On the mechanism of the palladium-catalyzed tert-butylhydroperoxide-mediated Wacker-type oxidation of alkenes using quinoline-2-oxazoline ligands

The mechanism of the tert-butylhydroperoxide-mediated, Pd(Quinox)-catalyzed Wacker-type oxidation was investigated to evaluate the hypothesis that a selective catalyst-controlled oxidation could be achieved by rendering the palladium coordinatively saturated using a bidentate amine ligand. The unique role of the Quinox ligand framework was probed via systematic ligand modifications. The modified ligands were evaluated through quantitative Hammett analysis, which supports a "push-pull" relationship between the electronically asymmetric quinoline and oxazoline ligand modules.     

Studies on the mechanism of allylic coupling reactions: a hammett analysis of the coupling of aryl silicate derivatives

Hammett analysis of the palladium-catalyzed allyl-aryl coupling reaction has demonstrated that the rate of the coupling reaction is enhanced by electron-withdrawing groups on the aryl siloxane. The positive slope of the Hammett plot indicated involvement of a charged transition state in which negative charge on the aryl ring is stabilized inductively. This result is consistent with either transmetalation or reductive elimination being the rate-determining step in the coupling process. Furthermore, the influence of ligand on the metal site has been assessed from competition studies as a function of ligand type, cone angle, and electronic effects. From the relative ratios of coupling products produced in the Hammett study, it is possible to gather insight into the role of the electronic as well as the steric effects of ligands on the mechanism of the coupling reaction.     

Rhodium-catalyzed asymmetric aqueous Pauson-Khand-type reaction

An interesting rhodium-catalyzed asymmetric aqueous Pauson-Khand-type reaction was developed. A chiral atropisomeric dipyridyldiphosphane ligand was found to be highly effective in this system. This operationally simple protocol allows both catalyst and reactants to be handled under air without precautions. Various enynes were transformed to the corresponding bicyclic cyclopentenones in good yield and enantiomeric excess (up to 95 % ee). A study of the electronic effects of the enyne substrates revealed a correlation between the electronic properties of the substrates and the ee value obtained in the product of the Pauson-Khand-type reaction. A linear free-energy relationship was observed from a Hammett study.     

Substituent effects on the exchange dynamics of ligands on 1.6 nm diameter gold nanoparticles

The kinetics of exchange ofphenylethanethiolate ligands (PhC2S) of monolayer-protected clusters (MPCs, average formula Au140(PhC2S)53) by para-substituted arylthiols (p-X-ArSH) are described. 1H NMR measurements of thiol concentrations show that the exchange reaction is initially rapid and gradually slows almost to a standstill. The most labile ligands, exchanging at the shortest reaction times, are thought to be those at defect sites (edges, vertexes) on the nanoparticle core surface. The pseudo-first-order rate constants derived from the first 10% of the exchange reaction profile vary linearly with in-coming arylthiol concentration, meaning that the labile ligands exchange in a second-order process, which is consistent with ligand exchange being an associative process. A linear Hammett relationship with slope p = 0.44 demonstrates a substituent effect in the ligand place exchange reaction, in which the bimolecular rate constants increase for ligands with electron-withdrawing substituents (1.4 x 10-2 and 3.8 x 10(-3) M(-1) s(-1) for X = NO2 and 4-OH, respectively). This is interpreted as the more polar Au-S bonds at the defect sites favoring bonding with more electron deficient sulfur moieties. At longer reaction times, where ligands exchange on nondefect (terrace) as well as defect sites, the extent of ligand exchange is higher for thiols with more electron-donating substituents. The difference between short-time kinetics and longer-time pseudoequilibria is rationalized based on differences in Au-S bonding at defect vs nondefect MPC core sites. The study adds substance to the mechanisms of exchange of protecting ligands on nanoparticles. The scope and limitations of 1H NMR spectroscopy for determining rate data are also discussed.     

