Well-Defined Ruthenium(II) Carboxylate as Catalyst for Direct C–H/C–O Bond Arylations with Phenols in Water

Cobalt-catalyzed reductive cross-coupling: a review

Transition-metal-catalyzed reductive cross-coupling is highly efficient for forming C-C bonds. It earns its limelight from its application by coupling unreactive electrophilic substrates to synthesize a variety of carbon-carbon bonds with various hybridizations (sp, sp2, and sp3), late-stage functionalization, and bioactive molecules' synthesis. Reductive cross-coupling is challenging to bring selectivity but promising approach. Cobalt is comparatively more affordable than other highly efficient metals e.g., palladium and nickel but cobalt catalysis is still facing efficacy challenges. Researchers are trying to harness the maximum out of cobalt's catalytic properties. Shortly, with efficiency achieved combined with the affordability of cobalt, it will revolutionize industrial applications. This review gives insight into the core of cobalt-catalyzed reductive cross-coupling reactions with a variety of substrates forming a range of differently hybridized coupled products.

Ubiquitous Role of Phosphine-Based Water-Soluble Ligand in Promoting Catalytic Reactions in Water

Catalysis has been at the forefront of the developments that has revolutionised synthesis and provided the impetus in the discovery of platform technologies for efficient C-C or C-X bond formation. Current environmental situation however, demands a change in strategy with catalysis being promoted more in solvents that are benign (Water) and for that the development of hydrophilic ligands (especially phosphines) is a necessity which could promote catalytic reactions in water, allow recyclability of the catalytic solutions and make it possible to isolate products using column-free techniques that involve lesser usage of hazardous organic solvents. In this review, we therefore critically analyse such catalytic processes providing examples that do follow the above-mentioned parameter.

Direct Synthesis of Biphenyl-2-carbonitriles by Rh(III)-Catalyzed C-H Hiyama Cross-Coupling in Water

This method represents an efficient rhodium(III)-catalyzed o-C-H arylation of readily available benzimidate derivatives with diverse arylsilanes in water as a sustainable solvent, enabling the straightforward synthesis of potentially useful biphenyl-2-carbonitrile derivatives. This silicon-based protocol employs benzimidates as both an efficacious directing group and the source of a nitrile group.

Cobalt-Catalyzed Enantiospecific Dynamic Kinetic Cross-Electrophile Vinylation of Allylic Alcohols with Vinyl Triflates

Asymmetric cross-electrophile coupling has emerged as a promising tool for producing chiral molecules; however, the potential of this chemistry with metals other than nickel remains unknown. Herein, we report a cobalt-catalyzed enantiospecific vinylation reaction of allylic alcohol with vinyl triflates. This work establishes a new method for the synthesis of enantioenriched 1,4-dienes. The reaction proceeds through a dynamic kinetic coupling approach, which not only allows for direct functionalization of allylic alcohols but also is essential to achieve high chemoselectivity. The use of cobalt enables the reactions to proceed with high enantiospecificity, which have failed to be realized by nickel catalysts.

Metal-catalysed C-Het (F, O, S, N) and C-C bond arylation

The formation of C-aryl bonds has been the focus of intensive research over the last decades for the construction of complex molecules from simple, readily available feedstocks. Traditionally, these strategies involve the coupling of organohalides (I, Br, Cl) with organometallic reagents (Mg, Zn, B, Si, Sn,…) such as Kumada-Corriu, Negishi, Suzuki-Miyaura, Hiyama and Sonogashira cross-couplings. More recently, alternative methods have provided access to these products by reactions with less reactive C-Het (F, O, S, N) and C-C bonds. Compared to traditional methods, the direct cleavage and arylation of these chemical bonds, the essential link in accessible feedstocks, has become increasingly important from the viewpoint of step-economy and functional-group compatibility. This comprehensive review aims to outline the development and advances of this topic, which was organized into (1) C-F bond arylation, (2) C-O bond arylation, (3) C-S bond arylation, (4) C-N bond arylation, and (5) C-C bond arylation. Substantial attention has been paid to the strategies and mechanistic investigations. We hope that this review can trigger chemists to discover more efficient methodologies to access arylation products by cleavage of these C-Het and C-C bonds.

