Ruthenium-Catalyzed Asymmetric Epoxidation of Olefins Using H2O2, Part I: Synthesis of New ChiralN,N,N-Tridentate Pybox and Pyboxazine Ligands and Their Ruthenium Complexes

Nickel-Catalyzed Deoxygenative Amidation of Alcohols with Carbamoyl Chlorides

We report a deoxygenative amidation reaction of alcohols with carbamoyl chlorides to afford amides through nickel-photoredox dual catalysis. Good to excellent yields can be obtained even for diverse complex sugar and steroid derivatives. The reaction is scalable, and the synthetic utility of the reaction was demonstrated by the homologation of alcohols to deliver several important γ-amino alcohols and a synthetically challenging bioactive compound intermediate.

Asymmetric Synthesis of Propargylic α-Stereogenic Tertiary Amines by Reductive Alkynylation of Tertiary Amides Using Ir/Cu Tandem Catalysis

The development of an asymmetric protocol for the reductive alkynylation of amides to access important α-stereogenic tertiary propargylic amines is reported using a tandem Ir-catalyzed hydrosilylation/enantioselective Cu-catalyzed alkynylation. The reaction utilizes a Cu/PyBox catalyst system in the alkynylation step to achieve asymmetry and affords excellent yields with moderate to good levels of enantiocontrol while employing low Ir-catalyst loadings (0.5 mol %).

Heteroleptic iron(ii) complexes of chiral 2,6-bis(oxazolin-2-yl)-pyridine (PyBox) and 2,6-bis(thiazolin-2-yl)pyridine ligands – the interplay of two different ligands on the metal ion spin sate

The spin-crossover properties of [Fe(LR)L][ClO4]2 (LR = a chiral PyBox {L1R} or ThioPyBox {L2R} derivative) show subtle differences depending on the tridentate ‘L’ co-ligand.

Complementing Pyridine-2,6-bis(oxazoline) with Cyclometalated N-Heterocyclic Carbene for Asymmetric Ruthenium Catalysis

A strategy for expanding the utility of chiral pyridine-2,6-bis(oxazoline) (pybox) ligands for asymmetric transition metal catalysis is introduced by adding a bidentate ligand to modulate the electronic properties and asymmetric induction. Specifically, a ruthenium(II) pybox fragment is combined with a cyclometalated N-heterocyclic carbene (NHC) ligand to generate catalysts for enantioselective transition metal nitrenoid chemistry, including ring contraction to chiral 2H-azirines (up to 97 % ee with 2000 TON) and enantioselective C(sp3 )-H aminations (up to 97 % ee with 50 TON).

Hydrogen-bonded metallo-supramolecular polymers based on ruthenium or iron complexes for the selective extraction of single-walled carbon nanotubes

The chemical functionalization of nano-carbon materials such as single-walled carbon nanotubes (SWNTs) and graphene by metal complexes has attracted much attention due to the multitude of potential applications in efficient energy-conversion and -storage devices. The solubilization and single-chirality separation of SWNTs by surface modifications is a useful approach to manipulate SWNTs in the liquid phase. In this study, several Ru and Fe complexes containing two terminal quadruple hydrogen-bonded (H-bonded) ureidopyrimidinedione (UPy) groups were synthesized (Ru-1, Ru-UPy, and Fe-UPy) to form H-bonded supramolecular polymers by self-association. In order to control the solubility of these complexes in nonpolar solvents, Ru-UPy and Fe-UPy were endowed with long alkyl side chain groups in the coordinated 2,6-bis(benzimidazol-2-yl)pyridine ligand, while Ru-1 and Ru-2 do not contain such long alkyl chain groups. AFM measurements revealed that Ru-1, Ru-UPy, and Fe-UPy form a fiber-like network morphology on HOPG surfaces, arising from the H-bonded aggregation. However, only Ru-UPy and Fe-UPy are able to solubilize SWNTs effectively upon simple sonication in chlorobenzene. After the solubilization of a CoMoCAT® SWNT in chlorobenzene using Ru-UPy or Fe-UPy, UV-Vis-NIR spectra showed sharp peaks at 996 and 1150 nm, which were attributed to (6, 5) and (7, 6)-SWNTs. The Raman spectra of the solubilized SWNTs revealed peaks that were attributed to the radial breathing mode (RBM), which suggests an enrichment of semiconducting SWNTs, i.e., Ru-UPy and Fe-UPy are able to selectively solubilize semiconducting SWNTs. Cyclic voltammograms of films of SWNTs covered with Ru-UPy or Fe-UPy on ITO electrodes showed a well-defined adsorbed Ru(ii/iii) or Fe(ii/iii) wave. Upon addition of acid, the redox response from the adsorbed H-bonded Ru-UPy and Fe-UPy disappeared and only SWNTs were left on the ITO electrode. Moreover, the Ru-UPy/SWNT and Fe-UPy/SWNT hybrids exhibited non-linear I-V characteristics.