Highly enantioselective hydroformylation of aryl alkenes with diazaphospholane ligands

Asymmetric, rhodium-catalyzed hydroformylation of terminal and internal aryl alkenes with diazaphospholane ligands is reported. Under partially optimized reaction conditions, high enantioselectivity (>90% ee) and regioselectivities (up to 65:1 alpha:beta) are obtained for most substrates. For terminal alkenes, both enantioselectivity and regioselectivity are proportional to the carbon monoxide partial pressure, but independent of hydrogen pressure. Hydroformylation of para-substituted styrene derivatives gives the highest regioselectivity for substrates bearing electron-withdrawing substituents. A Hammett analysis produces a positive linear correlation for regioselectivity.     

Effect of Ligand Fields on the Reactivity of O2‐Activating Iron(II)‐Benzilate Complexes of Neutral N5 Donor Ligands

Three new iron(II)‐benzilate complexes [(N4Py)Fe^II(benzilate)]ClO₄ ( 1 ), [(N4Py^Me2)Fe^II(benzilate)]ClO₄ ( 2 ) and [(N4Py^Me4)Fe^II(benzilate)]ClO₄ ( 3 ) of neutral pentadentate nitrogen donor ligands have been isolated and characterized to study their dioxygen reactivity. Single‐crystal X‐ray structures reveal a mononuclear six‐coordinate iron(II) center in each case, where benzilate binds to the iron center in monodentate mode via one carboxylate oxygen. Introduction of methyl groups in the 6‐positions of the pyridine rings makes the N4Py^Me2 and N4Py^Me4 ligand fields weaker compared to that of the parent N4Py ligand. All the complexes ( 1 – 3 ) react with dioxygen to decarboxylate the coordinated benzilate to benzophenone quantitatively. The decarboxylation is faster for the complex of the more sterically hindered ligand and follows the order 3 > 2 > 1 . The complexes display oxygen atom transfer reactivity to thioanisole and also exhibit hydrogen atom transfer reactions with substrates containing weak C?H bonds. Based on interception studies with external substrates, labelling experiments and Hammett analysis, a nucleophilic iron(II)‐hydroperoxo species is proposed to form upon two‐electron reductive activation of dioxygen by each iron(II)‐benzilate complex. The nucleophilic oxidants are converted to the corresponding electrophilic iron(IV)‐oxo oxidant upon treatment with a protic acid. The high‐spin iron(II)‐benzilate complex with the weakest ligand field results in the formation of a more reactive iron‐oxygen oxidant.     

脒基配体/PdCl2催化Suzuki偶联反应研究

合成和表征了一系列具有不同位阻和电子效应取代基的脒基配体.研究表明,脒基配体在有氧条件下,与氯化钯原位作用能有效地催化不同溴代反应物与苯硼酸的Suzuki偶联反应.考察了配体结构、反应温度和溶剂对反应的影响,结果表明,脒基配体上具有大体积和推电子取代基有利于偶联反应的进行;提高反应温度和引入质子性极性溶剂也是提高偶联反...     

Room-temperature combinatorial screening of cyclometallated iridium(III) complexes for a step towards molecular control of colour purity

A library of emission spectra of 90 bis-cyclometallated iridium complexes has been obtained using a simple combinatorial approach performed at room temperature. Trends in emission maxima are rationalized using Hammett parameters and invoking inter ligand energy transfer (ILET) processes. The screening approach allowed us to observe trends in the broadness of emission spectra opening the way for a rational approach to the engineering of the emission colour purity at a molecular level. Finally limitations to the screening strategy are discussed using a case study that involves two different monodentate ligands.     

Supramolecular chiral phosphorous ligands based on a [2]pseudorotaxane complex for asymmetric hydrogenation

A new type of supramolecular chiral phosphorus-based ligands was prepared from easily available monodentate ligands through complexation between dibenzylammonium salt and dibenzo[24]crown-8 macrocycle. Rhodium complexes with these supramolecular ligands were prepared, and the supramolecular bidentate ligand-containing catalyst has demonstrated better catalytic activity for all substrates, and higher enantioselectivity except for the ortho-substituted substrates than those obtained from the parent monodentate ligand in the asymmetric hydrogenation of α-dehydroamino acid esters.     

