Publications

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    ABSTRACT: Palladium nanoparticles of similar size of ~2 nm were synthesized on different silica-based materials all functionalized with thiol groups i.e., Aerosil-380, SBA-15, Plugged SBA-15 and m-MCF. The resulting materials were used to study the influence of confinement of Pd nanoparticles in functionalized silica support on the Heck and the Suzuki reaction. In the case of the Heck reaction, for all catalysts it was proven that leached Pd species were responsible for activity. However, the catalysts based on ordered mesoporous silica were still able to restrict Pd particle growth giving rise to an enhanced stability. For the Suzuki reaction, stronger alkaline conditions were required and catalysts based on plugged SBA-15 showed a higher stability than those based on SBA-15 and m-MCF which both collapsed after the first cycle. At almost identical Pd particle size, ordered mesoporous materials enhance stability and particle growth is slowed down but not fully suppressed.
    Catal. Sci. Technol. 01/2015;
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    ABSTRACT: Azole antifungal agents, which are widely used as antifungal antibiotics, inhibit cytochrome P450 sterol 14α demethylase (CYP51). In this research, a group of 1-methylimidazole derivatives were synthesized for evaluation as antibacterial and antifungal agents. Antimicrobial evaluation revealed that some of these compounds exhibited significant antimicrobial activities against tested pathogenic fungi and bacteria, wherein compounds 3 and 8 were most potent. To find the action mechanism, all of these compounds were subjected to docking studies using the AutoDock 4.2 program. The results show that all of the azoles (2 – 5, 7, and 8) interacted with 14α-DM, wherein azole – heme coordination, hydrogen binding, and -cation interactions were involved in the drug – receptor interaction. These studies offer some useful references in order to understand the action mechanism; moreover, performing the molecular design or modification of this series as a lead compound can assist in identifying new and potent antimicrobial agents.
    Pharmaceutical Chemistry Journal 11/2014; 27 Nov.(2014). · 0.30 Impact Factor
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    ABSTRACT: Reaction of [PdClMe(P^N)2 ] with SnCl2 followed by Cl-abstraction leads to apparent PdC bond activation, resulting in methylstannylene species trans-[PdCl{(P^N)2 SnClMe}][BF4 ] (P^N=diaryl phosphino-N-heterocycle). In contrast, reaction of Pt analogues with SnCl2 leads to PtCl bond activation, resulting in methylplatinum species trans-[PtMe{(P^N)2 SnCl2 }][BF4 ]. Over time, they isomerise to methylstannylene species, indicating that both kinetic and thermodynamic products can be isolated for Pt, whereas for Pd only methylstannylene complexes are isolated. Oxidative addition of RSnCl3 (R=Me, Bu, Ph) to M(0) precursors (M=Pd or Pt) in the presence of P^N ligands results in diphosphinostannylene pincer complexes trans-[MCl{(P^N)2 SnCl(R)}][SnCl4 R], which are structurally similar to the products from SnCl2 insertion. This showed that addition of RSnCl3 to M(0) results in formal SnCl bond oxidative addition. A probable pathway of activation of the tin reagents and formation of different products is proposed and the relevancy of the findings for Pd and Pt catalysed processes that use SnCl2 as a co-catalyst is discussed. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Chemistry 11/2014; 21(4). · 5.70 Impact Factor
  • ChemInform 09/2014; 45(40).
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    ABSTRACT: Pd nanoparticles were synthesized on nonfunctionalized and functionalized SBA-15 grafted with thiol or amine groups. The resulting materials were used as catalysts to study the influence of these functional groups for a similar Pd particle size of approximately 2 nm on the Heck reaction that uses iodide substrates. The nonfunctionalized catalysts lost their activity quickly because of Pd leaching and particle growth. However, if a mixed solvent that consisted of toluene and DMF was used and thiol groups were grafted on SBA-15, we were able to reduce the Pd leaching and particle growth to thus obtain a more stable catalyst in which an important role of the ligands was to recapture Pd. However, heterogeneity tests, such as a hot filtration test and poisoning experiments, gave a strong indication that Pd still leached from the support and that homogeneous Pd species were mainly responsible for the activity. Pd on S-functionalized SBA-15 was able to catalyze the Heck reaction using different substrates to achieve good activity and selectivity in most cases.
