Oxidative Addition Research Articles

Photosensitized Gold-Catalyzed Cross-Couplings of Aryl Bromides.

Recently, ligand-promoted Au(I)/Au(III)-catalyzed cross-coupling reactions with aryl iodides have garnered considerable attention. Here, we report the first visible-light-driven gold-catalyzed cross-couplings of challenging aryl bromides. In the presence of a (P, N)-gold(I) catalyst and an acridinium photocatalyst under blue LED irradiation, C-O coupling of aryl bromides with carboxylic acids was achieved, and soon it was found that this photoinduced gold-catalyzed cross-coupling of aryl bromides was appliable for other C-C, C-N, and C-S bond formation. Experimental and computational studies suggest that this visible-light-driven gold-catalyzed cross-couplings of aryl bromides involves two discrete photoinduced energy transfer (EnT) events: first, energy transfer (EnT) from a photosensitizer produces an excited-state gold(I) complex that allows the bottleneck oxidative addition of aryl bromides to form an aryl Au(III) complex and second, the reductive elimination of aryl-Au(III) complex to regenerate Au(I). Collectively, the new synergistic catalytic method developed here highlights the tremendous potential of photochemical gold catalysis via excited-state organogold complexes, as well as its potential to facilitate drug discovery due to the biocompatibility and mildness of the reaction conditions.

Using Common Alcohols to Transform N-H NHC-Palladium Complexes into Reactive Catalysts for the Suzuki-Miyaura Reaction with Aryl Chlorides.

We demonstrate that the generation of mixed NHC/phosphinite Pd catalysts from 2-phosphinoimidazole ligands using bulky alcohols provides highly active Pd catalysts for cross-coupling reactions. Mechanistic studies suggest that the rapid loss of bulkier phosphinites from the precatalyst leads to more rapid oxidative addition. These catalysts demonstrate a broad substrate scope in Suzuki-Miyaura reactions with aryl chlorides. Conversion of a methanol-derived catalyst into a tert-butanol-derived catalyst in situ also enables selective and sequential cross-coupling of bromochloroarenes.

DFT Study of the Mechanism of Selective Hydrogenation of Acetylene by Rhodium Single-Atom Catalyst Supported on HY Zeolite.

The selectivity of acetylene hydrogenation by the Rh single-atom catalyst (SAC) supported on HY zeolite was investigated using density functional theory (DFT) and a 5/83T quantum mechanics/molecular mechanics (QM/MM) embedded cluster model. The calculated activation barrier (ΔG‡) for the oxidative addition of dihydrogen to the Rh metal center (15.9 kcal/mol) is lower in energy than that for the σ-bond metathesis of dihydrogen to the Rh-C bond (22.7 kcal/mol) and the Rh-O bond (28.4 kcal/mol). The activation barriers of the oxidative addition of subsequent dihydrogen molecules are significantly higher than that of the oxidative addition of the first dihydrogen molecule. These findings align with the experimentally observed activity and selectivity of the atomically dispersed Rh catalyst supported on HY zeolite. Natural bond orbital (NBO), molecular orbital (MO) and fuzzy bond order analyses were used to examine the interaction between the Rh metal center and acetylene versus ethylene ligands. The occupancies of the Rh lone pairs, π-bonding and π*-antibonding orbitals of acetylene and ethylene are consistent with the expected stronger interaction between the Rh metal center and acetylene compared to ethylene on the HY zeolite support.

Theoretical investigation on Cu/Pd-catalyzed domino radical cyclization and C−H carbonylation of α-bromocarbonyl leading to carbonyl-containing quinolin-2(1H)‑one scaffold

Our DFT calculations provide the first theoretical investigation on Cu/Pd-catalyzed domino radical cyclization and C−H carbonylation of α-bromocarbonyl. The SET reduction was induced by CuI attacking C−Br of α-bromocarbonyl generating CuIIBr followed by intramolecular radical addition generating six-membered ring. The alkenyl bromide was formed assisted by CuIIBr together with CuI regeneration. Next, alkenyl PdII was giv-en via oxidative addition of Pd0 to C-Br. The five-membered cyclic PdII was obtained via intramolecular C−H activation. Finally, two paths are competitive to furnish six-membered acyl PdII via insertion of CO to C-Pd bond. The polycyclic carbonyl-containing quinolin-2(1H)-one was yielded via reductive elimination squeezing recovered Pd0 from six-membered ring. CO insertion is determined to be rate-limiting. The positive solvation effect is suggested by decreased absolute and activation energies in solution compared with in gas. These results are supported by Multiwfn analysis on FMO composition of specific TSs, and MBO value of vital bonding, breaking.