Synthesis and characterization of mononuclear ruthenium(III) pyridylamine complexes and mechanistic insights into their catalytic alkane functionalization with m-chloroperbenzoic acid

A series of mononuclear RuIII complexes [RuCl2(L)]+, where L is tris(2-pyridylmethyl)amine (TPA) or one of four TPA derivatives as tetradentate ligand, were prepared and characterized by spectroscopic methods, X-ray crystallography, and electrochemical measurements. The geometry of a RuIII complex having a non-threefold-symmetric TPA ligand bearing one dimethylnicotinamide moiety was determined to show that the nicotine moiety resides trans to a pyridine group, but not to the chlorido ligand. The substituents of the TPA ligands were shown to regulate the redox potential of the ruthenium center, as indicated by a linear Hammett plot in the range of 200 mV for RuIII/RuIV couples with a relatively large rho value (+0.150). These complexes act as effective catalysts for alkane functionalization in acetonitrile with m-chloroperbenzoic acid (mCPBA) as terminal oxidant at room temperature. They exhibited fairly good reactivity for oxidation of cyclohexane (C--H bond energy 94 kcal mol(-1)), and the reactivity can be altered significantly by the electronic effects of substituents on TPA ligands in terms of initial rates and turnover numbers. Catalytic oxygenation of cyclohexane by a RuIII complex with 16O-mCPBA in the presence of H2 18O gave 18O-labeled cyclohexanol with 100% inclusion of the 18O atom from the water molecule. Resonance Raman spectra under catalytic conditions without the substrate indicate formation of a RuIV==O intermediate with lower bonding energy. Kinetic isotope effects (KIEs) in the oxidation of cyclohexane suggest that hydrogen abstraction is the rate-determining step and the KIE values depend on the substituents of the TPA ligands. Thus, the reaction mechanism of catalytic cyclohexane oxygenation depends on the electronic effects of the ligands.     

Highly efficient synthesis of a class of novel chiral-bridged atropisomeric monophosphine ligands via simple desymmetrization and their applications in asymmetric Suzuki-Miyaura coupling reaction

A series of novel chiral-bridged atropisomeric monophosphine ligands were synthesized via convenient and simple pathways. The prepared ligands, especially for ligand 7d, were found to be highly effective in the Pd-catalyzed Suzuki-Miyaura coupling reaction. The steric hindrance and electronic effect of substrates on the reactivity and enantioselectivity were explored preliminarily.     

Catalytic asymmetric dihydroxylation of substituted trans-stilbene derivatives: implications of the variation of enantioselectivities on the mechanism of OsO4 addition to olefins

The OsO₄ catalyzed asymmetric dihydroxylation of substituted trans-stilbene derivatives using 9-O-acetyldihydrocinchonidine as chiral ligand gives the corresponding diols with lower enantioselectivity in the case of substrates containing electron-donating and electron-withdrawing substituents. The Hammett correlations of the enantiomeric ratios exhibit non-linear plots, in accordance with the conclusion that the reaction involves a 1,3-dipolar type [3+2] cycloaddition transition state.     

Specific interaction of the triethylammonium ion with substituted pyridines in nitrobenzene and inortho-Dichlorobenzene

The dissociation constants K_d of triethylammonium picrate have been determined from conductimetric measurements in nitrobenzene (NB) and in ortho-dichlorobenzene (ODCB) with and without pyridines being used as ligands. In the absence of any ligand, the enhancement of the dissociation in NB as compared with that in ODCB cannot be explained solely by the increase of the dielectric constant, but shows the existence of specific interactions of the former solvent with the ions. The results obtained in the presence of ligands are consistent with the formation of a 1:1 cation-pyridine hydrogen-bonded complex. The constantk ₁ ⁺ for the formation of theis complex has been determined for six substituted pyridines in both solvents. Logk ₁ ⁺ varies linearly with the Hammett parameter σ_H of the substituent. The slope ρ of the Hammett relation is ?1.86 in NB and ?2.90 in ODCB. The difference can be ascribed to specific interactions between NB and the pyridines which become stronger when the pyridine is more basic.     