    ChemCatChem 09/2014; · 5.18 Impact Factor
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    ABSTRACT: Homogeneous metal complexes often display superior activity and selectivity in catalysis of chemical transformations. Heterogenization of these complexes by immobilization on solid supports has been used to facilitate recovery, but this is often associated with a decrease in catalytic performance. We here describe a novel approach of sizing and engineering the cavity structure of nanoporous materials as “nanoreactors” to assemble metal complexes by the “ship-in-the-bottle” synthesis to combine the best of homogeneous and heterogeneous catalysts. Catalysis occurred by free metal complexes in confined liquid in these nanoreactors, while the catalysts were recyclable as being heterogeneous at the macroscopic scale. Subnanometer tailoring of window sizes (0.5–3.7 nm) of the cavities (16–22 nm) allowed control over loading (6–70 mg-metal complex/g-support) and a high turnover frequency (40–600 h–1) for the hydrolytic kinetic resolution of 1,2-epoxyhexane. Most importantly, the ‘heterogeneous homogeneous catalysts’ showed enhanced thermal stability and were stable upon reuse approaching excellent turnover numbers of 100,000. We showed that engineering and sizing of nanoreactors is a powerful approach to control performance of confined catalysts, and this method is generally applicable in host–guest catalysis.
    ACS Catalysis 09/2014; 4(10):3791. · 7.57 Impact Factor
  • Organometallics 05/2014; 33(11):2914–2918. · 4.25 Impact Factor
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    Yuxing Huang, Marc‐Etienne Moret, Robertus J. M. Klein Gebbink
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    ABSTRACT: Biaryls are a common motif in both natural and synthetic chemicals. Several methods have recently been reported for the preparation of these compounds using direct arylation catalyzed by iron, other base metals, or transition-metal-free systems. To date, these methods have all required inert and/or forcing conditions, which limited their application in routine organic synthesis. In this article we report a protocol for direct arylation using a convenient FeCl3/1,10-phenanthroline system as precatalyst. The reaction proceeds under mild conditions (100 °C), under air, and with non-distilled solvent.
    European Journal of Organic Chemistry 05/2014; 2014(18). · 3.15 Impact Factor
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    Martin Lutz, Yuxing Huang, Marc Etienne Moret, Robertus J M Klein Gebbink
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    ABSTRACT: The title compound, [(C2H5)4N][FeCl4], has at room temperature a disordered structure in the high-hexagonal space group P63mc. At 230 K, the structure is merohedrally twinned in the low-hexagonal space group P63. The volume has increased by a factor of 9 with respect to the room-temperature structure. At 170 and 110 K, the structure is identical in the orthorhombic space group Pca21 and twinned by reticular pseudomerohedry. The volume has doubled with respect to the room-temperature structure. All three space groups, viz. P63mc, P63 and Pca21, are polar and the direction of the polar axis is not affected by the twinning. In the P63 and Pca21 structures, all cations and anions are well ordered.
    Acta crystallographica. Section C, Structural chemistry. 05/2014; 70(Pt 5):470-6.
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    ABSTRACT: The synthesis of a series of pyridine-based PNP-pincer rhenium-oxo complexes, with phenyl (1, 3), tert-butyl (2), or cyclohexyl (4) groups on the phosphorus atoms and either a ReO2X (1, 2) or a ReCl2O (3, 4) core is reported. The structures of these compounds were characterized using 1H, 13C, and 31P NMR and using X-ray crystallography for 1 and 4. Compounds 2 and 3 crystallize as the corresponding ReO(OH)X compounds upon their reaction with moisture during the crystallization process. The complexes were tested as catalysts in the oxidation reaction of limonene with H2O2, the oxygen atom transfer reaction from pyridine N-oxide to triphenylphosphine, and the dehydration reaction of 1-phenylethanol, but all showed poor catalytic performance. All complexes can be deprotonated at the benzylic arm, and 2 can be deprotonated twice, accompanied by dearomatization of the pyridine ring. 3 and 4 are highly stable in their oxidized forms. The noninnocent behavior shown here could open up possibilities for cooperative catalysis.
