Autocatalysis for C-H bond activation by ruthenium(II) complexes in catalytic arylation of functional arenes

α-Halo carbonyls enable meta selective primary, secondary and tertiary C–H alkylations by ruthenium catalysis

A catalytic meta selective C–H alkylation of arenes is described using a wide range of α-halo carbonyls as coupling partners.

Introducing the 4-Phenyl-1,2,3-Triazole Moiety as a Versatile Scaffold for the Development of Cytotoxic Ruthenium(II) and Osmium(II) Arene Cyclometalates

Herein we report the synthesis, anticancer potency in vitro, biomolecule interaction, and preliminary mode of action studies of a series of cyclometalated 1,2,3-triazole-derived ruthenium(II) (2a-e) and osmium(II) (3a-e) organometallics of the general form [(η⁶-p-cym)RuCl(κ²-C^N-L)] with varying substituents in postion 1 of the 1,2,3-triazole moiety. These cyclometalates were characterized by standard analytical methods and their structures unambiguously assigned by single crystal X-ray crystallography. The anticancer activity of these novel compounds was tested in the human tumor cell lines A549 (non-small cell lung cancer), SW480 (colon adenocarcinoma), and CH1/PA-1 (ovarian teratocarcinoma), and preliminary structure-activity relationships were derived from the obtained data sets. Various representatives exhibit promising antineoplastic effects with IC₅₀ values down to the low micromolar range. The compounds readily formed stable DMSO adducts after aquation in DMSO-containing solution, but employing DMSO as solubilizer in cytotoxicity assays had no pronounced effect on the cytotoxicity, compared to analogous experiments with DMF for most compounds. We isolated and characterized selected DMSO adducts as triflate salts and found that they show activities in the same range as the parent chlorido metalacycles in MTT assays with the use of DMSO. Osmium(II) cyclometalates exhibited higher antiproliferative activities than their ruthenium(II) counterparts. The IC₅₀ values within each metal series decreased with increasing lipophilicity, which was attributed to higher cellular accumulation. Investigations on their mode of action revealed that the prepared organometallics were unable to inhibit topoisomerase IIα. Still, the most cytotoxic representatives 2b and 3b showed pronounced effects on cell cycle distribution.

Ru-Catalyzed One-Pot Diannulation of Heteroaryls: Direct Access to π-Conjugated Polycyclic Amides

A novel Ru-catalyzed oxidative double annulation of heteroarenes with symmetrical and unsymmetrical alkynes is reported. A general method for the unsymmetrical annulation of heteroarenes with two distinct alkynes is showcased for the first time. Methylphenyl sulfoximine (MPS) plays an important role in the annulations of heteroarenes and allows the construction of structurally complex π-conjugated heteroarene-fused polycyclic amide skeletons via the formation of multiple C-C and C-N bonds in a single operation. The reaction exhibits excellent substrate scope and tolerates a wide range of functional groups.

Ruthenium-Catalyzed C-H Arylation of Diverse Aryl Carboxylic Acids with Aryl and Heteroaryl Halides

Ruthenium ligated to tricyclohexylphosphine or di-tert-butylbipyridine catalyzes the arylation of carboxylic acids with diverse aryl halides (iodide, bromide, and triflate; aryl and heteroaryl). In addition, arylations with 2-iodophenol formed benzochromenones, carboxylate was shown to be a stronger donor than an amide, and the arylation of a pyridine carboxylate was demonstrated. Stoichiometric studies demonstrated that the added ligand is required for reaction with the electrophile but not the C–H bond.

Catalytic meta-selective C-H functionalization to construct quaternary carbon centres

A catalytic meta-selective C-H functionalization of 2-phenylpyridines using a range of tertiary halides is described. The protocol is simple to perform and uses commercially available reagents to construct challenging quaternary carbon centres in a regioselective manner. Preliminary studies suggest the C-H functionalization proceeds through a radical process directed via a remote σ-activation.

Ru-Catalyzed C-H Arylation of Fluoroarenes with Aryl Halides

Although the ruthenium-catalyzed C–H arylation of arenes bearing directing groups with haloarenes is well-known, this process has never been achieved in the absence of directing groups. We report the first example of such a process and show that unexpectedly the reaction only takes place in the presence of catalytic amounts of a benzoic acid. Furthermore, contrary to other transition metals, the arylation site selectivity is governed by both electronic and steric factors. Stoichiometric and NMR mechanistic studies support a catalytic cycle that involves a well-defined η⁶-arene-ligand-free Ru(II) catalyst. Indeed, upon initial pivalate-assisted C–H activation, the aryl-Ru(II) intermediate generated is able to react with an aryl bromide coupling partner only in the presence of a benzoate additive. In contrast, directing-group-containing substrates (such as 2-phenylpyridine) do not require a benzoate additive. Deuterium labeling and kinetic isotope effect experiments indicate that C–H activation is both reversible and kinetically significant. Computational studies support a concerted metalation–deprotonation (CMD)-type ruthenation mode and shed light on the unusual arylation regioselectivity.