First examples of improved catalytic asymmetric C-C bond formation using the monodentate ligand combination approach

[reaction: see text] Using a combination of chiral monodentate phosphoramidite ligands in the rhodium-catalyzed conjugate addition of boronic acids to three different substrates, we have shown for the first time that the ligand combination approach is applicable for C-C bond formation. Chiral catalysts based on hetero-combinations of ligands are found to be more effective than the homo-combinations. (31)P NMR experiments show that the hetero-combinations are formed in excess over the homo-combinations.     

Structure–property relationship of phenoxyimine ligands/metal chloride initiating systems for controlled cationic polymerizations of alkyl vinyl ethers

The catalyst structure–property relationships of the phenoxyimine complexes in controlled cationic polymerization of vinyl ethers were investigated based on the Hammett correlation. The correlation analyses of a series of experiments using the phenoxyimine ligands/TiCl₄ initiating systems indicated that the substituents on the N‐aryl phenoxyimine ligands affected the polymerization rate and stereoselectivity. Importantly, a linear correlation was observed between the Hammett substituent constants and the polymerization rates, which indicates that the Lewis acidity of the complex is affected by the electron‐withdrawing and ‐donating effects of the substituents. The tacticity of product polymers correlated to the Hammett substituent constants. Unlike the relationship with the polymerization rates, the σ^– values, which account for the enhanced resonance effects, were more appropriate for the relationship with the tacticity than the normal σ values. In contrast, the polymerization behavior using o‐substituted ligands exhibited a trend different from those using p‐ or m‐substituted ligands. The structural change, which was caused by the rotation of the C? N bonding, most likely triggered the acceleration effect in the case of the o‐substituents. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2021–2029     

Assembly of coordination nanostructures via ligand derivatization of oxide surfaces

A scheme is presented for the construction of coordination nanostructures on oxide surfaces (glass, Si/SiO2, quartz), based on application of epoxy-terminated monolayers as anchors for covalent grafting of ligands. Two ligands bearing amine groups were reacted with epoxysilane monolayers on oxide surfaces, providing ligand-terminated substrates. The ligands employed were (i) a pyridine moiety, used for subsequent binding of cobalt tetraphenylporphine (CoTPP), and (ii) deferoxamine (DFX), which contains hydroxamic acid moieties, used for subsequent construction of various Zr4+-based coordination layers. The results suggest that a dense ligand layer was obtained in both cases, allowing the formation of coordination overlayers on the oxide surfaces. The growth of coordinated layers was similar to analogous overlayers assembled on Au substrates, indicating that high ligand coverage is achieved by the epoxy-amine surface reaction. Epoxy-based functionalization of oxide substrates is a mild and efficient method for preparing high-quality coordination overlayers. Moreover, the method makes use of commercially available silane and amine reactants, providing the basis for wide application.     

Influence of structural changes in ferrocene phosphane aminophosphane ligands on their catalytic activity

New phosphane aminophosphane ligands based on [3]ferrocenophane skeleton were synthesized using a direct double lithiation followed by phosphanylation. Influence of ligand structure on catalytic performance was evaluated by performing a series of Pd-catalyzed allylic substitution on different substrates. Enantioselectivities up to 55% ee were obtained with bridged ligand compared to 33% ee with analogous non-bridged BoPhoz ligand.     

