Recyclable Enantioselective Catalysts Based on Copper(II) Complexes of 2‐(Pyridine‐2‐yl)imidazolidine‐4‐thione: Their Application in Asymmetric Henry Reactions

This paper describes the preparation of enantioselective catalysts based on derivatives of imidazolidine‐4‐thione and their subsequent anchoring by means of a sulfur atom on a polymeric carrier. First, we verified the catalytic activity and enantioselectivity in the Henry reaction of the homogeneous variants of the catalysts, i.e., the copper(II) complexes of 2‐(pyridine‐2‐yl)imidazolidine‐4‐thiones and 4‐benzylsufanyl‐2‐(pyridine‐2‐yl)imidazolines themselves. It was found that these catalysts exhibit high enantioselectivity (up to 98% ee). Subsequently, the imidazolidine‐4‐thione catalysts were immobilized by anchoring to polymeric carriers based on a copolymer of styrene and 4‐vinylbenzyl chloride. These heterogeneous catalysts were analogously tested with regard to their catalytic activity and enantioselectivity in the Henry reaction, and moreover, the possibility of their separation and reuse was studied. It was found that all the prepared immobilized catalysts are highly enantioselective (up to 97% ee). Their recycling ability was tested in Henry reaction of 2‐methoxybenzaldehyde with nitromethane. It was found that they can be recycled more than ten times without any decrease of their enantioselectivity. Therefore, they present a better means of catalysis than the original copper(II) complexes of imidazolidine‐4‐ones from both economic as well as ecological points of view. Thus, such immobilized catalysts exhibit high application potential for the asymmetric Henry reaction.

     

Multinuclear Cu(II) Schiff Base Complex as Efficient Catalyst for the Chemical Coupling of CO2 and Epoxides: Synthesis, X-ray Structural Characterization and Catalytic Activity

Abstract  

The synthesis, X-ray structure, spectroscopic and catalytic properties of sterically hindered Schiff-base ligands (L₁H = N-[allylamine]-3,5-di-tert-butyl salicylaldimine, L₂H=N-[2-amino-5-methyl pyridine]-3,5-di-tert-butyl salicylaldimine and L₃H=N-[2-amino-6-methyl pyridine]-3,5-di-tert-butyl salicylaldimine), and their mononuclear Cu(II) complex for L₁H with multinuclear Cu(II) complexes for L₂H and L₃H, were described. The copper(II) complexes of these ligands were synthesized by treating an methanolic solution of the appropriate ligand with an appropriate amount of CuCl₂·2H₂O. The ligands and their copper(II) complexes were characterized by FT-IR, UV–Vis, ¹H-NMR, elemental analysis, measurement of room temperature magnetic moment, and X-ray structural determination. The reaction of the L₂H and L₃H ligands in a 1:1 mol ratio with CuCl₂·2H₂O afforded ionic copper metal(II) complexes in the presence of NEt₃. The Cu(II) metal complexes tested as catalysts for the formation of cyclic organic carbonates from carbon dioxide and liquid epoxides which served as both reactant and solvent. [Cu₃(L₂)₄]Cl₂·CuCl₂ complex which has 5-methyl substituent on the pyridine ring showed high catalytic activity for chemical coupling carbon dioxide with epoxides (propylene oxide (PO), epichlorohydrine (EC) and 1,2-epoxy butane (EB)) selectively.     

Correlating Enantioselectivity with Activation Energies in the Asymmetric Hydrogenation of Acetophenone Catalysed by Noyori-Type Complexes

Abstract  

The reduction of acetophenone catalysed by Ru(diphosphine)(diamine) complexes, with various diphosphine and diamine ligands, has been investigated theoretically using density functional theory. The results show a correlation between differences in the calculated activation energy (ΔE _a) and enantiomeric excess (ee) in ruthenium-based asymmetric hydrogenation catalysts, suggesting that computational procedures may be able to predict the effect on the ee of ligand substitutions.     

Pyrrolidine-Based Chiral Quaternary Alkylammonium Ionic Liquids as Organocatalysts for Asymmetric Michael Additions

Abstract  

A series of chiral pyrrolidine-type quaternary alkylammonium ionic liquids were synthesized and served as efficient catalysts for asymmetric Michael additions of aldehydes and ketones to nitroolefins, and the corresponding adducts were obtained in excellent enantioselectivities (>99% ee) and diastereoselectivities (>99% dr) under solvent-free reaction conditions. Furthermore, the catalyst 7c could be recovered for next run of the reaction with similar yield and selectivity.     