A diversity-oriented synthesis of bioactive benzanilides via a regioselective C(sp2)-H hydroxylation strategy

A diversity-oriented synthesis of bioactive benzanilides via C(sp2)-H hydroxylation has been studied. Different regioselectivity was observed with Ru(ii) and Pd(ii) catalysts. The reaction demonstrates excellent regioselectivity, good tolerance of functional groups, and high yields. A wide range of ortho-hydroxylated-benzanilides can be readily synthesized with excellent regioselectivity via this new synthetic strategy. Computational investigations revealed that the regioselectivity was controlled mainly by both steric and electronic factors. Steric effects determine the regioselective outcomes in the Ru-catalyzed reaction, while electronic effects are dominant in the Pd-catalyzed reaction.

Ruthenium-catalyzed direct arylations with aryl chlorides

Aryl chlorides are readily available at lower cost than the corresponding bromides and iodides, but are much more challenging as substrates in metal-catalyzed cross-couplings.

[RuCl2(η6-p-cymene)] complexes bearing phosphinous acid ligands: preparation, application in C–H bond functionalization and mechanistic investigations

A series of [RuCl₂(η⁶-p-cymene)] complexes bearing phosphinous acid (PA) ligands have been straightforwardly prepared from the dimer [RuCl₂(p-cymene)]₂ and secondary phosphine oxides (SPOs) and fully characterized.

A novel iridium/acid co-catalyzed transfer hydrogenative C(sp3)–H bond alkylation to access functionalized N-heteroaromatics

A transfer hydrogenative coupling strategy, enabling direct alkylation of C(sp³)–H bonds and atom-economic access to functionalized N-heteroaromatics, has been demonstrated.

Half-sandwich cycloruthenated complexes from aryloxazolines: synthesis, structures, and catalytic activities

The half-sandwich cycloruthenated complexes [Ru(p-cymene)LCl] show promising catalytic activity in nitroarene reduction.

Mechanistic insight into ruthenium catalysed meta-sulfonation of 2-phenylpyridine

The mechanism of the ruthenium catalysed meta-sulfonation has been studied and a novel cycloruthenated phenylpyridine complex tosylated at the para position to the metal has been structurally characterised.

An air-stable half-sandwich RuIIcomplex as an efficient catalyst for [3+2] annulation of 2-arylcyclo-2-enones with alkynes

An air-stable half-sandwich Ru^IIcomplex, [Ru(p-cymene)(MeCN)₃](SbF6)₂, catalyzed a [3+2] annulation reaction of 2-arylcyclo-2-enones with alkynesviaC(sp²)–H/alkene functionalization to construct synthetically challenging spiro carbocyclic indenes bearing a quaternary carbon.

Temperature-controlled redox-neutral ruthenium(ii)-catalyzed regioselective allylation of benzamides with allylic acetates

Temperature controlled regioselective synthesis of ortho allyl and vinyl benzamides under redox-free conditions was reported. Detailed mechanistic investigation of the C–H bond activation and isomerization reactions was carried out.

A potential role of a substrate as a base for the deprotonation pathway in Rh-catalysed C–H amination of heteroarenes: DFT insights

DFT studies suggest that basic substrates assist the C–H activation step in Rh-catalysed reactions and transport protons towards the protodemetallation step.

The amide C–N bond of isatins as the directing group and the internal oxidant in Ru-catalyzed C–H activation and annulation reactions: access to 8-amido isocoumarins

The amide C–N bond of isatins was used as the oxidizing directing group for 4C–H activation and annulation reactions with alkynes.

Acylsilanes in Iridium-Catalyzed Directed Amidation Reactions and Formation of Heterocycles via Siloxycarbenes

Exposing ortho-amido aroylsilanes to visible light or heat leads to cyclization reactions that provide N-heterocyclic compounds via siloxycarbenes as key intermediates. The previously unreported starting materials have been prepared by directed amidations of aromatic acylsilanes in the presence of an iridium catalyst followed by N-alkylation.

Crystal structure of aceto-nitrile-[η(6)-1-methyl-4-(1-methyl-eth-yl)benzene][1-(pyrimidin-2-yl)-3H-indol-1-ium-2-yl-κ(2) N,C]ruthenium(II) bis-(hexa-fluorido-anti-monate)

In the title compound, [Ru(C10H14)(C12H9N3)(CH3CN)][SbF6]2, the ruthenium(II) cation is η (6)-coordinated by the para-cymene ligand with a Ru-centroid(η (6)-benzene) distance of 1.746 (2) Å. Furthermore, ruthenium coordinations to the C and N atoms of the pyrimidyl indole ligand are found to be 1.986 (4) and 2.082 (3) Å, respectively. The typical piano-stool coordination environment is saturated with an aceto-nitrile solvent mol-ecule with a Ru-N distance of 2.044 (3) Å. The indolyl ligand is protonated at the C3 position with the N=C imine bond length appropriate to that of related 3H-indole-based complexes. In the crystal, the complex cation is linked to the SbF6 (-) ions through weak C-H⋯F hydrogen bonds.