Unusually stable palladium(IV) complexes: detailed mechanistic investigation of C-O bond-forming reductive elimination

This communication describes the synthesis of a family of unusually stable palladium(IV) complexes containing two chelating 2-phenylpyridine ligands and two benzoates. These complexes undergo clean C-O bond-forming reductive elimination upon heating, and the mechanism of this catalytically relevant process has been studied in detail. Solvent effects, crossover experiments, Eyring plots (which show DeltaS of -1.4 +/- 1.9 and 4.2 +/- 1.4 in CDCl3 and DMSO, respectively), and Hammett analysis (which shows rho = -1.36 +/- 0.04 upon substitution of the para-benzoate substituent) all suggest that reductive elimination does not proceed via initial dissociation of a benzoate ligand. Instead, an unusual mechanism involving pre-equilibrium dissociation of the N-arm of the phenylpyridine ligand is proposed.     

Furanoside thioether-phosphinite ligands for Pd-catalyzed asymmetric allylic substitution reactions: Scope and limitations

A series of readily available thioether-phosphinite ligands has been tested in the Pd-catalyzed allylic substitution reactions of several acyclic and cyclic allylic substrates (S1-S7). This series of ligands have been designed to uncover their important structural features and to determine the scope of the thioether-phosphinite ligands in these catalytic reactions. Systematic variation of the electronic and steric properties at the thioether moiety provide useful information about the ligand parameters that control the enantiodiscrimination. By carefully selecting the ligand parameters, good enantioselectivities with high activities were obtained for hindered linear substrates S1 and S2 (ee’s up to 95%) and for unhindered cyclic substrates S4 and S5 (ee’s up to 91%).     

Formation, characterization, and reactivity of bis(mu-oxo)dinickel(III) complexes supported by a series of bis[2-(2-pyridyl)ethyl]amine ligands.

Bis(mu-oxo)dinickel(III) complexes supported by a series of bis[2-(2-pyridyl)ethyl]amine ligands have been successfully generated by treating the corresponding bis(mu-hydroxo)dinickel(II) complexes or bis(mu-methoxo)dinickel(II) complex with an equimolar amount of H(2)O(2) in acetone at low temperature. The bis(mu-oxo)dinickel(III) complexes exhibit a characteristic UV-vis absorption band at approximately 410 nm and a resonance Raman band at 600-610 cm(-1) that shifted to 570-580 cm(-1) upon (18)O-substitution. Kinetic studies and isotope labeling experiments using (18)O(2) imply the existence of intermediate(s) such as peroxo dinickel(II) in the course of formation of the bis(mu-oxo)dinickel(III) complex. The bis(mu-oxo)dinickel(III) complexes supported by the mononucleating ligands (L1(X) = para-substituted N,N-bis[2-(2-pyridyl)ethyl]-2-phenylethylamine; X = OMe, Me, H, Cl) gradually decompose, leading to benzylic hydroxylation of the ligand side arm (phenethyl group). The kinetics of the ligand hydroxylation process including kinetic deuterium isotope effects (KIE), p-substituent effects (Hammett plot), and activation parameters (Delta H(H)(*) and Delta S(H)(*)) indicate that the bis(muxo)dinickel(III) complex exhibits an ability of hydrogen atom abstraction from the substrate moiety as in the case of the bis(mu-oxo)dicopper(III) complex. Such a reactivity of bis(mu-oxo)dinickel(III) complexes has also been suggested by the observed reactivity toward external substrates such as phenol derivatives and 1,4-cyclohexadiene. The thermal stability of the bis(mu-oxo)dinickel(III) complex is significantly enhanced when the dinucleating ligand with a longer alkyl strap is adopted instead of the mononucleating ligand. In the m-xylyl ligand system, no aromatic ligand hydroxylation occurred, showing a sharp contrast with the reactivity of the (mu-eta(2):eta(2)-peroxo)dicopper(II) complex with the same ligand which induces aromatic ligand hydroxylation via an electrophilic aromatic substitution mechanism. Differences in the structure and reactivity of the active oxygen complexes between the nickel and the copper systems are discussed on the basis of the detailed comparison of these two systems with the same ligand.