Recent development in transition metal-catalysed C-H olefination

Transition metal-catalysed functionalizations of inert C-H bonds to construct C-C bonds represent an ideal route in the synthesis of valuable organic molecules. Fine tuning of directing groups, catalysts and ligands has played a crucial role in selective C-H bond (sp2 or sp3) activation. Recent developments in these areas have assured a high level of regioselectivity in C-H olefination reactions. In this review, we have summarized the recent progress in the oxidative olefination of sp2 and sp3 C-H bonds with special emphasis on distal, atroposelective, non-directed sp2 and directed sp3 C-H olefination. The scope, limitation, and mechanism of various transition metal-catalysed olefination reactions have been described briefly.

Ruthenium (II) Catalyzed C(sp2 )-H Bond Alkenylation of 2-Arylbenzo[d]oxazole and 2-Arylbenzo[d]thiazole with Unactivated Olefins

Functionalization of the bio-relevant heterocycles 2-arylbenzo[d]oxazole and 2-arylbenzo[d]thiazole has been achieved through Ru(II)-catalyzed alkenylation with unactivated olefins leading to selective formation of the mono-alkenylated products. This approach has a broad substrate scope with respect to the coupling partners, affords high yields, and works for gram scale synthesis using a readily available Ru-based catalyst. Mechanistic studies reveal a C-H activation pathway for the dehydrogenative coupling leading to the alkenylation. However, the results of the ESI-MS-guided deuterium kinetic isotope effect studies indicate that the C-H activation stage may not be the rate-determining step of the reaction. The use of a radical scavenging agent such as TEMPO did not show any detrimental effect on the reaction outcome, eliminating the possibility of the involvement of a free-radical pathway.

Alkenes as hydrogen trappers to control the regio-selective ruthenium(ii) catalyzed ortho C–H silylation of amides and anilides

A convenient and practical pathway to versatile silylated amides and anilides is described via efficient and selective ruthenium(ii) catalyzed ortho C–H silylation with different alkenes as the hydrogen acceptors.

Green strategies for transition metal-catalyzed C–H activation in molecular syntheses

Sustainable strategies for the activation of inert C–H bonds towards improved resource-economy.

Regioselective C-H Alkenylation and Unsymmetrical Bis-olefination of Heteroarene Carboxylic Acids with Ruthenium Catalysis in Water

An efficient weak carboxylate-assisted oxidative cross-dehydrogenative C-H/C-H coupling (CDC) of heteroarenes with readily available olefins has been devised employing water as green solvent under ruthenium(II) catalysis. The reaction is operationally simple, accommodates a large variety of heteroaromatic carboxylic acids as well as olefins, and facilitates a diverse array of high-value olefin-tethered heteroarenes in high yields (up to 87%). The potential of this ortho-C-H bond activation strategy has also been exploited toward tunable synthesis of densely functionalized heteroarenes through challenging unsymmetrical bis-olefination process in a one-pot sequential fashion. Mechanistic investigation demonstrates a reversible ruthenation process and C-H metalation step might not be involved in the rate-determining step.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Cross-Dehydrogenative Coupling/Annulation of Arene Carboxylic Acids and Alkenes in Water with Ruthenium(II) Catalyst and Air

A cross-dehydrogenative coupling of arene carboxylic acids with olefins is reported with ruthenium(II) catalyst employing air and water as green oxidant and solvent, respectively. It offers a robust synthesis of valuable phthalide molecules. A one-pot sequential strategy is also disclosed to access Heck-type products that are apparently difficult to make directly from arene carboxylic acids.