Ruthenium-Catalyzed Oxidant-Free Allylation of Aromatic Ketoximes with Allylic Acetates at Room Temperature

Substituted aromatic ketoximes reacted efficiently with allylic acetates in the presence of {[RuCl2(p-cymene)]2} and AgSbF6 in 1,2-dichloroethane at ambient temperature, providing ortho-allyl aromatic ketoximes in a highly regioselective manner without an oxidant. In the reaction, the acetate group of allyl acetate acts as a base to activate the C-H bond of aromatics. Later, ortho-allyl aromatic ketoximes were converted into ortho-allyl aromatic ketones in the presence of HCl.

Monoselective o-C-H Functionalizations of Mandelic Acid and α-Phenylglycine

Pd-catalyzed C-H functionalization of mandelic acid and α-phenylglycine is reported. We have developed different protocols for the arylation, iodination, acetoxylation, and olefination of these substrates based on two different (Pd(II)/Pd(IV) and Pd(II)/Pd(0)) catalytic cycles. Four crucial features of these protocols are advantageous for practical applications. First, the α-hydroxyl and amino groups are protected with simple protecting groups such as acetates (Ac, Piv) and carbamates (Boc, Fmoc), respectively. Second, these protocols do not involve installation and removal of a directing group. Third, monoselectivity is accomplished. Fourth, no epimerization occurs at the vulnerable α-chiral centers.

Carboxylate-assisted C-H activation of phenylpyridines with copper, palladium and ruthenium: a mass spectrometry and DFT study

The transition state of metal carboxylate mediated C–H activation is associated with carbon–metal bond formation supported by electron-poor carboxylates.

The C–H activation of 2-phenylpyridine, catalyzed by copper(ii), palladium(ii) and ruthenium(ii) carboxylates, was studied in the gas phase. ESI-MS, infrared multiphoton dissociation spectroscopy and quantum chemical calculations were combined to investigate the intermediate species in the reaction. Collision induced dissociation (CID) experiments and DFT calculations allowed estimation of the energy required for this C–H activation step and the subsequent acetic acid loss. Hammett plots constructed from the CID experiments using different copper carboxylates as catalysts revealed that the use of stronger acids accelerates the C–H activation step. The reasoning can be traced from the associated transition structures that suggest a concerted mechanism and the key effect of the carbon–metal bond pre-formation. Carboxylates derived from stronger acids make the metal atom more electrophilic and therefore shift the reaction towards the formation of C–H activated products.

Transition-metal-catalyzed C-N bond forming reactions using organic azides as the nitrogen source: a journey for the mild and versatile C-H amination

Owing to the prevalence of nitrogen-containing compounds in functional materials, natural products and important pharmaceutical agents, chemists have actively searched for the development of efficient and selective methodologies allowing for the facile construction of carbon-nitrogen bonds. While metal-catalyzed C-N cross-coupling reactions have been established as one of the most general protocols for C-N bond formation, these methods require starting materials equipped with functional groups such as (hetero)aryl halides or their equivalents, thus generating stoichiometric amounts of halide salts as byproducts. To address this aspect, a transition-metal-catalyzed direct C-H amination approach has emerged as a step- and atom-economical alternative to the conventional C-N cross-coupling reactions. However, despite the significant recent advances in metal-mediated direct C-H amination reactions, most available procedures need harsh conditions requiring stoichiometric external oxidants. In this context, we were curious to see whether a transition-metal-catalyzed mild C-H amination protocol could be achieved using organic azides as the amino source. We envisaged that a dual role of organic azides as an environmentally benign amino source and also as an internal oxidant via N-N2 bond cleavage would be key to develop efficient C-H amination reactions employing azides. An additional advantage of this approach was anticipated: that a sole byproduct is molecular nitrogen (N2) under the perspective catalytic conditions. This Account mainly describes our research efforts on the development of rhodium- and iridium-catalyzed direct C-H amination reactions with organic azides. Under our initially optimized Rh(III)-catalyzed amination conditions, not only sulfonyl azides but also aryl- and alkyl azides could be utilized as facile amino sources in reaction with various types of C(sp(2))-H bonds bearing such directing groups as pyridine, amide, or ketoxime. More recently, a new catalyst system using Ir(III) species was developed for the direct C-H amidation of arenes and alkenes with acyl azides under exceptionally mild conditions. As a natural extension, amidation of primary C(sp(3))-H bonds could also be realized on the basis of the superior activity of the Cp*Ir(III) catalyst. Mechanistic investigations revealed that a catalytic cycle is operated mainly in three stages: (i) chelation-assisted metallacycle formation via C-H bond cleavage; (ii) C-N bond formation through the in situ generation of a metal-nitrenoid intermediate followed by the insertion of an imido moiety to the metal carbon bond; (iii) product release via protodemetalation with the concomitant catalyst regeneration. In addition, this Account also summarizes the recent advances in the ruthenium- and cobalt-catalyzed amination reactions using organic azides, developed by our own and other groups. Comparative studies on the relative performance of those catalytic systems are briefly described.