Gregor Schnakenburg

University of Bonn, Bonn, North Rhine-Westphalia, Germany

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Publications (210)902.81 Total impact

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    ABSTRACT: The proton-catalyzed transformation of 2-thioureidobenzonitriles to 4-acylimino-4H-3,1-benzothiazines was accomplished by treatment with carboxylic anhydrides, acid chlorides or alkyl chloroformates. The reaction involves a cyclization to 4-imino-3,1-benzothiazinium salts, whose thioimidate structure is trapped by a subsequent reaction with the acyl donor. 2-Ureidobenzonitriles do not undergo such an intermolecular tandem reaction. The different reaction behavior of both types of substrates was verified by NMR monitoring and employing 18O-enriched water.
    No preview · Article · Jan 2016 · RSC Advances
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    ABSTRACT: The marine-derived fungus Stachylidium sp. was isolated from the sponge Callyspongia sp. cf. C. flammea. Culture on a biomalt medium supplemented with sea salt led to the isolation of two new, most unusual N-methylated peptides, i.e., the tetrapeptides endolide A and B (1 and 2). Both of these contain the very rare amino acid 3-(3-furyl)-alanine. In radioligand binding assays endolide A (1) showed affinity to the vasopressin receptor 1A with a Ki of 7.04 μM, whereas endolide B (2) exhibited no affinity to the latter receptor, but was selective toward the serotonin receptor 5HT2b with a Ki of 0.77 μM.
    No preview · Article · Jan 2016 · Organic Letters
  • I Begum · G Schnakenburg · R Streubel
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    ABSTRACT: The facile regioselective synthesis of the P(iii) substituted thiazol-2-thione is presented. Reaction of with hydrogenperoxide-urea, elemental sulfur and selenium resulted in P(v) chalcogenide thiazol-2-thiones . All compounds were characterized using (31)P, (1)H, (13)C NMR, IR and elemental analyses and, additionally, by the single-crystal X-ray diffraction technique. Oxidative desulfurization of the 5-phosphinoylated thiazol-2-thione using hydrogenperoxide led to the first C-phosphanoyl substituted thiazolium salt (). Deprotonation of and in situ reaction with the cyclooctadiene rhodium(i) chloride dimer yielded thiazol-2-ylidene rhodium(i) complex which was confirmed by NMR spectroscopy and ESI-MS spectrometry.
    No preview · Article · Jan 2016 · Dalton Transactions
  • C Murcia García · A Espinosa Ferao · G Schnakenburg · R Streubel
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    ABSTRACT: P-CPh3 substituted oxaphosphirane complexes were prepared using Li/Cl phosphinidenoid complexes (M = Cr, Mo, W) and benzaldehyde. Employing and benzophenone resulted in the formation of oxaphospholane complexes and , the former bearing a benzo[c]-1,2-oxaphospholane and the latter a novel pentacyclic P-ligand. According to DFT studies the latter P-heterocycle arises from formal dimerization of a transient benzofused 2-phosphafurane complex , one of the fragments undergoing water-catalyzed [1,3]H shift () and the other () formed via elimination of HCPh3.
    No preview · Article · Jan 2016 · Dalton Transactions
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    ABSTRACT: The reaction of MesP[double bond, length as m-dash]CPh2 with the least sterically demanding N-heterocyclic carbene (NHC = IMe) results in formation of the 'abnormal' (C(4)-substituted) 4-phosphino-NHC (). In contrast, reaction with Me2IMe gives the unprecedented 'normal' C(2) adduct, Me2IMe → P(Mes)[double bond, length as m-dash]CPh2 (). Particularly striking is the asymmetric and weak bonding of the NHC to the P[double bond, length as m-dash]C moiety in . DFT calculations indicate that the P[double bond, length as m-dash]C natural bond order in (1.54) still reflects significant π-character to the bond (cf. MesP[double bond, length as m-dash]CPh2: NBO = 1.98). Further computational analysis suggests that π-delocalization into the remote C-phenyl substituents is key to stabilizing the NHC adduct.