Cyclometalated N‐Heterocyclic Carbene‐Platinum Catalysts for the Enantioselective Cycloisomerization of Nitrogen‐Tethered 1,6‐Enynes

Platinum(II) complexes which combine six‐membered N‐heterocyclic carbene‐containing metallacyclic units and monodentate chiral phosphines have been prepared. The key step of their synthesis is the intramolecular oxidative addition of N‐2‐iodobenzylimidazolylidene‐platinum(0)‐diene complexes in the presence of the chiral phosphorus ligands. The platinum(II) metallacycles have been used as well‐defined pre‐catalysts for the enantioselective cycloisomerization of nitrogen‐tethered 1,6‐enynes into 3‐azabicyclo[4.1.0]hept‐4‐enes. High enantiomeric excesses have been obtained with either Monophos or phenyl‐Binepine based catalysts (ees=82–96%), although phenyl‐Binepine outperforms Monophos in these reactions. The absolute configuration of the final 3‐azabicyclo[4.1.0.]heptenes has been established by X‐ray diffraction studies. The method has been extended then to the cycloisomerization of dienynes with enantiotopic vinyl groups. An (S)‐phenyl‐Binepine‐platinum(II) complex allows total diastereoselectivity and high enantioselectivity levels to be attained in these reactions (ees up to 95%) which represent the first enantioselective desymetrizations achieved via enyne cycloisomerizations.     

Titanium complexes with β-ketoiminate chelate ligands for ethylene polymerization: The significant influence of substituents on structures and catalytic activities

Four titanium complexes having β-ketoiminate chelate ligands with fluorine or alkyl groups [(Ar)NC(CH₃)C(H)C(CH₃)O]₂TiCl₂ (3a: Ar = 2.6-F₂C₆H₃; 3b: Ar = C₆F₅; 3c: Ar = 2.6-Me₂C₆H₃; 3d: Ar = 2.6-ⁱPr₂C₆H₃) have been synthesized and characterized by ¹H NMR and EA. Complexes 3a, 3c and 3d were further characterized by X-ray diffraction analysis and demonstrated distorted octahedral coordination structure around the titanium center. The substituents in ligands greatly affect the coordination mode, resulting in three different isomeric structures. These complexes are active catalysts for polymerization of ethylene with MMAO as cocatalyst. The substituents in ligands have also great influences on catalytic activity. The complexes with alkyl groups have lower activity, while the complexes with fluorine atoms have middle or high catalytic activity.     

Rigid and Flexible Lanthanide Complexes with an Infinite Ln–O–Ln Framework: Synthesis,Structure and Properties

Abstract  

A series of new lanthanide coordination polymers, Ln(pydc)(Hglu) (Ln = Nd(1), Pr(2); H₂glu = glutaric acid; 2,5-H₂pydc = 2,5-pyridine dicarboxylic acid), have been synthesized hydrothermally from the self-assembly of the lanthanide ions (Ln³⁺) with rigid 2,5-pyridine dicarboxylic acid and flexible glutaric acid. They were characterized by elemental analysis, IR spectroscopy and single-crystal X-ray diffraction. Structural analyses reveal that both of the complexes were crystallized in monoclinic space group P2₁/c and have intricate 3D net-structures, which contain an infinite 1D Ln–O–Ln chain structure. In addition, the phase purities of the bulk samples were identified by X-ray powder diffraction. The thermogravimetric analysis of 1 is discussed in detail.     

Homogenous and Heterogeneous Catalytic Activity of Azo-Linked Schiff Base Complexes of Mn(II), Cu(II) and Co(II)

Abstract  

Azo linked Schiff-base[L] complexes of Mn(II)(1), Cu(II)(2) and Co(II)(3) obtained by template method, in the reaction of 4-(benzeneazo) salicylaldehyde with 1,2-propanediamine in the present of metal acetate, respectively. Complexes are used as catalyst for oxidation of cyclohexene with tert-butylhydroperoxide (TBHP); oxidation of cyclohexene catalyzed by these complexes gave 2-cyclohexene-1-one and 2-cyclohexene-1-ol as major products. Conversion of cyclohexene achieved was 95–100% with (1), (2) and (3), with selectivity of 57, 92 and 100% for 2-cyclohexene-1-one, respectively. The encapsulated Cu(II) complex (Cu–NaY) catalyzes the oxidation of cyclohexene using TBHP as oxidant in good yield. (Cu–NaY) under optimized reaction condition gave three reaction products. A maximum of 100% conversion of cyclohexene has been achieved where selectivity of 2-cyclohexene-1-one was 83%.     

