition/cyclization cascades

The development of operationally straightforward and cost-effective routes for the assembly of heterocycles from simple inputs is important for many scientific endeavors, including pharmaceutical, agrochemical, and materials research. In this article we describe the development of a new air-stable cationic Co(III) catalyst for convergent, one-step benchtop syntheses of N-aryl-2H-indazoles and furans by C-H bond additions to aldehydes followed by in situ cyclization and aromatization. Only a substoichiometric amount of AcOH is required as an additive that is both low-cost and convenient to handle. The syntheses of these heterocycles are the first examples of Co(III)-catalyzed additions to aldehydes, and reactions are demonstrated for a variety of aromatic, heteroaromatic, and aliphatic derivatives. The syntheses of both N-aryl-2H-indazoles and furans have been performed on 20 mmol scales and should be readily applicable to larger scales. The reported heterocycle syntheses also demonstrate the use of directing groups that have not previously been applied to Co(III)-catalyzed C-H bond functionalizations. Additionally, the synthesis of furans demonstrates the first example of Co(III)-catalyzed functionalization of alkenyl C-H bonds.

Recent advances in the ruthenium-catalyzed hydroarylation of alkynes with aromatics: synthesis of trisubstituted alkenes

The hydroarylation of alkynes with amide, azole, carbamate, phosphine oxide, amine, acetyl, sulfoxide and sulphur substituted aromatics in the presence of a ruthenium catalyst via chelation-assisted C–H bond activation is discussed.

Transition metal-catalyzed C–H bond functionalizations by the use of diverse directing groups

In this review, a summary of transition metal-catalyzed C–H activation by utilizing the functionalities as directing groups is presented.

Rhodium-catalyzed hydroarylation of alkynes via tetrazole-directed C–H activation

A rhodium-catalyzed hydroarylation of alkynes with aryl tetrazoles through tetrazole-directed C–H activation was developed.

Co(III)-catalyzed C-H activation/formal SN-type reactions: selective and efficient cyanation, halogenation, and allylation

The first cobalt-catalyzed cyanation, halogenation, and allylation via C-H activation have been realized. These formal SN-type reactions generate valuable (hetero)aryl/alkenyl nitriles, iodides, and bromides as well as allylated indoles using a bench-stable Co(III) catalyst. High regio- and mono-selectivity were achieved for these reactions. Additionally, allylation proceeded efficiently with a turnover number of 2200 at room temperature, which is unprecedented for this Co(III) catalyst. Alkenyl substrates and amides have been successfully utilized in Cp*Co(III)-catalyzed C-H activation for the first time.

Cobalt-catalyzed, aminoquinoline-directed C(sp²)-H bond alkenylation by alkynes

A method for cobalt-catalyzed, aminoquinoline- and picolinamide-directed C(sp(2))-H bond alkenylation by alkynes was developed. The method shows excellent functional-group tolerance and both internal and terminal alkynes are competent substrates for the coupling. The reaction employs a Co(OAc)2⋅4 H2O catalyst, Mn(OAc)2 co-catalyst, and oxygen (from air) as a terminal oxidant.

Room-temperature ortho-alkoxylation and -halogenation of N-tosylbenzamides by using palladium(II)-catalyzed C-H activation

The N-tosylcarboxamide group can direct the room-temperature palladium-catalyzed C-H alkoxylation and halogenation of substituted arenes in a simple and mild procedure. The room-temperature stoichiometric cyclopalladation of N-tosylbenzamide was first studied, and the ability of the palladacycle to react with oxidants to form C-X and C-O bonds under mild conditions was demonstrated. The reaction conditions were then adapted to promote room-temperature ortho-alkoxylations and ortho-halogenations of N-tosylbenzamides using palladium as catalyst. The scope and limitation of both alkoxylations and halogenations was studied and the subsequent functional transformation of the N-tosylcarboxamide group through nucleophilic additions was evaluated. This methodology offers a simple and mild route to diversely functionalized arenes.