    Organometallics 04/2014; 33(9):2201–2209. · 4.25 Impact Factor
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    ABSTRACT: A series of new chiral pyridinyl prolinate (RPyProR) ligands and their corresponding Fe(ii) triflate and chloride complexes are reported. The ligands possess an NN'O coordination motif, as found in the active site of non-heme iron enzymes with the so-called 2-His-1-carboxylate facial triad. The coordination behaviour of these ligands towards iron turned out to be dependent on the counter ion (chloride or triflate), the crystallization conditions (coordinating or non-coordinating solvents) and the presence of substituents on the ligand. In combination with Fe(ii)(OTf)2, coordinatively saturated complexes of the type [Fe(L)2](OTf)2 are formed, in which the ligands adopt a meridional coordination mode. The use of FeCl2 in a non-coordinating solvent leads to 5-coordinated complexes [Fe(L)(Cl)2] with a meridional N,N',O ligand. Crystallization of these complexes from a coordinating solvent leads to 6-coordinated [Fe(L)(solv)(Cl)2] complexes (solv = methanol or acetonitrile), in which the N,N',O ligand is coordinated in a facial manner. For RPyProR ligands bearing a 6-Me substituent on the pyridine ring, solvent coordination and, accordingly, ligand rearrangement are prevented by steric constraints. The complexes were tested as oxidation catalysts in the epoxidation of alkene substrates in acetonitrile with hydrogen peroxide as the oxidant under oxidant limiting conditions. The complexes were shown to be especially active in the epoxidation of styrene type substrates (styrene and trans-beta-methylstyrene). In the best case, complex [Fe(6-Me-PyProNH2)Cl2] () allowed for 65% productive consumption of hydrogen peroxide toward epoxide and benzaldehyde products.
    Dalton Transactions 03/2014; · 4.10 Impact Factor
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    ABSTRACT: The limited availability of catalytic reaction components may represent a major hurdle for the practical application of many catalytic procedures in organic synthesis. In this work, we demonstrate that the mixture of isomeric iron complexes [Fe(OTf)2(mix-BPBP)] (mix-1), composed of Λ-α-[Fe(OTf)2(S,S-BPBP)] (S,S-1), Δ-α-[Fe(OTf)2(R,R-BPBP)] (R,R-1) and Δ/Λ-β-[Fe(OTf)2(R,S-BPBP)] (R,S-1), is a practical catalyst for the preparative oxidation of various aliphatic compounds including model hydrocarbons and optically pure natural products using hydrogen peroxide as an oxidant. Among the species present in mix-1, S,S-1 and R,R-1 are catalytically active, act independently and represent ca. 75% of mix-1. The remaining 25% of mix-1 is represented by mesomeric R,S-1 which nominally plays a spectator role in both C–H and CC bond oxidation reactions. Overall, this mixture of iron complexes displays the same catalytic profile as its enantiopure components that have been previously used separately in sp3 C–H oxidations. In contrast to them, mix-1 is readily available on a multi-gram scale via two high yielding steps from crude DL/meso-2,2′-bipyrrolidine. Next to its use in C–H oxidation, mix-1 is active in chemospecific epoxidation reactions, which has allowed us to develop a practical catalytic protocol for the synthesis of epoxides.
    Organic & Biomolecular Chemistry 03/2014; 12(13). · 3.49 Impact Factor
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    ABSTRACT: The limited availability of catalytic reaction components may represent a major hurdle for the practical application of many catalytic procedures in organic synthesis. In this work, we demonstrate that the mixture of isomeric iron complexes [Fe(OTf)2(mix-BPBP)] (mix-), composed of Λ-α-[Fe(OTf)2(S,S-BPBP)] (S,S-), Δ-α-[Fe(OTf)2(R,R-BPBP)] (R,R-) and Δ/Λ-β-[Fe(OTf)2(R,S-BPBP)] (R,S-), is a practical catalyst for the preparative oxidation of various aliphatic compounds including model hydrocarbons and optically pure natural products using hydrogen peroxide as an oxidant. Among the species present in mix-, S,S- and R,R- are catalytically active, act independently and represent ca. 75% of mix-. The remaining 25% of mix- is represented by mesomeric R,S- which nominally plays a spectator role in both C-H and C[double bond, length as m-dash]C bond oxidation reactions. Overall, this mixture of iron complexes displays the same catalytic profile as its enantiopure components that have been previously used separately in sp(3) C-H oxidations. In contrast to them, mix- is readily available on a multi-gram scale via two high yielding steps from crude dl/meso-2,2'-bipyrrolidine. Next to its use in C-H oxidation, mix- is active in chemospecific epoxidation reactions, which has allowed us to develop a practical catalytic protocol for the synthesis of epoxides.