    No preview · Article · Jan 2016 · Chemical Communications
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    ABSTRACT: The oxidative treatment of (m)ethyl 2-aryl cinnamates equipped with methoxy groups in position 4 of the phenyl moiety promote the formation of cyclohexadienone substructures. This dealkylative oxidative C-C coupling gives access to spirocyclic compounds and avoids the construction of the corresponding phenanthrenes. Furthermore, the transformation can be expanded to other spirocyclic systems. The dealkylative oxidative coupling of 2-aryl cinnamates containing a methoxy group results in the formation of spirocyclic products. Solely the position of the methoxy group determines that spiro compounds are formed upon treatment with molybdenum pentachloride and not the analogous phenanthrene products.
    No preview · Article · Dec 2015 · European Journal of Organic Chemistry
  • Niklas Struch · Gregor Schnakenburg · Arne Lützen
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    ABSTRACT: Mononuclear complexes are good model systems for evaluating the effects of different ligand systems on the magnetic properties of iron(II) centres. A novel crystal structure of the title compound, [Fe(C 18 H 24 N 10 )](BF 4 ) 2 ·CH 3 OH, with one molecule of methanol per formula unit exhibits a strictly sixfold coordination sphere associated with a low-spin configuration at the metal centre. The incorporated methanol solvent molecule promotes extended hydrogen-bonding networks between the tetrafluoridoborate anions and the cationic units. A less constrained crystal structure regarding close contacts between the tetrafluoridoborate anions and the cationic units allows a spin transition which is inhibited in the previously published hydrate of the title compound.
    No preview · Article · Dec 2015 · Acta crystallographica. Section C

  • No preview · Article · Dec 2015 · ChemInform
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    ABSTRACT: The reaction of Li/Cl phosphinidenoid tungsten(0) complex 2 [R = CH(SiMe3)2] with 1,3,4,5-tetramethylimidazol-2-ylidene (3) yielded the zwitterionic phosphaalkene complex 4. A comparative study on chloro- (5) or dichloro(organo)phosphane complexes 1 [R = CH(SiMe3)2] and NHC 3 revealed the formation of 4 at ambient temperature. First evidence for the formation of phosphinidenoid complex 7, having a C-Cl imidazolium cation, and its reaction with benzaldehyde to give oxaphosphirane complex 9 are presented. Furthermore, the first example of a formal exchange at the phosphorus center of an oxaphosphirane complex to give zwitterion 4 is demonstrated.
    No preview · Article · Nov 2015
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    ABSTRACT: Two novel supramolecular meso-helicates have been synthesized and characterized by single-crystal X-ray diffraction, variable-temperature magnetic susceptibility measurements, NMR and UV/ Vis spectroscopy, mass spectrometry and density functional theory (DFT) simulations. Both compounds show a considerably high stabilization of the high-spin state of the metal centres compared with other compounds with similar types of ligands. Whereas the pyridyl complex [Fe2L32] [L-2 = N, N'-bis(pyridin-2-ylmethylene) benzene-1,3-diamine] exhibits the beginning of a spin transition at around 350 K, the imidazolyl complex [Fe2L31] [L-1 = N, N'-bis(1H-imidazol-4-ylmethylene) benzene-1,3-diamine] still exhibits a high-spin configuration at 20 K. We applied DFT to characterize the molecular as well as solid states of both compounds. Although the low-spin state of both systems is stabilized by crystal packing, an additional stabilization of the high-spin state is induced by intramolecular interactions. This effect can be ascribed to mechanical strain in the backbone of the ligands hailing from very short CH-p interactions and being similar in effect to sterically demanding methyl groups.
