Cu(II) 2-quinoxalinol salen catalyzed oxidation of propargylic,benzylic, and allylic alcohols using tert-butyl hydroperoxide in aqueous solutions

The synthesis of carbonyl compounds by oxidation of alcohols is a key reaction in organic synthesis. Such oxidations are typically conducted using catalysts featuring toxic metals and hazardous organic solvents. Considering green and sustainable chemistry, a copper(II) complex of sulfonated 2-quinoxalinol salen (sulfosalqu) has been characterized as an efficient catalyst for the selective oxidation of propargylic, benzylic, and allylic alcohols to the corresponding carbonyl compounds in water when in combination with the oxidant tert-butyl hydroperoxide. The reactions proceed under mild conditions (70 °C in water) to produce yields up to 99% with only 1 mol % of catalyst loading. This reaction constitutes of a rare example of propargylic alcohol oxidation in water, and it makes this process greener by eliminating the use of hazardous organic solvents. Excellent selectivity was achieved with this catalytic protocol for the oxidation of propargylic, benzylic, and allylic alcohols over aliphatic alcohols. The alcohol oxidation is thought to go through a radical pathway.     

Cu(II)-catalyzed functionalizations of aryl C-H bonds using O2 as an oxidant

Cu(II)-catalyzed acetoxylation and halogenation of aryl C-H bonds are developed. ortho-Selectivity was observed with a wide range of 2-arylpyridine substrates. Both mono- and difunctionalizations are achieved by tuning the reaction conditions. Excellent functional group tolerance and use of O2 as a stoichiometric oxidant are significant advantages over our recently developed Pd-catalyzed C-H functionalization reactions. These newly discovered reaction conditions are also applicable for cyanation, amination, etherification, and thioetherification of aryl C-H bonds. Mechanistic investigations are carried out to gain insights into the Cu(II)-catalyzed C-H functionalization reactions.     

Sigma-chelation-directed C-H functionalizations using Pd(II) and Cu(II) catalysts: regioselectivity, stereoselectivity and catalytic turnover

Recent progress in sigma-chelation directed C-H functionalization from the authors' group is described to illustrate the challenges and opportunities in the development of synthetically-useful catalytic reactions involving C-H activation as the key step. Emphasis is placed on strategies for developing catalysis under mild conditions and controlling regio- and stereoselectivity.     

Iron(II)-catalyzed oxidation of sp3 C-H bonds adjacent to a nitrogen atom of unprotected arylureas with tert-butyl hydroperoxide in water

With a FeSO(4)/TBHP system in water, direct oxidation of sp(3) C-H bonds adjacent to nitrogen of arylureas to give both unprecedented tert-butoxylated and hydroxylated products 2 was revealed. Under elevated temperatures, either 2-oxo-N-arylpyrrolidine-1-carboxamides 3 or 1,3-diarylureas 4 were attained, depending on the aliphatic ring size of the arylurea substrates.     

Chromium(VI) oxide-catalysed oxidations by tert-butyl hydroperoxide using benzotrifluoride as solvent

Chromium(VI) oxide-catalysed oxidations of alcohols and activated methylenes by aqueous tert-butyl hydroperoxide have been studied using benzotrifluoride as solvent. Comparisons with previous studies showed that this solvent is a valuable alternative to methylene chloride for such reactions. © 2000 Académie des sciences / Éditions scientifiques et medicales Elsevier SAS     

Co(salen) catalyzed oxidation of oximes with t-butyl hydroperoxide

Co(salen) catalyzed the oxidation of oximes with t-butyl hydroperoxide to give the parent carbonyl compounds, providing a new convenient route to deoximation reaction. A plausible mechanism involves a substrate radical intermediate.

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Co() -, . . .

     

两种新型salen金属络合物WO2(salen)和MoO2-(salen)的合成

新合成了两种过渡金属salen络合物WO2(salen)和MoO2(salen).通过和已经有的文献比较,发现这两种络合物的分子结构是少见的β-cis构型.     

A polymer-supported diacetatobis(2-quinolylbenzimidazole)copper(II) complex as an efficient catalyst for oxidation of alcohols with tert-butyl hydroperoxide

A polymer-supported diacetatobis(2-quinolylbenzimidazole)copper(II) complex [PS–(QBIM)₂Cu(II)] was synthesized by functionalization of chloromethylated polystyrene cross-linked with 6.5 % divinyl benzene with 2-(2′-quinolyl)benzimidazole and subsequent treatment with Cu(OAc)₂ in methanol. The complex was characterized by physical, analytical and spectroscopic techniques. Electronic and ESR spectra, together with magnetic susceptibility measurements, indicated that the complex was paramagnetic with distorted octahedral geometry around the copper. The complex was found to be active toward oxidation of various alcohols including phenol, benzyl alcohol and cyclohexanol using 70 % aqueous tert-butyl hydroperoxide under mild conditions. Under the optimized reaction conditions, cyclohexanol gave 100 % conversion to cyclohexanone, benzyl alcohol gave 98 % yield of benzaldehyde and phenol gave 89 % yield of catechol and 4 % of hydroquinone. The complex was recycled five times without much loss in catalytic activity.     

