New mechanistic insight into the coupling reactions of CO2 and epoxides in the presence of zinc complexes

Coupling reactions of CO(2) and epoxide to produce cyclic carbonates were performed in the presence of a catalyst [L(2)ZnX(2)] (L=pyridine or substituted pyridine; X=Cl, Br, I), and the effects of pyridine and halide ligands on the catalytic activity were investigated. The catalysts with electron-donating substituents on pyridine ligands exhibit higher activity than those with unsubstituted pyridine ligands. On the other hand, the catalysts with electron-withdrawing substituents at the 2-position of the pyridine ligands show no activity; this demonstrates the importance of the basicity of the pyridine ligands. The catalytic activity of [L(2)ZnX(2)] was found to decrease with increasing electronegativity of the halide ligands. A series of highly active zinc complexes bridged by pyridinium alkoxy ions of the general formula [((mu-OCHRCH(2)L)ZnBr(2))(n)] (n=2 for R=CH(3); n=3 for R=H; L=pyridine or substituted pyridine) were synthesized and characterized by X-ray crystallography. The dinuclear zinc complexes obtained from propylene oxide adopt a square-planar geometry for the Zn(2)O(2) core with two bridging pyridinium propoxy ion ligands. Trinuclear zinc complexes prepared from ethylene oxide adopt a boat geometry for the Zn(3)O(3) core, in which three zinc and three oxygen atoms are arranged in an alternate fashion. These zinc complexes bridged by pyridinium alkoxy ions were also isolated from the coupling reactions of CO(2) and epoxides performed in the presence of [L(2)ZnBr(2)]. Rapid CO(2) insertion into the zincbond;oxygen bond of the zinc complexes bridged by pyridinium alkoxy ions leads to the formation of zinc carbonate species; these which yield cyclic carbonates and zinc complexes bridged by pyridinium alkoxy ions upon interaction with epoxides. The mechanistic pathways for the formation of active species and cyclic carbonates are discussed on the basis of results from structural and spectroscopic analyses.     

Recent advance in heterocyclic organozinc and organomanganese compounds; direct synthetic routes and application in organic synthesis

A practical synthetic route for the preparation of 2-pyridyl and 3-pyridyl derivatives has been accomplished by utilizing a simple coupling reaction of stable 2-pyridylzinc bromides and 3-pyridylzinc bromides. The organozincs used in this study were easily prepared via the direct insertion of active zinc into the corresponding bromopyridines. The subsequent coupling reactions with a variety of different electrophiles have afforded the corresponding coupling products. Using highly active manganese, a variety of Grignard-type organomanganese reagents have been obtained. The subsequent coupling reactions of the resulting organomanganese reagents with several electrophiles have also been accomplished under mild conditions.     

Quaternary Centers by Nickel‐Catalyzed Cross‐Coupling of Tertiary Carboxylic Acids and (Hetero)Aryl Zinc Reagents

This work bridges a gap in the cross‐coupling of aliphatic redox‐active esters with aryl zinc reagents. Previously limited to primary, secondary, and specialized tertiary centers, a new protocol has been devised to enable the coupling of general tertiary systems using nickel catalysis. The scope of this operationally simple method is broad, and it can be used to simplify the synthesis of medicinally relevant motifs bearing quaternary centers.     

A novel organozinc reagent 4-coumarinylzinc bromide; preparation and application in the synthesis of 4-substituted coumarin derivatives

A novel organozinc reagent, 4-coumarinylzinc bromide, was prepared by the direct oxidative addition of active zinc to 4-bromocoumarin. The resulting organozinc bromide underwent the palladium-catalyzed cross-coupling reactions with a variety of aryl halides and acid chlorides affording the corresponding coupling products in good yields under mild conditions.     

5-(1,3-Dioxolan-2-yl)-2-furanylzinc bromide; direct preparation, and its application for the synthesis of 5-substituted furan derivatives

5-(1,3-Dioxolan-2-yl)-2-furanylzinc bromide was easily prepared by the direct insertion of active zinc to 2-bromo-5-(1,3-dioxolane)furan under mild conditions. Of interest, the resulting organozinc was successfully coupled with aryl halides and acid chlorides affording the corresponding coupling products in good to excellent yields.     