Fe-Catalysed Coupling Reactions Between Alkynes and Alcohols

In this review, we discussed about the synthetic developments in the field of Fe-catalysis for the formation of - bonds through the coupling of alkynes and alcohols, for the period of 13 years (2008-2020). These strategies fulfil important Green Chemistry principles, as they are highly atom economic (up to 100 %), no toxic by-products (only water), employs highly abundant and low toxic alcohols (no need for any pre-functionalization hence step economy) and cheaper Fe-catalysts. Having these advantages, one can predict that in the coming years a large number of fascinating new iron-catalyzed reactions will be developed for the organic synthesis. We hope that this review article will be highly useful for the synthetic community to design and develop new Fe-catalyzed coupling reactions and keeps the content in the right prospect.

Preparation of Neutral trans - cis [Ru(O2CR)2P2(NN)], Cationic [Ru(O2CR)P2(NN)](O2CR) and Pincer [Ru(O2CR)(CNN)P2] (P = PPh3, P2 = diphosphine) Carboxylate Complexes and their Application in the Catalytic Carbonyl Compounds Reduction

The diacetate complexes trans-[Ru(κ¹-OAc)₂(PPh₃)₂(NN)] (NN = ethylenediamine (en) (1), 2-(aminomethyl)pyridine (ampy) (2), 2-(aminomethyl)pyrimidine (ampyrim) (3)) have been isolated in 76-88% yield by reaction of [Ru(κ²-OAc)₂(PPh₃)₂] with the corresponding nitrogen ligands. The ampy-type derivatives 2 and 3 undergo isomerization to the thermodynamically most stable cationic complexes [Ru(κ¹-OAc)(PPh₃)₂(NN)]OAc (2a and 3a) and cis-[Ru(κ¹-OAc)₂(PPh₃)₂(NN)] (2b and 3b) in methanol at RT. The trans-[Ru(κ¹-OAc)₂(P₂)₂] (P₂ = dppm (4), dppe (5)) compounds have been synthesized from [Ru(κ²-OAc)₂(PPh₃)₂] by reaction with the suitable diphosphine in toluene at 95 °C. The complex cis-[Ru(κ¹-OAc)₂(dppm)(ampy)](6) has been obtained from [Ru(κ²-OAc)₂(PPh₃)₂] and dppm in toluene at reflux and reaction with ampy. The derivatives trans-[Ru(κ¹-OAc)₂P₂(NN)] (7-16; NN = en, ampy, ampyrim, 8-aminoquinoline; P₂ = dppp, dppb, dppf, (R)-BINAP) can be easily synthesized from [Ru(κ²-OAc)₂(PPh₃)₂] with a diphosphine and treatment with the NN ligands at RT. Alternatively these compounds have been prepared from trans-[Ru(OAc)₂(PPh₃)₂(NN)] by reaction with the diphosphine in MEK at 50 °C. The use of (R)-BINAP affords trans-[Ru(κ¹-OAc)₂((R)-BINAP)(NN)] (NN = ampy (11), ampyrim (15)) isolated as single stereoisomers. Treatment of the ampy-type complexes 8-15 with methanol at RT leads to isomerization to the cationic derivatives [Ru(κ²-OAc)P₂(NN)]OAc (8a-15a; NN = ampy, ampyrim; P₂ = dppp, dppb, dppf, (R)-BINAP). Similarly to 2, the dipivalate trans-[Ru(κ¹-OPiv)₂(PPh₃)₂(ampy)] (18) is prepared from [Ru(κ²-OPiv)₂(PPh₃)₂] (17) and ampy in CHCl₃. The pincer acetate [Ru(κ¹-OAc)(CNN^OMe)(PPh₃)₂] (19) has been synthesized from [Ru(κ²-OAc)₂(PPh₃)₂] and HCNN^OMe ligand in 2-propanol with NEt₃ at reflux. In addition, the dppb pincer complexes [Ru(κ¹-OAc)(CNN)(dppb)] (CNN = AMTP (20), AMBQ^Ph (21)) have been obtained from [Ru(κ²-OAc)₂(PPh₃)₂], dppb, and HAMTP or HAMBQ^Ph with NEt₃, respectively. The acetate NN and pincer complexes are active in transfer hydrogenation with 2-propanol and hydrogenation with H₂ of carbonyl compounds at S/C values of up to 10000 and with TOF values of up to 160000 h^-1.