Hydride Rebound: A Frustrated Lewis Pair (FLP)-Type Cooperative Mechanism for H2 Activation by a Potassium Aluminyl Compound.

Combining experiment and theory, the mechanisms of H2 activation by the potassium-bridged aluminyl dimer K2[Al(NON)]2 (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tertbutyl-9,9-dimethylxanthene) and its monomeric K+-sequestered counterpart have been investigated. These systems show diverging reactivity towards the activation of dihydrogen, with the dimeric species undergoing formal oxidative addition of H2 at each Al centre under ambient conditions, and the monomer proving to be inert to dihydrogen addition. Noting that this K+ dependence is inconsistent with classical models of single-centre reactivity for carbene-like Al(I) species, we rationalize these observations instead by a cooperative frustrated Lewis pair (FLP)-type mechanism (for the dimer) in which the aluminium centre acts as the Lewis base and the K+ centres as Lewis acids. In contrast to previous theoretical work on this precise system by Schaefer and co-workers, the potassium ions are shown to play explicit roles in stabilizing a nascent 2-bridging hydride, formed by heterolytic H-H bond cleavage (with accompanying protonation of the aluminium-centred lone pair). K-to-Al hydride "rebound" into the vacant aluminium-centred p-orbital then completes the net addition of H2 via sequential H+/H- transfer. The experimentally determined kinetic isotope effect (kH/kD=2.6) reflects a high degree of bond activation in the transition state (as predicted quantum chemically).

The Reductive Addition–Oxidative Elimination Mechanism

AbstractThe oxidative addition–reductive elimination (OARE) mechanism of reactive molecules on metal atoms is a cornerstone of modern chemistry. However, the complementary reductive addition–oxidative elimination (RAOE) mechanism is barely considered, despite a first reduction reaction between metal atoms and the incoming organic reactant makes chemical sense in a plethora of processes. Here we show, in a chronological order, early precedents in the literature which indicated the possibility of a general RAOE mechanism, the few systems explicitly reported so far (including a catalytic system) and some other reactions where a RAOE mechanism would satisfactorily explain the mechanistic evidences found. These examples, together, strongly suggest that researchers should consider the RAOE mechanism during their investigations, and not simply adjust their conclusions to the omnipresent OARE mechanism. This new line of thinking might open new avenues in the design of chemical reactions, particularly catalytic ones.

A Comparative Study of CH and mCH Mechanisms in Hydrosilylation Reactions

The cyclic process of Si-H oxidation addition, olefin insertion, and reductive elimination was discussed by density functional theory (DFT) and high-precision quantum chemical calculations. All the calculations were performed at the B3LYP-D3/def2-TZVP level. Two fundamental mechanisms of Chalk-Harrod (CH) and modified Chalk-Harrod (mCH) were explored, in the perspective of computational chemistry. Pt(PH3)2 was used as model for the renowned Speier's catalyst, while HSiR (R=(CH3)3 and CH3(OSi(CH3)3)2 and CH2=CHR’ (R’=H, CH2OH and CH2OCH2CH2OH) were used as the reactant models. Significant findings include the identification of the olefin insertion as the rate-determining step in both mechanisms.The energy barrier of rate-determining step was 27.6 kcal/mol and 41.0 kcal/mol, respectively, according to the CH mechanism and the mCH mechanism, when HSi(CH3)3 and CH2=CH2 were used as the reactant models. But the barrier of rate-determining step become to 27.2 kcal/mol and 28.4 kcal/mol, when HSiCH3(OSi(CH3)3)2 and CH2=CHCH2OCH2CH2OH were used as the reactant models. The difference in energy barriers betweeen the CH and mCH mechanisms become closer as the reactant models increase. The reaction between polyhydrosiloxane and allyl polyether may very potentially follow the mCH mechanism rather than CH mechanism.

Influence of neodymium doping on radiation shielding properties of yttrium lead borotellurite glass system

The need for effective shielding materials has grown as radiation-based technologies have been more widely used. Glasses doped with rare earth elements like neodymium oxide, are promising candidates due to their enhanced radiation attenuation properties. Using Phy-X software, this research examines how neodymium oxide doping affects the shielding properties of yttrium lead borotellurite glass. X-ray diffraction confirmed the amorphous structure, while density measurements showed increased density with neodymium oxide addition. The Phy-X software calculated radiation shielding parameters for gamma energies from 10⁻³ to 10⁵ MeV. The results demonstrate that neodymium oxide improves gamma irradiation attenuation and enhances the glass system's radiation shielding capabilities.