Low-valent cobalt catalysis: new opportunities for C-H functionalization

Rapid progress in the fields of organometallic chemistry and homogeneous catalysis has made it possible for synthetic chemists to consider using ubiquitous yet unreactive C-H bonds as starting points to construct complex organic molecules. However, a majority of the C-H functionalization reactions currently in use require noble transition metal catalysts and harsh reaction conditions, so researchers have placed a priority on the development of mild and cost-effective catalysts. Given this situation, we wondered whether earth-abundant first-row transition metals could emulate the reactivity of a noble transition metal catalyst and carry out similar C-H functionalization reactions at a lower cost and under milder conditions. We also wondered whether we could use first-row transition metals to achieve hitherto unknown, but useful, C-H functionalization reactions. This Account summarizes our research on the development of three different types of C-H functionalization reactions using low-valent cobalt catalysts: (1) hydroarylation of alkynes and olefins, (2) ortho C-H functionalization with electrophiles, and (3) addition of arylzinc reagents to alkynes involving 1,4-cobalt migration. Although synthetic chemists have previously paid little attention to cobalt in designing catalytic C-H functionalization reactions, earlier studies, particularly those on stoichiometric cyclometalation, inspired us as we developed the hydroarylation and ortho C-H functionalization reactions. In these transformations, we combined a cobalt precatalyst, a ligand (such as phosphine or N-heterocyclic carbene (NHC)), and Grignard reagent to generate low-valent cobalt catalysts. These novel catalysts promoted a series of pyridine- and imine-directed hydroarylation reactions of alkynes and olefins at mild temperatures. Notably, we observed branched-selective addition to styrenes, which highlights a distinct regioselectivity of the cobalt catalyst compared with typical rhodium and ruthenium catalysts. The combination of a cobalt-NHC catalyst and a Grignard reagent allows directed aromatic C-H functionalizations with electrophiles such as aldimines, aryl chlorides, and alkyl chlorides or bromides. This second reaction has a particularly broad scope, allowing us to introduce secondary alkyl groups at the ortho position of aryl imines, a difficult reaction to carry out by other means. Serendipitously, we found that a cobalt-Xantphos complex catalyzed the third type of C-H functionalization: the addition of an arylzinc reagent to an alkyne to afford ortho-alkenylarylzinc species through a 1,4-cobalt migration. This "migratory arylzincation" allowed us to quickly construct a diverse group of functionalized benzothiophenes and benzoselenophenes. Collectively, our studies of cobalt catalysis have provided cost-effective catalysts and milder conditions for existing C-H functionalizations and have led to some unprecedented, attractive chemical transformations.

Cobalt mediated C–H bond functionalization: emerging tools for organic synthesis

This review provides a perspective on C–H bond functionalization mediated by cobalt complexes used in either stoichiometric or catalytic amounts, without the contribution of any other transition metal, for organic synthesis applications.

Pd-Catalyzed [3+2] cycloaddition of ketoimines with alkynes via directed sp3 C–H bond activation

The palladium-catalyzed oxidative [3+2] cycloaddition of ketoimines with alkynes provided an efficient access to multisubstituted pyrroles via Csp³–H activation.

Rh(iii)-catalyzed regioselective hydroarylation of alkynes via directed C–H functionalization of pyridines

Rh(iii)-catalyzed C-3 selective alkenylation of pyridines via hydroarylation of alkynes has been developed. The reaction shows high regioselectivity, high yield and good functional group tolerance, providing a convenient strategy for the synthesis of trisubstituted alkenes.

Ruthenium-catalyzed highly regio- and stereoselective hydroarylation of aromatic sulfoxides with alkynes via C–H bond activation

A ruthenium-catalyzed hydroarylation of aromatic sulfoxides with alkynes in the presence of AgSbF₆ and pivalic acid yielding trisubstituted alkenes is described.

Oxidative addition and C–H activation chemistry with a PNP pincer-ligated cobalt complex

The bis(phosphino)pyridine (PNP) cobalt(i) methyl complex, (^iPrPNP)CoCH₃is a rich platform for the oxidative addition of non-polar reagents such as H₂, the C–H bonds of arenes and terminal alkynes.