Synthesis and Structural Analysis of Palladium(II) Complexes Containing Neutral or Anionic C 2-Symmetric Bis(oxazoline) Ligands: Effects of Substituents in the 5-Position

A series of neutral and cationic palladium(II) complexes containing C ₂-symmetric bis(oxazoline) (BOX) ligands, (BOX)PdCl₂ (2a-d), (BOX)Pd(Me)Cl (3a-d), and [(BOX)PdMe(2,6-Me₂C₅H₃N)]⁺PF₆ ^- (4a-d) [BOX: 2,2'-(2-propylidene)bis{(4R)-4-phenyl-5,5-dimethyl-2-oxazoline}, 2,2'-methylenebis{(4R)-4-phenyl-5,5-dimethyl-2-oxazoline}, 2,2'-methylenebis{(4R)-4,5,5-triphenyl-2-oxazoline}, and 2,2'-methylenebis{(4R,5S)-4,5-diphenyl-2-oxazoline}], were prepared, and their structures were determined by X-ray crystallography. It was found that substituents at the 5-position (Ph, Me) in addition to substituents on the bridgehead carbon directly affect the structure around palladium, especially the BOX bite angle and the dihedral angles between the phenyl rings at the 4-position and the N₂Pd plane. Treatment of the bridged methylene proton in the BOX ligand (1b-d) with KH afforded the anionic BOX ligand; also, the neutral Pd complexes, (BOX)PdMe(2,6-Me₂C₅H₃N) (5b-d), could thus be prepared by reaction with Pd(Me)Cl(cod) (cod = 1,5-cyclooctadiene); 5b-d showed strong coordination to Pd, as demonstrated by X-ray crystallographic analysis.

Spin States of Homochiral and Heterochiral Isomers of [Fe(PyBox)2 ]2+ Derivatives

The following iron(II) complexes of 2,6-bis(oxazolinyl)pyridine (PyBox; L^H ) derivatives are reported: [Fe(L^H )₂ ][ClO₄ ]₂ (1); [Fe((R)-L^Me )₂ ][ClO₄ ]₂ ((R)-2; L^Me =2,6-bis{4-methyloxazolinyl}pyridine); [Fe((R)-L^Ph )₂ ][ClO₄ ]₂ ((R)-3) and [Fe((R)-L^Ph )((S)-L^Ph )][ClO₄ ]₂ ((RS)-3; L^Ph =2,6-bis{4-phenyloxazolinyl}pyridine); and [Fe((R)-L^iPr )₂ ][ClO₄ ]₂ ((R)-4) and [Fe((R)-L^iPr )((S)-L^iPr )][ClO₄ ]₂ ((RS)-4; L^iPr =2,6-bis{4-isopropyloxazolinyl}pyridine). Solid (R)-3⋅MeNO₂ exhibits an unusual very gradual, but discontinuous thermal spin-crossover with an approximate T1/2 of 350 K. The discontinuity around 240 K lies well below T1/2 , and is unconnected to a crystallographic phase change occurring at 170 K. Rather, it can be correlated with a gradual ordering of the ligand conformation as the temperature is raised. The other solid compounds either exhibit spin-crossover above room temperature (1 and (RS)-3), or remain high-spin between 5-300 K [(R)-2, (R)-4 and (RS)-4]. Homochiral (R)-3 and (R)-4 exhibit more twisted ligand conformations and coordination geometries than their heterochiral isomers, which can be attributed to steric clashes between ligand substituents [(R)-3]; or, between the isopropyl substituents of one ligand and the backbone of the other ((R)-4). In solution, (RS)-3 retains its structural integrity but (RS)-4 undergoes significant racemization through ligand redistribution by ¹ H NMR. (R)-4 and (RS)-4 remain high-spin in solution, whereas the other compounds all undergo spin-crossover equilibria. Importantly, T1/2 for (R)-3 (244 K) is 34 K lower than for (RS)-3 (278 K) in CD₃ CN, which is the first demonstration of chiral discrimination between metal ion spin states in a molecular complex.