Toward rational control of metal stoichiometry in heterobimetallic coordination complexes: synthesis and characterization of Pb(Hsal)2(Cu(salen*))2, [Pb(NO3)(Cu(salen*))2](NO3), Pb(OAc)2(Cu(salen*)), and [Pb(OAc)(Ni(salen*))2](OAc)

The ability of the transition metal complex M(salen)* (M = Ni, Cu) to form Lewis acid-base adducts with lead(II) salts has been explored. The new complexes Pb(Hsal)(2)(Cu(salen*))(2) (1), [Pb(NO(3))(Cu(salen*))(2)](NO(3)) (2), Pb(OAc)(2)(Cu(salen*)) (3), and [Pb(OAc)(Ni(salen*)(2)](OAc) (4) (Hsal = O(2)CC(6)H(4)-2-OH, salen* = bis(3-methoxy)salicylideneimine) have been synthesized and characterized spectroscopically and by single-crystal X-ray diffraction. The coordination environment of the lead in the heterobimetallic complex is sensitive both to the initial lead salt and to the transition metal salen* complex that is employed in the synthesis. As a result, we have been able to access both 2:1 and 1:1 adducts by varying either the lead salt or the transition metal in the heterobimetallic coordination complex. In all cases, the salen* complex is associated with the lead center via dative interactions of the phenolic oxygen atoms. The relationship between the coordination requirements of the lead and the chemical nature of the anion is examined. In compound 1, the Pb(2+) ion is chelated by two Cu(salen*) moieties, and both salicylate ligands remain attached to the lead center and bridge to the Cu(2+) ions. The two Cu(salen*) groups are roughly parallel and opposed to each other as required by crystallographic inversion symmetry at lead. In contrast, the two Cu(salen*) groups present in 2 and 4 attached to the lead ion show considerable overlap. Furthermore, only one nitrate ion in 2 and one acetate ion in 4 remain bonded to the lead center. Compound 3 is unique in that only one Cu(salen*) group can bind to lead. Here, both acetate ligands remain attached, although one is chelating bidentate and the other is monodentate.     

Diruthenium(II,III) bis(tetramethyl-1,3-benzenedipropionate) as a novel catalyst for tert-butyl hydroperoxide oxygenation

The reaction of Ru2(OAc)4Cl with 2.2 equiv of H2esp (esp = alpha,alpha,alpha',alpha'-tetramethyl-1,3-benzenedipropionate) resulted in a new compound, Ru2(esp)2Cl (1), that is soluble in organic media. 1 is an active catalyst for the oxygenation of organic sulfides by tert-butyl hydroperoxide (TBHP) in both an acetonitrile solution or neat (solvent-free) conditions. Solvent-free reactions display the quantitative utility of TBHP and hence excellent chemical selectivity for sulfoxide formation.     

A Cu(II)-mediated C-H oxygenation of sterically hindered tripyridine ligands to form triangular Cu(II)3 complexes

Two sterically hindered tris-pyridyl methane ligands, tris(6-methyl-2-pyridyl)methane (L1) and bis(6-methyl-2-pyridyl)pyridylmethane (L2), are newly synthesized. Under aerobic conditions, Ln (n = 1 or 2) reacts with CuX2 (X = Cl or Br), oxygenated at the methine position to LnOH or LnOMe. The former alcoholate ligand creates trinuclear Cu(II) complexes [Cu3(X)(LnO)3](PF6)2 [(X, n) = (Br, 1) 1, (C1, 1) 2, (Br, 2) 3, or (C1, 2) 4] in which the alkoxide oxygen atoms bridge copper centers. The crystal structures of 1-4 are presented along with their magnetic susceptibility data. The weak antiferromagnetic coupling between the Cu(II) centers in this trinuclear arrangement is due to weak interaction of the magnetic orbitals (dz2) which are oriented along three alternate sides in a hexagon of the Cu3O3 core in 1-4. Under anaerobic conditions, L1 reacts with CuBr2 to form a square pyramidal complex [CuL1Br2] (9) with the ligand facially capping. [Cu(Br)2(L1OMe)] (10) was obtained after the suspension of 9 in MeOH was stirred under air for 48 h. In the presence of cyclohexene, 9 is converted to [Cu(Br)(L1)]m (m = 1 or 2) 5 quantitatively to give trans- 1,2-dibromocyclohexane, indicating that Br2 is generated during the reaction. The FAB MS spectrum of [18O]-1 prepared by the reaction of L1 with CuBr2 under 18O2 shows that the ligand of [18O]-1 is L1(18O-.) L1(18OH), L1OCD3, and bis(6-methyl-2-pyridyl) ketone were obtained from reaction of L1 with CuBr2 in CD3OD under 18O2. These results indicate that the origins of the O atom in L1OH and L1OMe are O2 and MeOH, respectively. On the basis of these results, a mechanism of the oxygenation of L1 in the present system will be proposed.     