Optically active poly(amide-imide)/zinc oxide hybrid nanocomposites based on hydroxyphenyl benzamide segments: Compatibility by using 3-methacryloxypropyltrimethoxysilane coupling agent

In this study, a new series of optically active poly(amide-imide)/zinc oxide hybrid nanocomposites were fabricated by the sonication-assisted method. The compatibility of the organic and inorganic parts was effectively improved by reducing in aggregation of nanoparticles by surface modification using the coupling agent. The coupling agent 3-methacryloxypropyltrimethoxysilane was selected to improve the compatibility between the polymer matrix and ZnO nanoparticles. Characterizations with FTIR spectroscopy, FE-SEM, X-ray diffraction and AFM confirm the success in synthesis of nanocomposites.     

5-Bromo-2-pyridylzinc reagent; direct preparation and its coupling reactions

A facile synthetic route to the direct preparation of 5-bromo-2-pyridylzinc iodide has been developed. Treatment of 5-bromo-2-iodopyridine with active zinc gave rise to the selective oxidative addition to C-I bond under mild conditions. The resulting organozinc iodide has been used in the variety of coupling reactions affording the corresponding cross-coupling product.     

Regularities of adsorption of zinc acetate from aqueous solutions onto the surface of modified activated carbons

The dependence of the characteristics of Zn(OAc)₂/C catalysts for vinyl acetate synthesis on the solution circulation rate, on the temperature and initial concentration of zinc acetate solution, and on the procedures for modification of activated carbons with oxidants was studied with the aim to achieve uniform distribution of the supported active component (zinc acetate). Oxidation of activated carbons with hydrogen peroxide and nitric acid increases the adsorption rate and the amount of adsorbed zinc acetate. Treatment of the support with acetic acid leads to an increase in the adsorption capacity for zinc acetate, to more uniform distribution of the active component over the surface, and to enhancement of the catalyst activity. The hydrodynamic regime of stirring in the two-phase system consisting of the support and zinc acetate solution is an important factor determining the activity and stability of the zinc acetate catalyst for vinyl acetate synthesis.     

亚临床肝性脑病患者的血清锌,铜含量的测定及临床意义

检测了３０例慢性活动性肝炎患者、３０例肝硬化患者、３０例亚临床肝性脑病患者血清中的铜、锌含量，并与正常对照组进行比较分析，探讨了微量元素铜、锌的血清含量改变及其影响因素，分析其临床意义。结果表明体内的铜锌含量与肝脏功能有密切的关系。在慢性活动性肝炎时，虽然肝脏功能有一定的损害，但由于肝脏的代偿能力强，铜，锌含量仍可保持在正常水平，而在肝硬化、亚临床肝性脑病时，铜锌明显降低，而铜锌含量的降低又导致机     

On the mechanism of palladium-catalyzed coupling of haloaryls to biaryls in water with zinc

[reaction: see text] Kinetics and process parameters of coupling and hydro-dehalogenation reactions of chloroaryls are studied in the presence of zinc, water, and catalytic Pd/C. Good yields are obtained for the coupling of chlorobenzene, 4-chlorotoluene, and 4-chloro-1,1,1-trifluorotoluene. It is shown that water is actually one of the reagents, reacting with zinc in the presence of palladium to give zinc oxide and hydrogen gas, which then regenerates the Pd0 catalyst for the coupling reaction.     

Solid-State 67Zn NMR spectroscopic studies and ab initio molecular orbital calculations on a synthetic analogue of carbonic anhydrase

The tris(pyrazolyl)hydroborato zinc complexes [Tp(But,Me)]ZnX (where X = Br, Cl, and OH) have been examined by low-temperature solid-state (67)Zn NMR spectroscopy. The value of the quadrupole coupling constant, Cq, for the zinc increased monotonically with the electronegativity of the bound substituent X, e.g., Br < Cl < OH. Calculations on the methylimidazole complex [(MeImH)(3)Zn(OH)](+) as a model for the active site of carbonic anhydrase indicate that the computed electric field gradient tensor is in good agreement with the experimental and calculated values for [Tp(But,Me)]ZnOH.     