Rhodium(III)-Catalyzed Oxidative ortho-Olefination of Phenyl Carbamates with Alkenes: Elucidation of Acceleration Mechanisms by Using an Unsubstituted Cyclopentadienyl Ligand

It has been established that an unsubstituted cyclopentadienyl (Cp) Rh(III) complex is an effective catalyst for the oxidative ortho-olefination of phenyl carbamates with both acrylates and styrenes under mild conditions. In addition, diolefination of a protected BINOL (1,1'-binaphthalene-2,2'-diol) proceeded in high yields and disubstituted acrylates could participate in this catalysis. Experimental and theoretical mechanistic studies elucidated that an electron-deficient nature of the unsubstituted CpRh(III) complex accelerates both the electrophilic aryl C-H rhodation and the rate-limiting alkene insertion steps.

Rhodium-Catalyzed Electrooxidative C-H Olefination of Benzamides

Metal-catalyzed chelation-assisted C-H olefinations have emerged as powerful tools for the construction of functionalized alkenes. Herein, we describe the rhoda-electrocatalyzed C-H activation/alkenylation of arenes. The olefinations of challenging electron-poor benzamides were thus accomplished in a fully dehydrogenative fashion under electrochemical conditions, avoiding stoichiometric chemical oxidants, and with H2 as the only byproduct. This versatile alkenylation reaction also features broad substrate scope and used electricity as a green oxidant.

C7-Indole Amidations and Alkenylations by Ruthenium(II) Catalysis

C7-H-functionalized indoles are ubiquitous structural units of biological and pharmaceutical compounds for numerous antiviral agents against SARS-CoV or HIV-1. Thus, achieving site-selective functionalizations of the C7-H position of indoles, while discriminating among other bonds, is in high demand. Herein, we disclose site-selective C7-H activations of indoles by ruthenium(II) biscarboxylate catalysis under mild conditions. Base-assisted internal electrophilic-type substitution C-H ruthenation by weak O-coordination enabled the C7-H functionalization of indoles and offered a broad scope, including C-N and C-C bond formation. The versatile ruthenium-catalyzed C7-H activations were characterized by gram-scale syntheses and the traceless removal of the directing group, thus providing easy access to pharmaceutically relevant scaffolds. Detailed mechanistic studies through spectroscopic and spectrometric analyses shed light on the unique nature of the robust ruthenium catalysis for the functionalization of the C7-H position of indoles.

Cu-Catalyzed C-H Alkenylation of Benzoic Acid and Acrylic Acid Derivatives with Vinyl Boronates

An efficient Cu-catalyzed C-H alkenylation with acyclic and cyclic vinyl boronates was realized for the first time under mild conditions. The scope of the vinyl borons and the compatibility with functional groups including heterocycles are superior than Pd-catalyzed C-H coupling with vinyl borons, providing a reliable access to multisubstituted alkenes and dienes. Subsequent hydrogenation of the product from the internal vinyl borons will lead to installation of secondary alkyls.

Ruthenium(II)-Catalyzed Regioselective Ortho C-H Allenylation of Electron-Rich Aniline and Phenol Derivatives

Ortho C-H allenylation of electron-rich benzene derivatives with propargylic alcohol derivatives has been a challenge, due to their great innate tendency toward a para C-H allenylation via an SN₂'-type substitution process. Here, we described a Ru(II)-catalyzed regioselective ortho C-H allenylation of electron-rich aniline and phenol derivatives, which allows the previously challenging synthesis of a broad range of ortho allenylated aniline and phenol derivatives. More significantly, highly optically active fully substituted allenes can also be prepared with high enantiomeric excess via a highly efficient chirality transfer. No para C-H allenylation product was observed in the current catalytic system, thus showing a complete reversibility of the regioselectivity.

Nickela-electrocatalyzed C-H Alkoxylation with Secondary Alcohols: Oxidation-Induced Reductive Elimination at Nickel(III)

Nickela-electrooxidative C-H alkoxylations with challenging secondary alcohols were accomplished in a fully dehydrogenative fashion, thereby avoiding stoichiometric chemical oxidants, with H2 as the only stoichiometric byproduct. The nickela-electrocatalyzed oxygenation proved viable with various (hetero)arenes, including naturally occurring secondary alcohols, without racemization. Detailed mechanistic investigation, including DFT calculations and cyclovoltammetric studies of a well-defined C-H activated nickel(III) intermediate, suggest an oxidation-induced reductive elimination at nickel(III).