Design, synthesis, anticancer evaluation and molecular docking studies of different aryl derivatives of azaindole-pyrimidine-1,3,4-oxadiazoles

The aryl-azaindole-pyrimidine-1,3,4-oxadiazole derivatives (12a–j) were synthesized by Suzuki coupling reaction between bromo-azaindole-1,3,4-oxadiaole intermediate 10 and various aryl boronic acids (11a–j) by using of Pd(dppf)Cl2 and K2CO3 in 1,4-dioxane/H2O. Here, the Suzuki coupling mechanism starts with the oxidative addition followed by transmetallation and ends with reductive elimination. These derivatives were screened in vitro anticancer applications against four human cancer cell lines including MCF-7, A549, Colo-205, and A2780 by employing of MTT method, the well-known chemotherapeutic agent as etoposide used as positive control. Among them, compound 12a bearing 3,4,5-trimethoxy substituent on the aryl moiety displayed good activity as compared with positive control against MCF-7, A549, Colo-205, and A2780 cell lines with IC50 values of 1.10 ± 0.84 µM, 1.07 ± 0.067 µM, 1.20 ± 0.95 µM, and 1.34 ± 0.66 µM respectively. Compounds 12a and 12b primarily engage in hydrophobic interactions such as pi-pi stacked, amide-pi stacked, pi-alkyl, and alkyl interactions. Specifically, nucleotides DG13, DA12, and arg503 display pi-pi stacked and amide-pi stacked interactions.

Enantioselective reductive cross-couplings to forge C(sp2)–C(sp3) bonds by merging electrochemistry with nickel catalysis

Motivated by the inherent benefits of synergistically combining electrochemical methodologies with nickel catalysis, we present here a Ni-catalyzed enantioselective electroreductive cross-coupling of benzyl chlorides with aryl halides, yielding chiral 1,1-diaryl compounds with good to excellent enantioselectivity. This catalytic reaction can not only be applied to aryl chlorides/bromides, which are challenging to access by other means, but also to benzyl chlorides containing silicon groups. Additionally, the absence of a sacrificial anode lays a foundation for scalability. The combination of cyclic voltammetry analysis with electrode potential studies suggests that NiI species activate aryl halides via oxidative addition and alkyl chlorides via single electron transfer.

Stable Nickel Oxidative Addition Complexes for the Versatile Elaboration of Bioactive Molecules

Stable Nickel Oxidative Addition Complexes for the Versatile Elaboration of Bioactive Molecules

Enantioselective Pd- and Cu-Catalyzed Hydrofunctionalization of Methylenecyclopropanes via a Distal C-C Bond Cleavage.

The enantioselective ring-opening reactions of methylenecyclopropanes (MCPs) involving C-C bond activation via oxidative addition of transition metals have been rarely reported. Here, we disclose a Pd/Cu-catalyzed enantio- and regioselective coupling between cyclic imino esters and MCPs to produce α-allylated 2H-pyrrole derivatives. In this reaction, azomethine ylide formed by a chiral copper catalyst with ketimine ester would serve as a nucleophile to react with activated MCPs via palladium catalysis. This bimetallic catalyst system exhibited a broad substrate scope and high regio- and enantioselectivities.

Sustainable Lightweight Concrete Designed with Modified Solidified Wastewater Sludge as Partial Replacement of Cement

The requirement for high-quality drinking water and the treatment of wastewater prior to discharge into the environment results in the generation of sludge. As with any high-volume materials, beneficial reuse applications are being sought to promote sustainable environmental solutions. This research examined the possibilities of producing sustainable lightweight concrete using modified solidified wastewater sludge as a partial replacement of cement. Wastewater sludge was modified by the addition of aluminum oxide and magnesium silicate hydrate. The properties of the modified wastewater sludge were examined, as well as the influence of the partial cement replacement with the sludge in lightweight concrete. Besides testing the physical and mechanical properties of four mortar mixtures, an additional analysis of the willingness of final users to accept novel material containing wastewater sludge was addressed. The results obtained for the mortar samples indicate that 20% cement replacement is the upper limit for the modified sludge’s application. The lightweight concrete prepared with the modified sludge (in the amount of 20%) was tested in a hardened state. The water permeability was reduced by 33.3% with the addition of the modified sludge. Both tested concrete mixtures showed good frost resistance. The maximal measured reduction in the compressive strengths was 7.6%. Citizens’ perceptions and responses regarding the beneficial reuse of materials emphasize the importance of comprehensive education for their future acceptance.