Towards the Development of Fiber-Based Oxidovanadium(IV) Catalysts for the Oxidation of Thioanisole

Abstract  

The catalytic activity of a series of bis-coordinated oxidovanadium(IV) complexes with 2-(4,5-diphenyl-1H-imidazol-2-yl)-4R-phenol ligands (R = NO₂, Br, H, MeO) was investigated for the oxidation of thioanisole by hydrogen peroxide in acetonitrile at 25°C. The substituent groups had a significant effect on the catalytic activity and followed the order NO₂ > Br > H > MeO, with the nitro (NO₂) derivative achieving >99% conversion after 10 min while the methoxy (MeO) derivative yielded the same result in 60 min. The nitro- and methoxy-substituted homogeneous catalysts were incorporated into polystyrene and electrospun to form nonwoven mats with fiber diameters in the range 0.33–1.98 μm. These vanadium incorporated fibers were subjected to a continuous flow system at a flow rate of 1 mL/h for the oxidation of thioanisole and the nitro-substituted catalyst showed excellent catalytic activity with conversions dropping from 99 to 93% in the 10th fraction while the drop was down to 88% for the methoxy-substituted catalyst.     

A New Series Lanthanide-2,6-Pyridinedicarboxylic Acid Complexes Containing Low Dimensionality: Synthesis,Structure and Photoluminescent Properties

Abstract  

Based on the 2,6-pyridinedicarboxylate acid ligand, ten lanthanide complexes with formula, (Hdipa)₃[Ln(L)₃] (Ln = Eu [1], Gd [2], Nd [3], Tb [4], Ce [5], Sm [6], Pr [7], Dy [8] and Er [9]) and [Nd(L)(HL)(H₂O)₂]·4H₂O (10), (where H₂L = 2,6-pyridinedicarboxylic acid and dipa = N-(1-methylethyl)-2-propanamine) have been prepared by different synthetic methods. Structural analyses reveal that complexes 1–3 are isomorphous, zero-dimensional structures, which are further connected to 3D H-bonding networks via extensive intermolecular hydrogen bonds. In the structures of these complexes, the dipa plays a key role in balancing electric charge. For complex 10, the 1D Ln–O–C–O–Ln polymeric chains are linked into a stable 3D H-bonding framework through numerous intermolecular and intramolecular hydrogen bonds. The luminescent properties of complexes 1, 4, 6 and 8 were investigated in detail.     

Syntheses, Structures, and Luminescent Properties of Two Novel Coordination Polymers with Mixed Ligands

Two novel metal–organic coordination polymers {[Pb₂(MIP)₂(BDC)₂]·H₂O} _n 1 and [Cd(MIP)(NDC)] _n 2 [MIP = 2-(3-methoxyphenyl)-1H-imidazo [4,5-f] [1, 10] phenanthroline, BDC = terephthalic acid, NDC = naphthalene-1,4-dicarboxylic acid] were obtained from hydrothermal reaction and characterized by elemental analysis, thermogravimetric (TG) analysis, infrared spectrum (IR) and single-crystal X-ray diffraction. The coordination polymers crystallize in triclinic, space group P-1 with a = 1.0335(2), b = 1.4224(3), c = 1.8156(4) nm, β = 106.088(3)° for complex 1 and the complex 2 crystallizes in monoclinic, space group P2(1)/n with a = 1.2562(8), b = 1.4800(9), c = 1.3723(8) nm, β = 97.257(1)°, respectively. The metal ions Pb(II) are located in [:PbN2O4] pentagonal bipyramidal geometry in complex 1. The metal ions Cd(II) in complex 2 act as distorted octahedral geometry, being surrounded by four carboxylate oxygen atoms from three NDC ligands and two donor nitrogen atoms from one MIP molecule. Moreover, there are hydrogen bonds in the two complexes, and it is noteworthy that the existence of hydrogen bonds and π–π interactions reinforce the structural stability of the title complexes, which have been proved by TG analysis. The luminescent properties for the ligand MIP, NDC and complex 2 are also discussed in detail.     