Effect of Cp-Ligand Methylation on Rhodium(III)-Catalyzed Annulations of Aromatic Carboxylic Acids with Alkynes: Synthesis of Isocoumarins and PAHs for Organic Light-Emitting Devices

An efficient protocol was developed for the synthesis of π-extended isocoumarins and polycyclic aromatic hydrocarbons based on the oxidative coupling of aromatic carboxylic acids with internal alkynes catalyzed by (cyclopentadienyl)rhodium complexes. The coupling chemoselectivity strongly depends on whether Cp or the methylated Cp* ligands are used. The pentamethyl derivative [Cp*RhCl₂ ]₂ predominantly gives isocoumarins, while the non-methylated complex [CpRhI₂ ]_n produces naphthalene derivatives. The polyaromatic carboxylic acids (such as 1-naphthoic acid, 1-pyrenecarboxylic acid, fluorene-1-carboxylic acid, and dibenzofuran-4-carboxylic acid) are suitable for this approach. A mixture of Cp*H/RhCl₃ can be used as a catalyst instead of [Cp*RhCl₂ ]₂ . The structures of 3,4-diphenylindeno[1,2-h]isochromen-1(11H)-one and 7,10-dimethyl-8,9-diphenylbenzo[pqr]tetraphene were determined by X-ray diffraction. In addition, the optical properties of the prepared compounds were studied. 7,8-Diphenyl-10H-phenaleno[1,9-gh]isochromen-10-one was employed as an emissive layer for OLED manufacturing. The OLED emits yellow-green light with a maximum intensity 1740 cd ⋅ m^-2 at 15 V.

The ruthenium(ii)-catalyzed C–H olefination of indoles with alkynes: the facile construction of tetrasubstituted alkenes under aqueous conditions

An environmentally-friendly and facile protocol for the construction of tetrasubstituted alkenes has been established with Ru(ii)-catalyzed C–H bond functionalizations under mild conditions.

Solvent-free ruthenium-catalysed triflate coupling as a convenient method for selective azole-o-C-H monoarylation

Metal-catalysed ortho-directed C-H functionalization usually faces selectivity issues in the competition between mono- and disubstitution processes. We report herein the ruthenium-catalysed N-directed C-H monoarylation of arylpyrazoles with a selectivity of up to 96% or that generally reaches values above 80%. This selectivity is an effect of solvent-free conditions associated with sulfonate reagents, in the absence of frequently used acidic additives.

para-Selective C-H Olefination of Aniline Derivatives via Pd/S,O-Ligand Catalysis

Herein we report a highly para-selective C-H olefination of aniline derivatives by a Pd/S,O-ligand-based catalyst. The reaction proceeds under mild reaction conditions with high efficiency and broad substrate scope, including mono-, di-, and trisubstituted tertiary, secondary, and primary anilines. The S,O-ligand is responsible for the dramatic improvements in substrate scope and the high para-selectivity observed. This methodology is operationally simple, scalable, and can be performed under aerobic conditions.

Micellar Catalysis for Ruthenium(II)-Catalyzed C-H Arylation: Weak-Coordination-Enabled C-H Activation in H2 O

Chemoselective C-H arylations were accomplished through micellar catalysis by a versatile single-component ruthenium catalyst. The strategy provided expedient access to C-H-arylated ferrocenes with wide functional-group tolerance and ample scope through weak chelation assistance. The sustainability of the C-H arylation was demonstrated by outstanding atom-economy and recycling studies. Detailed computational studies provided support for a facile C-H activation through thioketone assistance.

Recent Advances in C–H Bond Functionalization with Ruthenium-Based Catalysts

The past decades have witnessed rapid development in organic synthesis via catalysis, particularly the reactions through C–H bond functionalization. Transition metals such as Pd, Rh and Ru constitute a crucial catalyst in these C–H bond functionalization reactions. This process is highly attractive not only because it saves reaction time and reduces waste,but also, more importantly, it allows the reaction to be performed in a highly region specific manner. Indeed, several organic compounds could be readily accessed via C–H bond functionalization with transition metals. In the recent past, tremendous progress has been made on C–H bond functionalization via ruthenium catalysis, including less expensive but more stable ruthenium(II) catalysts. The ruthenium-catalysed C–H bond functionalization, viz. arylation, alkenylation, annulation, oxygenation, and halogenation involving C–C, C–O, C–N, and C–X bond forming reactions, has been described and presented in numerous reviews. This review discusses the recent development of C–H bond functionalization with various ruthenium-based catalysts. The first section of the review presents arylation reactions covering arylation directed by N–Heteroaryl groups, oxidative arylation, dehydrative arylation and arylation involving decarboxylative and sp3-C–H bond functionalization. Subsequently, the ruthenium-catalysed alkenylation, alkylation, allylation including oxidative alkenylation and meta-selective C–H bond alkylation has been presented. Finally, the oxidative annulation of various arenes with alkynes involving C–H/O–H or C–H/N–H bond cleavage reactions has been discussed.