Intramolecular Oxidative Addition Triggered by One-Electron Oxidation of Molybdenum(iii) Tris(anilide): Generation of Molybdenum(v) Imido Aryl Bis(anilide) by Autocatalysis

Intramolecular Oxidative Addition Triggered by One-Electron Oxidation of Molybdenum(<scp>iii</scp>) Tris(anilide): Generation of Molybdenum(v) Imido Aryl Bis(anilide) by Autocatalysis

An open-shell Ir(II)/Ir(IV) redox couple outperforms an Ir(I)/Ir(III) pair in olefin isomerization.

Open-shell systems based on first-row transition metals and their involvement in various catalytic processes are well explored. By comparison, mononuclear open-shell complexes of precious transition metals remain underdeveloped. This is particularly true for IrII complexes, as there is very limited information available regarding their application in catalysis. Here we show that a family of IrII metalloradicals, featuring a C6H3-2,6-(OP(tBu)2)2 (POCOP) pincer ligand, effectively catalyses olefin isomerization-a key step in alkane metathesis-exhibiting up to 20 times higher activity than their IrI counterparts. Computational studies reveal that the IrII/IrIV redox cycling enables faster kinetics than the traditional IrI/IrIII pathway owing to reduced barriers for the oxidative addition and reductive elimination steps. Thus, this study presents a redox catalyst involving an IrII/IrIV pair, highlighting the capabilites of precious-metal systems that extend beyond traditional redox cycles. These findings emphasize the need for expanding catalytic design principles, especially for platinum-group metals.

C2-Selective Palladium-Catalyzed C-S Cross-Coupling of 2,4-Dihalopyrimidines.

Under most conditions, 2,4-dihalopyrimidines undergo substitution reactions at C4. Here we report that Pd(II) precatalysts supported by bulky N-heterocyclic carbene ligands uniquely effect C2-selective cross-coupling of 2,4-dichloropyrimidine with thiols. The regioselectivity of this reaction stands in stark contrast to ∼1500 previously reported Pd-catalyzed cross-couplings that favor C4 in the absence of other substituents on the pyrimidine ring. Selectivity in the catalytic system reported herein is extremely sensitive to the structure of the Pd(II) precatalyst, largely due to competing C4-selective nucleophilic aromatic substitution. C2-selectivity is high with most 1° thiols and thiophenols, and a range of substituted dichloropyrimidines can be used. The atypical selectivity of this transformation may facilitate diversity-oriented synthesis, as demonstrated for derivatives of an antiviral agent. Under these conditions, C2─Cl cleavage may not take place through a typical oxidative addition pathway.

Reactivity of a Stable and Highly Electron-Rich (η6-Diborabenzene)nickel(0) Synthon.

The reaction of the diborabenzene (DBB) nickel(0) pogo-stick complex [(η6-DBB)Ni(CO)] (II) with a large excess of [Ni(CO)4] yields the dark green, unstable dinickel(0) complex [(η6-DBB)Ni(μ-CO)Ni(CO)3] (1), which loses one CO ligand to yield the purple, bimetallic Ni02 half-sandwich complex [(η6-DBB)Ni2(μ-CO)(η1-CO)2] (2). The addition of the chromium aminoborylene complex [(OC)5Cr{BN(TMS)2}] (TMS = trimethylsilyl) to II does not result in the expected borylene transfer but in the formation of the black Ni0-Cr0 complex [(η6-DBB)Ni(μ-CO)Cr(CO)5] (3), alongside the dimeric iminoborane [(TMS)BN(TMS)]2 (4), which results from the rearrangement of the released BN(TMS)2 aminoborylene moiety. Furthermore, the oxidative addition of methyl triflate (MeOTf) to II leaves the (η6-DBB)Ni moiety intact and provides the ionic NiII half-sandwich complex [(η6-DBB)NiMe(CO)]OTf (5), while reaction with pentaphenylborole (PPB) yields the unique, dark-blue, unsymmetrical sandwich complex [(η6-DBB)Ni2(μ-CO)2(η5-PPB)] (6). DFT calculations point toward 6 being a dinickel(0) complex with a neutral aromatic DBB and a neutral antiaromatic PPB ligand.