Synthesis and characterization of copper(I) complexes from triphenylphosphine and isatin Schiff bases of semi- and thiosemicarbazide

The reaction of isatin-3-thiosemicarbazone (ITC, 1) or isatin-3-semicarbazone (ISC, 2) with nitrato bis(triphenylphosphine)copper(I) gave the four coordinate copper(I) complexes [Cu(PPh₃)₂(ITC)]NO₃ (3) and [Cu(PPh₃)₂(ISC)]NO₃ (4). The synthesized complexes were characterized by FT-IR, UV–VIS, Raman and elemental analysis. The crystal structure of 3 was investigated by single crystal X-ray diffraction. The ITC coordinates to the copper(I) ion in a bidentate fashion via the N(imine) and S atoms which along with two triphenylphosphine ligands form a tetracoordinate complex. The complex has a distorted tetrahedral coordination environment. Crystal data at 150.0 K: space group P2₁/c with a=12.5777(4), b=15.2062(5), c=21.9057(7) Å, β=95.628(3)^o, Z=4, R ₁=0.049.

     

2D Cationic Metal-Organic Frameworks of Ag+ with Mixed Ligands (Semi-Rigid Dipyridyl, 3-pmpmd, and Diphosphine, dppe)

Abstract  

With the complex 1 or 2 ([Ag(3-pmpmd)]_n·n(X) (X⁻ = BF₄ ⁻, 1; X = PF₆ ⁻, 2) from the semi-rigid 3-pmpmd (N,N′-bis(3-pyridylmethyl)-pyromellitic diimide) ligand and AgBF₄ or AgPF₆ as the precursor, two new coordination polymers [Ag₂(3-pmpmd)₂(dppe)(BF₄)₂]_n·4nDMF (3) and [Ag₂(3-pmpmd)₂(dppe)(PF₆)₂]_n·4nDMF (4) with the 2D cationic MOFs (metal-organic frameworks), have been obtained in the presence of the second dppe (Ph₂P(CH₂)₂PPh₂) ligand. In the 2D layer network, the 3-pmpmd ligands show the Z _T -mode and the Z _C -mode conformations, and the bridged dppe ligands have the same anti conformation. In the meantime, the functions of the two selected ligands, together with the supramolecular interactions from counter ions and solvates molecules within, should play a key role in the construction of the 2D noninterpenetrated network.     

Enantioselective Synthesis of Chiral Tetrahydroisoquinolines by Iridium‐Catalyzed Asymmetric Hydrogenation of Enamines

Chiral iridium complexes based on spiro phosphoramidite ligands are demonstrated to be highly efficient catalysts for the asymmetric hydrogenation of unfunctionalized enamines with an exocyclic double bond. In combination with excess iodine or potassium iodide and under hydrogen pressure, the complex Ir/(S_a,R,R)‐ 3a provides chiral N‐alkyltetrahydroisoquinolines in high yields with up to 98% ee. The L/Ir ratio of 2:1 is crucial for obtaining a high reaction rate and enantioselectivity. A deuterium labeling experiment showed that an inverse isotope effect exists in this reaction. A possible catalytic cycle including an iridium(III) species bearing two monophosphoramidite ligands is also proposed.     

Syntheses, structures and catalytic activities of ruthenium (II) carbonyl iodide complexes with CNC-pincer bis (carbene) ligand

Two new pincer bis (carbene) ligands and corresponding ruthenium (II) carbonyl iodide complexes have been synthesized and characterized. The structures of complexes 3 and 4 have been determined by X-ray crystallography. Complexes 3 and 4 have been proved to be efficient catalysts in the transfer hydrogenation of acetophenone.     

Ethylene Oligomerizations by Diazene Bridged Ni(II) Catalysts Derived from Pyrazole-Scaffold-Based Binucleating Ligands with Alkyl and Aryl Pendant Arms

Abstract  

The coordination and organometallic chemistry of a series of diazene (N₂)-bridged Ni(II) catalysts derived from pyrazole-scaffold-based ligands bearing alkyl and aryl pendent arms was investigated. Binucleating ligands were obtained as products of the condensation reaction between 3,5-dichloroformyl-1H-pyrazole and aliphatic/aromatic primary/secondary amines under anhydrous conditions. The Ni(II) catalysts were activated with ethyl aluminum sesquichloride (EASC) to oligomerize the ethylene mainly into C4, C6, C8, and C10 fractions with activities up to 1.2 and 0.5 × 10⁶ g (mol-Ni)⁻¹ bar⁻¹ h⁻¹ at 30 and 50 °C, respectively. All catalysts were found to be electrochemically active in the working potential range of −2 to +2 V. A change in the potential of Ni(II) was provoked by the N₄ donor bridging ligands, increasing the ethylene oligomerization activity.     