Nickel-catalyzed coupling of bromides of 1,6-methano[10]annulene and azulene. A facile synthesis of biannulene and bi-, ter-, quater-, and polyazulenes

The active nickel complex generated in situ by reduction of NiBr₂(PPh₃)₂ with zinc in the presence of Et₄NI is a useful reagent for the dehalogenative coupling of 2-bromo-1,6- methano[10]annulene and 1-bromo- and 1,3-dibromoazulenes.     

Nickel-catalyzed cross-electrophile allylation of vinyl bromides and the modification of anti-tumour natural medicine β-elemene

Herein, we present a facile and efficient allylation method via Ni-catalyzed cross-electrophile coupling of readily available allylic acetates with a variety of substituted alkenyl bromides using zinc as the terminal reductant. This Ni-catalyzed modular approach displays excellent functional group tolerance and a broad substrate scope, which the creation of a series of 1,4-dienes including several structurally complex natural products and pharmaceutical motifs. Moreover, the coupling strategy has the potential to realize enantiomeric control. The practicality of this transformation is demonstrated through the potent modification of the naturally antitumor active molecule β-elemene.

Herein, we present a facile and efficient allylation method via Ni-catalyzed cross-electrophile coupling of readily available allylic acetates with a variety of substituted alkenyl bromides using zinc as the terminal reductant.     

New and efficient dehalogenative coupling and reduction of α-haloketones with active nickel complex. Facile syntheses of 1,4-diketones and bicyclo[4.2.O]oct-3-ene-2,5-diones

The active nickel complex generated in situ by reduction of NiBr₂(PPh₃)₂ with zinc in the presence of Et₄NI is a useful reagent for the dehalogenative coupling of phenacyl halides to 1,4-diaryl-1,4-diketones and for the dechlorination of 3,4-dichlorobicyclo[4.2.0]-octane-2,5-diones to bicyclo[4.2.0]oct-3-ene-2,5-diones.     

Catalysis of cure reaction of ɛ-caprolactam-blocked polyisocyanate with diol using non-tin catalysts

Absract

Catalysis of cure reaction of ?-caprolactam-blocked polyisocyanate with 1,4-butane diol in the presence of DABCO - a tertiary amine, tin(II) 2-ethylhexanoate and non-tin organometallics like bismuth neodecanoate, calcium(II) 2-ethylhexanoate and zinc(II) 2-ethylhexanoate was investigated. The activity of these catalysts was studied by the determination of cure-time (gel-time) and analysis of absolute molecular weight of cured polymers. Cure-time results indicated that all the non-tin organometallics act as catalysts; they were more active than DABCO and except bismuth neodecanoate, other two non-tin compounds were also found to be equally active or more active than tin(II) 2-ethylhexanoate. Within the three organometallics studied, zinc (II) 2-ethylhexanoate showed highest catalytic activity, it reduced the cure-time more than 50% compared to the uncatalysed reaction. In the case of bismuth neodecanoate catalysed reaction, it was found that the viscosity was build-up to a certain limit and then decreased; this catalyst assisted chain degradation was confirmed by the analysis of molecular weight of polymers as a function of time. Catalytically active new species was formed when mixing the solution of DABCO to the solution of zinc (II) 2-ethylhexanoate and this formation was confirmed using FT-IR and UV-visible spectroscopy, but this species not showed synergism.     