Reactions in Water - A Greener Approach Using Ruthenium Catalysts

Reactions using transition metals as catalysts have emerged as an efficient method in the recent times. However, the selection of solvent plays a crucial role in this regard. Several solvents used traditionally suffer majorly with problems of toxicity; high boiling point etc. leading to drastic reaction conditions. Water being a non-toxic, non-inflammable and environmentally benign can replace the hazardous organic solvents in laboratory as well as industry. Maintaining a minimum catalyst loading percentage we can advantageously avail high levels of selectivity. Water was found to be a good solvent medium for several metal catalysed reactions. An intramolecular deprotonation mechanism is followed by the ruthenium (II) catalysts in water; thereby, facilitating the catalytic action of the metal. These studies can help the industrial chemists to utilize water as a solvent for their reactions towards improvement of their waste management procedure. This review mainly focuses on the several recent developments in the above direction.

Ruthenium(II)-Catalyzed Microwave-Promoted Multiple C-H Activation in Synthesis of Hexa(heteroaryl)benzenes in Water

A series of hexa(heteroaryl)benzenes were synthesized by the Ru(II)-carboxylate-catalyzed multiple C-H activation of benzenes carrying pyridyl, pyrimidyl, or pyrazolyl directing groups using N-heteroaryl bromides as coupling partners. The reactions proceeded with high selectivity under microwave irradiation in water. Iterative penta-arylation could be implemented via activation of C-H bonds of generated intermediates by cascade chelation assistance of in situ installed pyridyl groups. This strategy provides multidentate ligands for selective complexation of transition metals and potential building of photoredox systems.

Synthesis of Unsymmetrical 2,6-Diarylanilines by Palladium-Catalyzed C-H Bond Functionalization Methodology

3,5-Dimethylpyrazole was employed as a monodentate directing group for palladium-catalyzed ortho-sp² C-H arylation with aryl iodides. The reaction shows good functional group tolerance and outstanding selectivity for mono- ortho-arylation. Ozonolysis of ortho-arylated arylpyrazoles gave acylated biphenylamines that were further arylated to afford unsymmetrically substituted 2,6-diarylacetanilides.

Ruthenium(II)-Catalyzed meta C-H Mono- and Difluoromethylations by Phosphine/Carboxylate Cooperation

Ruthenium(II)-catalyzed meta-selective C-H (di)fluoromethylation was accomplished by phosphine and carboxylate cooperation. The remote C-H functionalization was characterized by ample substrate scope, thereby setting the stage for meta-(di)fluoromethylation through facile C-H cleavage.

Bioorthogonal Diversification of Peptides through Selective Ruthenium(II)-Catalyzed C-H Activation

Methods for the chemoselective modification of amino acids and peptides are powerful techniques in biomolecular chemistry. Among other applications, they enable the total synthesis of artificial peptides. In recent years, significant momentum has been gained by exploiting palladium-catalyzed cross-coupling for peptide modification. Despite major advances, the prefunctionalization elements on the coupling partners translate into undesired byproduct formation and lengthy synthetic operations. In sharp contrast, we herein illustrate the unprecedented use of versatile ruthenium(II)carboxylate catalysis for the step-economical late-stage diversification of α- and β-amino acids, as well as peptides, through chemo-selective C-H arylation under racemization-free reaction conditions. The ligand-accelerated C-H activation strategy proved water-tolerant and set the stage for direct fluorescence labelling as well as various modes of peptide ligation with excellent levels of positional selectivity in a bioorthogonal fashion. The synthetic utility of our approach is further demonstrated by twofold C-H arylations for the complexity-increasing assembly of artificial peptides within a multicatalytic C-H activation manifold.

Combination of RuCl3·xH2O with PEG – a simple and recyclable catalytic system for direct arylation of heteroarenes via C–H bond activation

A simple and recyclable catalytic system for direct arylation of heteroarenes via C–H bond activation was developed with a relatively inexpensive RuCl₃·xH₂O as a catalyst and PEG-400 as a green medium without any additive or ligand.

Polyureas derived from CO2 and diamines: highly efficient catalysts for C–H arylation of benzene

Heterogeneous polyureas were effective for C–H arylation of benzene in the presence of t-BuOK, producing biaryl products in high yields up to 97%.