Theoretical studies on magnetic interactions between Cu(II) ions in salen nucleobases

The magnetic and other physical properties between Cu²⁺ ions coordinated by salen–base pairs (Cu²⁺–DNA) are examined by using DFT calculations. In order to consider effects of entanglement and dis-entanglement of the double helix chain, three types of structural disorders i.e. distance, rotation angle and discrepancy in XY-plane, are changed in the model dimer structure. All calculated results show that J_ab values are weak anti-ferromagnetic couplings. It is also found that the J_ab values strongly depend on the salen structure.     

Indirect electroreductive cyclization and electrohydrocyclization using catalytic reduced nickel(II) salen

[Chemical reaction: See text] We describe efforts to achieve the electroreductive cyclization (ERC) and the electrohydrocyclization (EHC) reactions using catalytic nickel(II) salen as a mediator. While nickel(II) salen proved effective, the analogous cobalt complex as well as nickel(II) cyclam were not. The transformations were achieved in yields ranging from 60 to 94% using either a mercury pool or an environmentally preferable reticulated vitreous carbon (RVC) cathode. These examples represent the first instances wherein a nickel salen complex has been used in this manner. Clear differences between the voltammetric behavior of the ERC and EHC substrates were observed. The bisenoate 14, for example, displays a substantially larger catalytic current. When the structurally modified mediator 31 was used, the electron-transfer pathway shuts down. Instead, the reduced form of 31 behaves as an electrogenerated base, leading to the formation of the intramolecular Michael adduct 23. Presumably, the methyl groups of the modified ligand diminish the ability of the reduced form of the complex to serve as a nucleophile but not as a base. Aldehyde 23 was also characterized as a side product of the nickel(II) salen mediated electroreductive cyclization of 11. Given that it is absent from nonmediated processes, its formation is linked to the presence of the mediator. To account for the results, we favor the existence of a mechanistic continuum involving an equilibrium between nickel(II) salen (15) and two reduced forms, one being the metal-centered species 16, the other being a ligand-centered species 17. We postulate that one form may be more prominently involved with the chemistry than another, depending upon the electronic properties/requirements of the substrate, and suggest that the equilibrium will shift to accommodate the need. Thus, for a hard electrophile like an alkyl halide, the properties of 16 ought to dominate, whereas 17 ought to predominate as the reactive species accounting for the chemistry described herein since it properly matches a soft ligand-centered nucleophile with a soft electron deficient alkene.     

Mild propargylic oxidation using a diacetoxyiodobenzene/tert-butyl hydroperoxide protocol

A mild propargylic oxidation of alkynes is reported using a diacetoxyiodobenzene/tert-butyl hydroperoxide (DIB/TBHP) protocol. The reactions proceed smoothly at 0 °C and a number of α,β-unsaturated alkynoic ketones are obtained.     

Iron(II) and zinc(II) monohelical binaphthyl salen complexes

A new chiral binaphthyl salen ligand with rigid polyaromatic sidearms gives monohelical complexes (Fe(II) and Zn(II)) of predetermined handedness.     

Ring-opening of unsymmetrical 1,2-dioxines using cobalt(II) salen complexes

The regioselectivity of the metal-catalyzed ring opening of unsymmetrical 1,2-dioxines to cis-gamma-hydroxyenones was investigated using two different Co(II) salen complexes. Regioselectivity was determined by direct examination of the enone ratios and by derivitization with a stabilized phosphorus ylide. The steric influence of the substituents on the 1,2-dioxine was the primary influence on regioselectivity. Temperature played little role; however, solvent and selection of Co(II) complex could be used to mildly influence the outcome of the rearrangement for selected substrates. The origins of the selectivity for the reaction are discussed.     

Synthesis and characterization of biocompatible poly (L-lactide) using zinc (II) salen complex

Abstract

A biocompatible zinc (II) complex based on a tetradentate N,N,O,O-type salen ligand was synthesized, characterized and used for the solvent-free ring-opening polymerization (ROP) of L-lactide in bulk at 180?°C to prepare high molecular weight poly(L-lactide) (M_n : 82,600?Da; M_w : 140,000?Da; PDI: 1.70). Poly(L-lactide) (PLLA) was characterized using FTIR, ¹H NMR, ¹³C NMR, GPC, TGA, DSC, WAXD, and MALDI-ToF. Kinetic measurement was carried out and first-order behavior to monomer was observed. The k _app was found as 6?±?0.001?×?10^?4?s^?1. The biocompatibility of the PLLA was confirmed by in vitro cytotoxicity against NIH/3T3 fibroblast cell line and can be used in biomedical applications.     

Allylic and benzylic oxidation using cobalt(II) alkyl phosphonate modified silica

The allylic and benzylic oxidation of a range of substrates proceeds in good yield using catalytic quantities of cobalt(II) alkyl phosphonate modified silica and tert-butyl hydroperoxide.