Ring‐opening metathesis polymerization of norbornene catalyzed by tantalum and niobium complexes with chelating O‐donor ligands

BACKGROUND: In comparison with group 6 transition metals, such as tungsten and molybdenum, and group 8 metal ruthenium, group 5 metal‐based catalysts for ring‐opening metathesis polymerization (ROMP) have remained much less studied. The few reported ROMP catalysts of group 5 metals require multiple reaction steps to be synthesized, and are highly sensitive to air and moisture. RESULTS: A series of pentavalent tantalum and niobium complexes having catecholato, tropolonato, hinokitiolato, biphenolato and binaphtholato ligands were prepared and their catalytic activities for the ROMP of norbornene (NBE) were studied in the presence of trialkylaluminium as a co‐catalyst. Among these complexes, the tantalum complexes showed high activity upon activation with Buⁱ₃Al. In sharp contrast, the niobium complexes were effectively activated with Me₃Al. The polymers obtained with these complexes had high molecular weights (M_n > 10⁵ g mol⁻¹) and relatively narrow molecular weight distributions (M_w/M_n ≈ 2). CONCLUSION: We found that easily accessible and relatively stable tantalum and niobium complexes with such chelating O‐donor ligands showed high catalytic activity for ROMP of NBE depending on the kind of co‐catalyst. These findings could contribute to future development of ROMP catalysts. Copyright © 2008 Society of Chemical Industry     

N-Heterocyclic Carbene–Rhodium(I) Complexes Derived from Proline for the 1,4-Conjugate Addition of Arylboronic Acids to Enones in Neat Water

Abstract  

N-Heterocyclic carbene–rhodium(I) complexes derived from N-benzyl substituted proline have been successfully synthesized and were found to be efficient catalysts for the 1,4-conjugate addition of arylboronic acids to enones in neat water at 40 °C. Under the optimal reaction conditions, all reactions gave the addition products in good to high yields.     

Hydrogenation of dienes with cationic rhodium complexes

Methylcis-9,cis-12-octadecadienoate (methyl linoleate;c9,c12), itst10,t12 andt10,c12 isomers and methylcis-9-octadecenoate (methyl oleate;c9) were hydrogenated with rhodium complexes, the active species of which consisted of [RhL₂]⁺ and [RhL₂H₂]⁺ with ligands L=P(C₂H₅)₂C₆H₅ (catalyst A) P(i-C₄H₉)₃ (catalyst B) and P(CH₃)₃ (catalyst C). Using these catalysts the influence of steric effects on the reaction mechanism of hydrogenation of dienes was studied. The reactions were carried out in 2-propanol at atmospheric hydrogen pressure and ambient temperature. During hydrogenation ofc9 on catalysts A and B, geometrical isomerization mainly occurred, whereas on catalyst C some positional isomerization also took place.C9,c12 was almost exclusively hydrogenated via conjugated intermediates on catalyst A. On catalyst C, one of the double bonds was hydrogenated directly, in most cases. In the absence of hydrogen, catalysts A and B conjugatedc9,c12 very fast. The conjugation activity of catalyst C was much lower. Catalyst C showed a high 1,5-shift activity for the conjugatedcis, trans andtrans, cis intermediates during hydrogenation, in contrast to catalysts A and B, which showed a poor activity in this respect.T10,t12 was hydrogenated almost exclusively via 1,4-addition of hydrogen to thecisoid conformation, whereas only a slight preference was found in this mechanism over 1,2-addition for the hydrogenation oft10,c12. On the sterically unhindered catalysts A and C thetrans double bond int10,c12 was preferentially hydrogenated whereas on catalyst B, with its bulky ligands, thecis double bond was reduced faster than thetrans double bond.     

Aqueous Biphasic Hydroformylation of Higher Olefins Catalyzed by Rhodium Complexes with Amphiphilic Ligands of Sulfonated Triphenylphosphine Analog

The catalytic performances of rhodium complexes with three new amphiphilic phosphine ligands, bis-(3-sodium sulfonatophenyl)-(4-tert-butylphenyl)-phosphine (3), phenyl-(3-sodium sulfonatophenyl)-(4-tert-butyl-phenyl)-phosphine (4) and bis-(4-tert-butylphenyl)-(3-sodium sulfonatophenyl) phosphine (5), in hydroformylation of 1-hexene, 1-octene and 1-dodecene have been studied. The steric attributes of free ligands are investigated by Tolman's cone angle method through geometric optimizations. The results reveal that the new phosphines are surface-active as the typical surfactants and the corresponding rhodium complexes show significant enhancements in the reaction rate and higher selectivities toward the normal aldehydes in comparison with those obtained by triphenylphosphine trisulfonate (TPPTS)- and triphenylphosphine disulfonate (TPPDS) rhodium complexes under identical conditions.