    No preview · Article · Nov 2015 · European Journal of Inorganic Chemistry
  • Melina Klein · Gregor Schnakenburg · Arturo Espinosa Ferao · Rainer Streubel
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    ABSTRACT: The reaction of Li/Cl phosphinidenoid pentacarbonylmetal(0) complexes (R = CH(SiMe3)2; M = Cr, Mo, W) with bis(2-pyridyl)ketone led to overcrowded 3,3-bis(2-pyridyl)oxaphosphirane complexes . On heating (pyridine at 95 °C or THF at 60 °C) was transformed into complex which had a novel heterobicyclic P-ligand. In the case of the P-Cp* derivative the reaction led to a mixture of oxaphosphirane complex and complex , whereas only the novel heterobicyclic complex was formed in the case of the P-CPh3 substituted complex . Single-crystal X-ray analysis of confirmed the structure of the new ligand as an isomer of the oxaphosphirane which rearranged under loss of aromaticity of one pyridyl substituent. DFT calculations on the P-Me model derivative revealed that, despite the easier C-O bond cleavage in oxaphosphirane complex , only the P-C bond cleavage intermediate enables kinetically favoured P-N ring closure to give the bicyclic P-ligand in . Preliminary studies demonstrated that complex reacts with an in situ generated Ti(iii) complex to give the phosphaalkene complex , the hetero-dinuclear complex ·ZnCl2 and the P,N-chelate complex .
    No preview · Article · Oct 2015 · Dalton Transactions
  • Andreas Meyer · Gregor Schnakenburg · Olav Schiemann
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    ABSTRACT: The title compound, C 44 H 35 N 4 O 3 2.5(C 6 H 6 ) ( 1 ), consists of a terpyridine and a N -oxylpyrroline-3-formate group separated by an aromatic spacer, viz. 4-(phenylethynyl)-1,1′-biphenyl. It crystallized in the triclinic space group P -1 with two and a half benzene solvate molecules (one benzene molecule is located about an inversion center), while the dichloromethane solvate ( 2 ) of the same molecule [Ackermann et al. (2015). Chem. Commun. 51 , 5257–5260] crystallized in the tetragonal space group P 4 2 / n , with considerable disorder in the molecule. In ( 1 ), the terpyridine (terpy) group assumes an all- trans conformation typical for terpyridines. It is essentially planar with the two outer pyridine rings ( B and C ) inclined to the central pyridine ring ( A ) by 8.70 (15) and 14.55 (14)°, respectively. The planes of the aromatic spacer ( D , E and F ) are nearly coplanar with dihedral angles D / E , D / F and E / F being 3.42 (15), 5.80 (15) and 4.00 (16)°, respectively. It is twisted with respect to the terpy group with, for example, dihedral angle A / D being 24.48 (14)°. The mean plane of the N -oxylpyrroline is almost normal to the biphenyl ring F , making a dihedral angle of 86.57 (16)°, and it is inclined to pyridine ring A by 72.61 (15)°. The intramolecular separation between the O atom of the nitroxyl group and the N atom of the central pyridine ring of the terpyridine group is 25.044 (3) Å. In the crystal, molecules are linked by pairs of C—H...O hydrogen bonds, forming inversion dimers. The dimers stack along the c axis forming columns. Within and between the columns, the spaces are occupied by benzene molecules. The shortest oxygen–oxygen separation between nitroxyl groups is 4.004 (4) Å. The details of the title compound are compared with those of the dichloromethane solvate ( 2 ) and with the structure of a related molecule, 4′-{4-[(2,2,5,5-tetramethyl- N -oxyl-3-pyrrolin-3-yl)ethynyl]phenyl}-2,2′:6′,2′′-terpyridine ( 3 ), which has an ethynylphenyl spacer [Meyer et al. (2015). Acta Cryst. E 71 , 870–874].
    No preview · Article · Oct 2015
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    ABSTRACT: Eleven different derivatives of Tröger's base were synthesized and successfully resolved by (recycling) HPLC on chiral stationary (S,S)-Whelk-O1 or CHIRALPAK IA phase on a (semi-) preparative scale or synthesized in enantiomerically pure form. The absolute configuration was determined by X-ray crystal structure analysis, comparison of the quantum chemically calculated circular dichroism spectra with the experimental spectra, or comparison of specific rotations with literature data.