    Organic & Biomolecular Chemistry 01/2014; · 3.49 Impact Factor
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    ABSTRACT: The first Fe-based catalytic system for the oxidative cleavage of unsaturated fatty acids and esters to carboxylic acids is reported. The system comprises [Fe(OTf)2(6-Me-PyTACN)] (2) (6-Me-PyTACN = 1-[(6-methyl-2-pyridyl)methyl]-4,7-dimethyl-1,4,7-triazacyclononane, OTf = trifluoromethane sulfonate anion) as the catalyst (3 mol%) either with a combination of hydrogen peroxide and NaIO4 or exclusively with NaIO4 as the oxidant, and operates at 0 [degree]C or ambient temperature. Under these standard conditions (method A), methyl oleate is converted in a one-pot procedure into 50-55% of both nonanoic and azelaic acid, together with some epoxide and aldehyde intermediates as byproducts. These yields can be further improved by addition of sulfuric acid (method B) to hydrolyze the epoxide byproducts, by including a pH neutralization step and addition of more catalyst (1 mol%). Under the optimized conditions, both methyl oleate and oleic acid are converted into high yields of the corresponding carboxylic acids (80-85%). Overall, this catalytic system provides an alternative to the industrial ozonolysis of oleic acid and to catalytic Ru- and Os-based systems for the oxidative cleavage of unsaturated fatty acids and esters.
    Catalysis Science & Technology 01/2014; 4(3):708-716. · 4.76 Impact Factor
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    ABSTRACT: The synthesis and characterization of monomeric and dendritic Grubbs II and Hoveyda–Grubbs II-based complexes are reported. These complexes were synthesized via a route based on the connection of monomeric or dendritic N-alkyl-N′-mesitylimidazol-2-ylidene pre-ligands to Grubbs I or Hoveyda–Grubbs I complexes. The immobilization of a modified Grubbs II type catalyst on a G0 carbosilane dendrimer was successfully carried out. Together with monomeric Grubbs II and Hoveyda–Grubbs-analogs and several commercially available olefin metathesis catalysts, the soluble, homogeneous G0-dendritic Grubbs II complex was tested as catalyst in the ring closing metathesis of diethyl diallylmalonate. The immobilized complex proved to outperform its monomeric analog in this reaction at room temperature, whereas it was found to be slightly slower at reflux temperature.
    Inorganica Chimica Acta 01/2014; 409:163–173. · 2.04 Impact Factor
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    ABSTRACT: A one-pot method has been developed for the oxidative cleavage of internal alkenes into aldehydes by using 0.5 mol % of the nonheme iron complex [Fe(OTf)2 (mix-bpbp)] (bpbp=N,N'-bis(2-picolyl)-2,2'-bipyrrolidine) as catalyst and 1.5 equivalents of hydrogen peroxide and 1 equivalent of sodium periodate as oxidants. A mixture of diastereomers of the chiral bpbp ligand can be used, thereby omitting the need for resolution of its optically active components. The cleavage reaction can be performed in one pot within 20 h and under ambient conditions. Addition of water after the epoxidation, acidification and subsequent pH neutralization are crucial to perform the epoxidation, hydrolysis, and subsequent diol cleavage in one pot. High aldehyde yields can be obtained for the cleavage of internal aliphatic double bonds with cis and trans configuration (86-98 %) and unsaturated fatty acids and esters (69-96 %). Good aldehyde yields are obtained in reactions of trisubstituted and terminal alkenes (62-63 %). The products can be easily isolated by a simple extraction step with an organic solvent. The presented protocol involves a lower catalyst loading than conventional methods based on Ru or Os. Also, hydrogen peroxide can be used as the oxidant in this case, which is often disproportionated by second- and third-row metals. By using only mild oxidants, overoxidation of the aldehyde to the carboxylic acid is prevented.