Synthesis of a Ruthenium Complex Based on 2,6-Bis[1-(Pyridin-2-yl)-1H-Benzo[d]Imidazol-2-yl]Pyridine and Catalytic Oxidation of (1H-Benzo[d]-Imidazol-2-yl)Methanol to 1H-Benzo[d]Imidazole-2-Carbaldehyde with H2O2

2,6-Bis[1-(pyridin-2-yl)-1H-benzo[d]-imidazol-2-yl]pyridine (bpbp), which has been synthesised by intramolecular thermocyclisation of N2,N6-bis[2-(pyridin-2-ylamino)phenyl]pyridine-2,6-dicarboxamide, reacts with sodium pyridine-2,6-dicarboxylate (pydic) and RuCl3 to give [Ru(bpbp)(pydic)] which can catalyse the oxidation of (1H-benzo[d]imidazol-2-yl)methanol to 1H-benzo[d]imidazole-2-carbaldehyde by H2O2. The optimal reaction conditions were: molar ratios of catalyst to substrate to H2O2 set at 1: 1000: 3000; reaction temperature 50 °C; reaction time 5 h. The yield of (1H-benzo[d]imidazol-2-yl) methanol was 70%.

Synthesis of an oligomer ruthenium complex and its catalysis in the oxidation of alcohols

The catalyst showed high efficiency in the oxidation of alcohols to ketones or acids and can be recycled several times.

Synthesis and Applications of (ONO Pincer)Ruthenium-Complex-Bound Norvalines

Two (ONO pincer)ruthenium-complex-bound norvalines, Boc-[Ru(pydc)(terpy)]Nva-OMe (1; Boc=tert-butyloxycarbonyl, terpy=terpyridyl, Nva=norvaline) and Boc-[Ru(pydc)(tBu-terpy)]Nva-OMe (5), were successfully synthesized and their molecular structures and absolute configurations were unequivocally determined by single-crystal X-ray diffraction. The robustness of the pincer Ru complexes and norvaline scaffolds against acidic/basic, oxidizing, and high-temperature conditions enabled us to perform selective transformations of the N-Boc and C-OMe termini into various functional groups, such as alkyl amide, alkyl urea, and polyether groups, without the loss of the Ru center or enantiomeric purity. The resulting dialkylated Ru-bound norvaline, n-C11 H23 CO-l-[Ru(pydc)(terpy)]Nva-NH-n-C11 H23 (l-4) was found to have excellent self-assembly properties in organic solvents, thereby affording the corresponding supramolecular gels. Ru-bound norvaline l-1 exhibited a higher catalytic activity for the oxidation of alcohols by H2 O2 than parent complex [Ru(pydc)(terpy)] (11 a).

A tetranuclear ruthenium complex with bridging pyridine-2,4-dicarboxylato ligands forming a square metallamacrocycle

Treatment of [RuCl2(PPh3)3] with equimolar amounts of 2,4-pyridinedicarboxylic acid (2,4-dipicH2) in the presence of Et3N afforded a tetranuclear complex [Ru(μ-2,4-dipic)(PPh3)2]4 (1) as red crystals. The crystal and molecular structure of [Ru(μ-2,4-dipic)(PPh3)2]4·CHCl3·8H2O (1·CHCl3·8H2O) was determined by single-crystal X-ray diffraction. Each ruthenium center in 1 is six-coordinated with two phosphorus atoms from triphenylphosphine ligands, one nitrogen atom from a pyridyl moiety and three oxygen atoms from two 2,4-dipic2– ligands. 2,4-Pyridinedicarboxylate dianions (2,4-dipic2–) act as bridging ligands to form the stable tetranuclear metallamacrocyclic compound. The electrochemical properties of 1 were also investigated.

Interesting properties of p-, d-, and f-block elements when coordinated with dipicolinic acid and its derivatives as ligands: their use as inorganic pharmaceuticals

This is a review of the literature concerning the interesting properties of p-, d-, and f-block elements when coordinated with 2,6-pyridinedicarboxylic acid (dipicolinic acid, H2 dipic) and its derivatives as ligands, with a focus on their use as inorganic pharmaceuticals. Some of the complexes reported were used as insulin-like, bioimaging contrasting agents, antimicrobial agents, and anticancer agents.