The mononuclear metal center of type-I dihydroorotase from aquifex aeolicus

Background

Dihydroorotase (DHO) is a zinc metalloenzyme, although the number of active site zinc ions has been controversial. E. coli DHO was initially thought to have a mononuclear metal center, but the subsequent X-ray structure clearly showed two zinc ions, α and β, at the catalytic site. Aquifex aeolicus DHO, is a dodecamer comprised of six DHO and six aspartate transcarbamoylase (ATC) subunits. The isolated DHO monomer, which lacks catalytic activity, has an intact α-site and conserved β-site ligands, but the geometry of the second metal binding site is completely disrupted. However, the putative β-site is restored when the complex with ATC is formed and DHO activity is regained. Nevertheless, the X-ray structure of the complex revealed a single zinc ion at the active site. The structure of DHO from the pathogenic organism, S. aureus showed that it also has a single active site metal ion.

Results

Zinc analysis showed that the enzyme has one zinc/DHO subunit and the addition of excess metal ion did not stimulate catalytic activity, nor alter the kinetic parameters. The metal free apoenzyme was inactive, but the full activity was restored upon the addition of one equivalent of Zn²⁺ or Co²⁺. Moreover, deletion of the β-site by replacing the His180 and His232 with alanine had no effect on catalysis in the presence or absence of excess zinc. The 2.2 Å structure of the double mutant confirmed that the β-site was eliminated but that the active site remained otherwise intact.

Conclusions

Thus, kinetically competent A. aeolicus DHO has a mononuclear metal center. In contrast, elimination of the putative second metal binding site in amidohydrolyases with a binuclear metal center, resulted in the abolition of catalytic activity. The number of active site metal ions may be a consideration in the design of inhibitors that selectively target either the mononuclear or binuclear enzymes.

     

Bifunctional Catalysts Based on m‐Phenylene‐Bridged Porphyrin Dimer and Trimer Platforms: Synthesis of Cyclic Carbonates from Carbon Dioxide and Epoxides

Highly active bifunctional diporphyrin and triporphyrin catalysts were synthesized through Stille coupling reactions. As compared with a porphyrin monomer, both exhibited improved catalytic activities for the reaction of CO₂ with epoxides to form cyclic carbonates, because of the multiple catalytic sites which cooperatively activate the epoxide. Catalytic activities were carefully investigated by controlling temperature, reaction time, and catalyst loading, and very high turnover number and turnover frequency were obtained: 220 000 and 46 000 h^?1, respectively, for the magnesium catalyst, and 310 000 and 40 000 h^?1, respectively, for the zinc catalyst. Results obtained with a zinc/free‐base hybrid diporphyrin catalyst demonstrated that the Br^? ions on the adjacent porphyrin moiety also function as nucleophiles.     

Stereoselective Palladium-catalyzed Cross-coupling of (2-Amido-1-phenylpropyl)zinc Compounds with Aryl Bromides

A controllable diastereoselective C(sp²)―C(sp³) Negishi coupling reaction of secondary benzylic zinc reagents with aryl bromides to form medicinally important 2-arylphenylethylamines was demonstrated. In the presence of Pd(OAc)₂ and 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl(S-phos), open-chain(2-amido-1-phenylethyl)zinc reagent bearing a β-NHAc or NHCHO group undergoes coupling reaction to give syn-1-arylphenylethylamine mainly, whereas the zinc reagent bearing a sterically hindered β-amido group, for example NHCOC(CH₃)₂OTBS undergoes the coupling reaction to yield anti-1-arylphenylethylamines with a configuration inversion. In addition, a working mechanism for the stereoselective Negishi cross-coupling was also proposed.     

Regioselective Transition‐Metal‐Free Allyl–Allyl Cross‐Couplings

Readily prepared allylic zinc halides undergo S_N2‐type substitutions with allylic bromides in a 1:1 mixture of THF and DMPU providing 1,5‐dienes regioselectively. The allylic zinc species reacts at the most branched end (γ‐position) of the allylic system furnishing exclusively γ,α′‐allyl–allyl cross‐coupling products. Remarkably, the double bond stereochemistry of the allylic halide is maintained during the cross‐coupling process. Also several functional groups (ester, nitrile) are tolerated. This cross‐coupling of allylic zinc reagents can be extended to propargylic and benzylic halides. DFT calculations show the importance of lithium chloride in this substitution.