    Chemistry - A European Journal 09/2013; · 5.93 Impact Factor
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    ABSTRACT: A study of the influence of reaction conditions on the Pd/TOMPP-catalyzed (TOMPP = tris(2-methoxyphenyl)phosphine) telomerization of 1,3-butadiene with phenols revealed that the composition of the reaction medium strongly influences the regioselectivity of the telomer products. Mechanistic studies show this effect to be related to the stability of the key catalytic intermediate cis-[Pd((1–3η)-octa-2,7-dien-1-yl)(TOMPP)2]+ (E), which mainly depends on the solvent composition. The crotyl analogue cis-[Pd((1–3η)-but-2-en-1-yl)(TOMPP)2]BF4 (4) was synthesized as a model for E and was fully characterized, including by an X-ray crystal structure determination. Comparison of the crystal structures of 4 and [Pd((1–3,7,8η)-octa-2(E),7-dien-1-yl)(TOMPP)]BF4 (1), an analogue of intermediate C, shows that the difference in regioselectivity for nucleophilic attack of these complexes is not reflected in the structure of the allyl moiety. Furthermore, Pd/TOMPP-catalyzed n/iso equilibration reactions of telomers show that all product formation routes in the Pd-catalyzed telomerization mechanism are reversible. Telomer product equilibration studies resulted in complete conversion to 1,3,7-octatriene, with the selectivity changing over time in favor of formation of the Z isomer of octatriene.
    Organometallics 09/2013; 32(18):5047–5057. · 4.25 Impact Factor
  • Mozaffar Shakeri, Robertus J M Klein Gebbink, Petra E de Jongh, Krijn P de Jong
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    ABSTRACT: Ship shape! Chiral (salen)Co(III) complexes (spheres) inside plugged nanochannels of SBA-15 materials is achieved using a ship-in-a-bottle synthesis technique. The local concentration of the metal complexes and the catalytic activity (such as the hydrolytic kinetic resolution of 1,2-epoxyalkanes; see scheme) showed a strong dependence on the size of the window.
    Angewandte Chemie International Edition 08/2013; · 11.34 Impact Factor
  • Ties J Korstanje, Johann T B H Jastrzebski, Robertus J M Klein Gebbink
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    ABSTRACT: Rhenium-based complexes are powerful catalysts for the dehydration of various alcohols to the corresponding olefins. Here, we report on both experimental and theoretical (DFT) studies into the mechanism of the rhenium-catalyzed dehydration of alcohols to olefins in general, and the methyltrioxorhenium-catalyzed dehydration of 1-phenylethanol to styrene in particular. The experimental and theoretical studies are in good agreement, both showing the involvement of several proton transfers, and of a carbenium ion intermediate in the catalytic cycle.
    Chemistry - A European Journal 08/2013; · 5.93 Impact Factor
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    Vital A. Yazerski, Ane Orue, Tim Evers, Henk Kleijn, Robertus J.M. Klein Gebbink
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    ABSTRACT: A convenient approach for the anchoring of S,S-BnTsDPEN ligands (S,S-N-tosyl-1,2-diphenylethylenediamine) to branched carbosilane scaffolds was investigated. It is based on a high-yielding reductive amination reaction between commercially available S,S-TsDPEN and readily accessible carbosilanes furnished with benzaldehyde terminal fragments. These molecularly enlarged ligands, bearing four S,S-BnTsDPEN units, and their simplified monomeric and “dimeric” analogues were evaluated in iron(III)-catalyzed asymmetric trans-stilbene epoxidation reactions using hydrogen peroxide as an environmentally benign oxidant. The catalytic investigations showed a large degree of variation in the activity and stereoselectivity of the series of DPEN catalysts. In combination with ESI-MS investigations, these data revealed an important role of the ligand orientation in determining the overall activity of the catalyst system. Accordingly, a suitable design of the molecularly enlarged ligands resulted in fully retained activity and selectivity in catalysis. Finally, a number of strategies for the recovery and reuse of the best performing carbosilane-tethered DPEN ligands were explored.
    Catalysis Science & Technology 07/2013; 3(10):2810-2818. · 4.76 Impact Factor

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