    No preview · Article · Aug 2015 · Synthesis
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    ABSTRACT: An efficient method for the synthesis of the NHC-stabilised Si(I) halides Si2X2(Idipp)2 (2-X, X = Cl, Br, I; Idipp = C[N(C6H3-2,6-iPr2)CH]2) was developed, which involves the oxidation of Si2(Idipp)2 (1) with 1,2-dihaloethanes. Halogenation of 1 is a diastereoselective reaction leading exclusively to a racemic mixture of the RR and SS enantiomers of 2-X. Compounds 2-Br and 2-I were characterised by single-crystal X-ray crystallography and multinuclear NMR spectroscopy, and their electronic structures were analysed by quantum chemical methods. Dynamic NMR spectroscopy unraveled a fluxional process of 2-Br and 2-I in solution, which involved a hindered rotation of the NHC groups about the Si–CNHC bonds. Iodide abstraction from 2-I by [Li(Et2O)2.5][B(C6F5)4] selectively afforded the disilicon(I) salt [Si2(I)(Idipp)2][B(C6F5)4] (3). X-ray crystallography and variable-temperature NMR spectroscopy of 3 in combination with quantum chemical calculations shed light on the ground-state geometric and electronic structure of the [Si2(I)(Idipp)2]+ ion, which features a SiSi bond between a trigonal planar coordinated SiII atom with a Si–I bond and a two-coordinate Si0 center carrying a lone pair of electrons. The dynamics of the [Si2(I)(Idipp)2]+ ion were studied in solution by variable-temperature NMR spectroscopy and they involve a topomerisation, which proceeds according to quantum theory via a disilaiodonium intermediate (“π-bonded” isomer) and exchanges the two heterotopic Si sites.
    No preview · Article · Aug 2015 · Chemical Science
  • Andreas Meyer · Gregor Schnakenburg · Robert Glaum · Olav Schiemann
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    ABSTRACT: The surprisingly complicated crystal structure of (bis(terpyridine))copper(II) tetraphenylborate [Cu(tpy)2](BPh4)2 (tpy = 2,2':6',2″-terpyridine) consists of six crystallographically independent [Cu(tpy)2](2+) complexes. At ambient temperature, five out of six [Cu(II)N6] chromophores appear to be compressed octahedra, while at 100 K, four exhibit elongated and only two compressed octahedral geometry. Temperature dependent single crystal UV/vis (100, 298 K) and EPR measurements (20, 100, 298 K) as well as AOM calculations suggest that the octahedra which show apparently compressed octahedral geometry (XRD) result from dynamic Jahn-Teller behavior of elongated octahedra [Cu(II)N6]. The detailed correlation of structural and spectroscopic data allows an understanding of the strongly solvent-dependent structures of the [Cu(tpy)2](2+) complex in solution.
    No preview · Article · Aug 2015 · Inorganic Chemistry
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    ABSTRACT: One-electron oxidation of the disilicon(0) compound Si2 (Idipp)2 (1, Idipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with [Fe(C5 Me5 )2 ][B(Ar(F) )4 ] (Ar(F) =C6 H3 -3,5-(CF3 )2 ) affords selectively the green radical salt [Si2 (Idipp)2 ][B(Ar(F) )4 ] (1-[B(Ar(F) )4 ). Oxidation of the centrosymmetric 1 occurs reversibly at a low redox potential (E1/2 =-1.250 V vs. Fc(+) /Fc), and is accompanied by considerable structural changes as shown by single-crystal X-ray structural analysis of 1-B(Ar(F) )4 . These include a shortening of the SiSi bond, a widening of the Si-Si-CNHC angles, and a lowering of the symmetry, leading to a quite different conformation of the NHC substituents at the two inequivalent Si sites in 1(+) . Comparative quantum chemical calculations of 1 and 1(+) indicate that electron ejection occurs from the symmetric (n+ ) combination of the Si lone pairs (HOMO). EPR studies of 1-B(Ar(F) )4 in frozen solution verified the inequivalency of the two Si sites observed in the solid-state, and point in agreement with the theoretical results to an almost equal distribution of the spin density over the two Si atoms, leading to quite similar (29) Si hyperfine coupling tensors in 1(+) . EPR studies of 1-B(Ar(F) )4 in liquid solution unraveled a topomerization with a low activation barrier that interconverts the two Si sites in 1(+) . © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    No preview · Article · Aug 2015 · Chemistry - A European Journal
  • Philipp A. Ottersbach · Gregor Schnakenburg · Michael Gütschow
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    ABSTRACT: Abstract Cbz-protected azadipeptides, designed as structurally reduced model compounds, were synthesized and investigated with respect to the occurrence of atropisomerism. Methylation at the carbamate nitrogen caused mixtures of E and Z isomers in Cbz-sarcosyl-azaglycine-amide (9) and Cbz-sarcosyl-methylazaalanine-amide (10). A formal O/S exchange led to Cbz-glycyl-azaglycine-thioamide (11) and Cbz-glycyl-methylazaalanine-thioamide (12), respectively. The (MeN)2 fragment, present in 10 and 12 (but not 9 and 11), serves as a chirality-inducing element owing to a restricted rotation around the N-N axis.