A C-H Arylation-Based Enantioselective Synthesis of Planar Chiral Cyclophanes.

Despite the growing importance of planar chiral macrocyclophanes owing to their unique properties in different areas of chemistry, methods that are effective in controlling their planar chirality are restricted to certain molecular scaffolds. Herein, we report the first Pd(0)-catalyzed enantioselective intermolecular C-H arylation that induces planar chirality by installing bulky aryl groups through dynamic kinetic resolution (DKR). A computer-assisted approach allowed a fine-tuning of the structure of the employed chiral bifunctional phosphine-carboxylate ligands to achieve high enantioselectivities. A range of planar chiral macrocyclophanes were successfully synthesized from configurationally labile macrocyclic precursors via C-H arylation with various perfluoroarenes. Mechanistic investigations indicated that the reaction proceeds via reversible oxidative addition and enantiodetermining C-H activation. Selected macrocyclophane products showed interesting photophysical and chiroptical properties, which depended on both the nature of the ansa chains and the aromatic moieties.

Secondary Alkylation of Arenes via the Borono-Catellani Strategy.

A modular platform technology for the synthesis of α-aryl carbonyl derivatives via Borono-Catellani-type secondary alkylation of arenes is presented. This practical method features a broad substrate scope regarding aryl boronic acid catechol esters, secondary alkyl bromides, and diversified terminating reagents (e.g., olefins, alkynes, and Zn(CN)2), mild reaction conditions, and good functional-group tolerance. Importantly, the asymmetric version based on a dynamic kinetic asymmetric transformation (DyKAT) has also been realized by introducing the 1:1 diastereomeric mixture of α-bromo carboxamides bearing an Evans chiral auxiliary as the electrophiles, and constantly excellent diastereoselectivities (>20:1 d.r.) are obtained. Notably, the obtained chiral products can be readily transformed into diversified enantioenriched α-aryl propionic acid derivatives, laying a solid foundation for the discovery of new NSAIDs. Through mechanistic studies and DFT calculations, a critical stereoretentive oxidative addition step is revealed in this Borono-Catellani secondary alkylation reaction, and the origins of stereodiscrimination of this DyKAT are well explained.

Tetra-Cationic Distibane Stabilized by Bis(α-iminopyridine) and Its Reactivity.

The work establishes the salt of a tetra-cationic distibane, [L2Sb2][CF3SO3]4 = [1]2[OTf]4 (CF3SO3 = OTf), stabilized by a bis(α-iminopyridine) ligand L, defying the Coulombic repulsion. The synthetic approach involved a dehydrocoupling reaction when a mixture of L and Sb(OTf)3 in a 1:1 ratio was treated with Et3SiH/LiBEt3H as the hydride source. Compound [1]2[OTf]4 was also achieved from [LSbCl][OTf]2 as a precursor and using Et3SiH. Dissolution of [1]2[OTf]4 in polar solvents unveiled the formation of the persistent L-stabilized dicationic Sb(II) radical monomer [1][OTf]2, while the addition of Me3SiOTf regenerated the dimer in the salt [1]2[OTf]4. The homolytic cleavage of the Sb-Sb bond in [1]24+ has manifested in exchange reactions between [1]2[OTf]4 and Ph2Ch2 (Ch = S, Se), giving [LSb(SPh)][OTf]2 = [2][OTf]2 and [LSb(SePh)][OTf]2 = [3][OTf]2, respectively, in acetonitrile. Reaction between [1]2[OTf]4 and p-benzoquinone gave [L2Sb2(C6H4O2)][OTf]4 = [4][OTf]4. An interesting oxygen atom insertion reaction occurred when [1]2[OTf]4 was treated with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) to give [L2Sb2O][ OTf]4 = [5][OTf]4. The oxo-bridged compound [5][OTf]4 was also obtained from exposure of [1]2[OTf]4 in open air. The strong Mn-Mn bond in [Mn2(CO)10] could be cleaved by reacting with [1]2[OTf]4 in the presence of pyridine to form [LSbMn(CO)5][ OTf]2 = [6][OTf]2. On the other hand, the reaction between [Co2(CO)8] and [1]2[OTf]4 gave the oxidative addition product [L2Sb2Co(CO)3][OTf]3 = [7][OTf]3. The compounds were characterized both in the solid and solution states. Computational studies gave a comprehensive understanding of the experimental findings.