Dehydrogenative α-oxygenation of ethers with an iron catalyst

Selective α-oxidation of ethers under aerobic conditions is a long-pursued transformation; however, a green and efficient catalytic version of this reaction remains challenging. Herein, we report a new family of iron catalysts capable of promoting chemoselective α-oxidation of a range of ethers with excellent mass balance and high turnover numbers under 1 atm of O2 with no need for any additives. Unlike metalloenzymes and related biomimetics, the catalyst produces H2 as the only byproduct. Mechanistic investigations provide evidence for an unexpected two-step reaction pathway, which involves dehydrogenative incorporation of O2 into the ether to give a peroxobisether intermediate followed by cleavage of the peroxy bond to form two ester molecules, releasing stoichiometric H2 gas in each step. The operational simplicity and environmental friendliness of this methodology affords a useful alternative for performing oxidation, while the unique ability of the catalyst in oxygenating a substrate via dehydrogenation points to a new direction for understanding metalloenzymes and designing new biomimetic catalysts.

Ruthenium complexes with chiral bis-pinene ligands: an array of subtle structural diversity

A new chiral derivative of the N,N-bis(2-pyridylmethyl)ethylamine (bpea) ligand, Me-pinene[5,6]bpea [(-)-L1], has been prepared from a new aldehyde building block [Me-pinene-aldehyde, (-)-4] arising from the monoterpene chiral pool. The tridentate (-)-L1 ligand has been employed to prepare a new set of Ru-Cl complexes in combination with didentate 2,2'-bipyridine (bpy) with the general formula [RuCl((-)-L1)(bpy)](+). These complexes have been characterized in solution by cyclic voltammetry, UV-vis, and 1D and 2D NMR spectroscopy. Isomeric mixtures of trans,fac-C1a and anti,mer-C1c compounds are formed when (-)-L1 is reacted with a [Ru(bpy)(MeOH)Cl3] precursor. Density functional theory calculations of all of the potential isomers of this reaction have been performed in order to interpret the experimental results in terms of electronic and steric effects and also to unravel the observed isomerization pathway between anti,mer-C1c and trans,fac-C1a.

2,6-Bis[(S)-4-benzyl-4,5-dihydro-1,3-oxazol-2-yl]pyridine

The commercially available title compound, C₂₅H₂₃N₃O₂, has been known since 1993 [Nesper et al. (1993 ▶). Helv. Chim. Acta, 76, 2239–2249], but has not been structurally characterized until now. In the free ligand, the N atoms of both oxazoline rings point in opposite directions. The phenyl rings make dihedral angles of 30.56 (5) and 84.57 (3)° with the pyridine ring and 72.85 (3)° with each other.

m-Carborane-based chiral NBN pincer-metal complexes: synthesis, structure, and application in asymmetric catalysis

We have succeeded in synthesizing m-carborane-based chiral NBN-pincer ligands, 1,7-bis(oxazolinyl)-1,7-dicarba-closo-dodecaborane (Carbox) (7-9). The combination of bis(hydroxyamides) and 3 equiv of diethylaminosulfur trifluoride (DAST) is a key step for cyclization to form oxazoline rings in excellent yields. X-ray crystal structures of these ligands confirmed three donor sites, one central B and two flanking N atoms in fixed positions. The electrophilic halogenation of the Carbox pincer ligands with iodine and a catalytic amount of Lewis acid led to ring-opening of the oxazolines and afforded bis(haloamides) (13 and 14). The air- and moisture-stable Carbox pincer complexes of rhodium(III), nickel(II), and palladium(II) were synthesized by the oxidative addition of RhCl(3)·3H(2)O, Ni(COD)(2), and Pd(CH(3)CN)(4)[BF(4)](2) to the Carbox pincer ligands (7-9), respectively. The catalytic activity of the rhodium(III) complexes (18-20) was examined for the asymmetric conjugate reduction of α,β-unsaturated esters and reductive aldol reaction. Among these catalysts, [(S,S)-Carbox-iPr]Rh(OAc)(2)·H(2)O (18) showed the highest enantioselective catalytic ability for both asymmetric conjugate reduction and reductive aldol reaction.