Direct oxidative coupling of N-acyl pyrroles with alkenes by ruthenium(ii)-catalyzed regioselective C2-alkenylation

Ruthenium(ii)-catalyzed oxidative coupling by C2-alkenylation of N-acyl pyrroles with alkenes has been described. The acyl unit was found to be an effective chelating group for the activation of aryl C-H bonds ortho to the directing group. The alkenylation reaction of benzoyl pyrroles occurred regioselectively at the C2-position of the pyrrole ring, without touching the benzene ring. The reaction provides exclusively monosubstituted pyrroles under the optimized conditions. Disubstituted pyrroles could be obtained using higher loadings of the ruthenium(ii)-catalyst and the additives.

Ruthenium(ii)/acetate catalyzed intermolecular dehydrogenative ortho C-H silylation of 2-aryl N-containing heterocycles

The first application of a RuHCl(CO)(PPh3)3-OAc catalytic system on the selective intermolecular mono C-H silylation of 2-aryl N-heterocycles using HSiEt3 as the silylating reagent has been described. This protocol features good functional group tolerance and high regioselectivity, and has potential for gram scale-up, which provides a convenient and practical pathway for the synthesis of versatile organosilane compounds. This catalytic system can also be applied to the silylation of challenging sp3 C-H bonds.

Rhodium(i)-catalyzed C6-selective C-H alkenylation and polyenylation of 2-pyridones with alkenyl and conjugated polyenyl carboxylic acids

A versatile Rh(i)-catalyzed C6-selective decarbonylative C-H alkenylation of 2-pyridones with readily available, and inexpensive alkenyl carboxylic acids has been developed. This directed dehydrogenative cross-coupling reaction affords 6-alkenylated 2-pyridones that would otherwise be difficult to access using conventional C-H functionalization protocols. The reaction occurs with high efficiency and is tolerant of a broad range of functional groups. A wide scope of alkenyl carboxylic acids, including challenging conjugated polyene carboxylic acids, are amenable to this transformation and no addition of external oxidant is required. Mechanistic studies revealed that (1) Boc2O acts as the activator for the in situ transformation of the carboxylic acids into anhydrides before oxidative addition by the Rh catalyst, (2) a decarbonylation step is involved in the catalytic cycle, and (3) the C-H bond cleavage is likely the turnover-limiting step.

An intermolecular C-H oxidizing strategy to access highly fused carbazole skeletons from simple naphthylamines

Highly π-extended hetero-cyclic/aromatic skeletons are of great importance as they can be utilized in many organic material based technologies. Therefore, developing efficient, pre-activation-free, synthetic procedures for the rapid build-up of these complex structures remains a high priority objective. The herein presented approach delivers highly fused carbazole skeletons from simple naphthylamine derivatives.

2-Amino-5,6-difluorophenyl-1H-pyrazole-Directed PdII Catalysis: Arylation of Unactivated β-C(sp3)-H Bonds

Palladium-catalyzed arylation of unactivated β-C(sp³)-H bonds in carboxylic acid derivatives with aryl iodides is described for the first time using 2-amino-5,6-difluorophenyl-1H-pyrazole as an efficient and readily removable directing group. Two fluoro groups are installed at the 5- and 6-position of the anilino moiety in 2-aminophenyl-1H-pyrazole, clearly enhancing the directing ability of the auxiliary. In addition, the protocol employs Cu(OAc)₂/Ag₃PO₄ (1.2/0.3) as additives, evidently reducing the stoichiometric amount of expensive silver salts. Furthermore, this process exhibits high β-site selectivity, compatibility with diverse substrates containing α-hydrogen atoms, and excellent functional group tolerance.