    No preview · Article · Aug 2015 · Tetrahedron Letters
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    ABSTRACT: Reaction of Li/Cl phosphinidenoid complexes 2a,b with diphenyl thioketone or N,N′-tetramethylthiourea yielded the bicyclic 1,2-thiaphospholane complex 4b or alkylidene(thioxo)phosphane complex 6a, respectively. DFT studies provide insight into the reaction pathways, pointing to the initial thiocarbonyl–phosphane–ylide complex 3, which can form the bicyclic 1,2-thiaphospholane complex 4. However, cyclization to give thiaphosphirane complex 5 also appears to be feasible, which via subsequent combined C–S bond cleavage and P-to-S haptotropic metal complex shift yields complex 6.
    No preview · Article · Jul 2015 · Organometallics
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    ABSTRACT: An efficient two-step synthesis of the first NHC-stabilized disilavinylidene (Z)-(SIdipp)SiSi(Br)Tbb (2; SIdipp=C[N(C6 H3 -2,6-iPr2 )CH2 ]2 , Tbb=C6 H2 -2,6-[CH(SiMe3 )2 ]2 -4-tBu, NHC=N-heterocyclic carbene) is reported. The first step of the procedure involved a 2:1 reaction of SiBr2 (SIdipp) with the 1,2-dibromodisilene (E)-Tbb(Br)SiSi(Br)Tbb at 100 °C, which afforded selectively an unprecedented NHC-stabilized bromo(silyl)silylene, namely SiBr(SiBr2 Tbb)(SIdipp) (1). Alternatively, compound 1 could be obtained from the 2:1 reaction of SiBr2 (SIdipp) with LiTbb at low temperature. 1 was then selectively reduced with C8 K to give the NHC-stabilized disilavinylidene 2. Both low-valent silicon compounds were comprehensively characterized by X-ray diffraction analysis, multinuclear NMR spectroscopy, and elemental analyses. Additionally, the electronic structure of 2 was studied by various quantum-chemical methods. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Full-text · Article · Jul 2015 · Angewandte Chemie International Edition
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    Andreas Meyer · Jennifer Wiecek · Gregor Schnakenburg · Olav Schiemann
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    ABSTRACT: The terpyridine group of the title compound, C 31 H 27 N 4 O, assumes an all- transoid conformation and is essentially planar with the dihedral angles between the mean planes of the central pyridine and the two outer rings amounting to 3.87 (5) and 1.98 (5)°. The pyrroline- N -oxyl group commonly seen in such nitroxyls is found in the title structure and the mean plane of the pyrroline ring subtends a dihedral angle of 88.44 (7)° to the mean plane of the central pyridine ring. The intramolecular separation between the nitrogen atom of the central pyridine unit of the terpyridine group and the nitroxyl group is 14.120 (2) Å. In the crystal, the molecules are arranged in layers stacked along [001]. Slipped face-to-face π–π interactions between the pyridine rings are observed along this direction with the shortest centroid–centroid distances amounting to 3.700 (1) and 3.781 (1) Å. Furthermore, edge-on C—H...π interactions between the phenylene rings of neighbouring molecules are observed along this direction. A two-dimensional C—H...O hydrogen-bonded network is formed within the (010) plane. The shortest O...O separation between neighbouring molecules is 5.412 (3) Å.
    Full-text · Article · Jul 2015