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Eberhardt Herdtweck

Publications and source records attributed to Eberhardt Herdtweck.

At least 19 recordsLinked to original sources
PubMed2006

Solid-state NMR of a paramagnetic DIAD-FeII catalyst: sensitivity, resolution enhancement, and structure-based assignments.

A general protocol for the structural characterization of paramagnetic molecular solids using solid-state NMR is provided and illustrated by the characterization of a high-spin Fe(II) catalyst precursor. We show how good NMR performance can be obtained on a molecular powder sample at natural abundance by using very fast (>30 kHz) magic angle spinning (MAS), even though the individual NMR resonances have highly anisotropic shifts and very short relaxation times. The results include the optimization of broadband heteronuclear (proton-carbon) recoupling sequences for polarization transfer; the observation of single or multiple quantum correlation spectra between coupled spins as a tool for removing the inhomogeneous bulk magnetic susceptibility (BMS) broadening; and the combination of NMR experiments and density functional theory calculations, to yield assignments.

Algorithms↗
PubMed2006

Stereoselective allyl transfer to chiral alpha-methoxycarbaldehydes: a model study related to the C-9/C-15 fragment of geldanamycin.

The enantiomerically pure alpha-methoxycarbaldehyde 3 was prepared from l-leucine in five steps and 31% overall yield. The aldehyde was subjected to a diastereoselective BF3-mediated crotylation with silane 4 and to various reagent-controlled addition reactions. The configuration of aldol addition products 19 and 20 was proven by single crystal X-ray crystallography. Based on these data spectral comparison allowed an unambiguous assignment of the desired anti,syn-crotylation product 2b and of the syn,syn-crotylation product 2a. Unexpectedly, product 2a of the BF3-mediated crotylation is formally the product of chelation-control. The anti,syn-crotylation product 2b, which is a model for the C-9/C-15 fragment of geldanamycin, was obtained by a reagent-controlled crotylation with the chiral (Z)-crotylborane 23.

Aldehydes↗
PubMed2006

Strong hydrogen bonds in the ionic pair (1,3-diisopropyl-4,5-dimethyl-4-imidazolin-2-ylidene)ammonium trichloro(1,3-diisopropyl-4,5-dimethyl-4-imidazolin-2-ylideneamine)iron(II).

The title compound, (C11H22N3)[FeCl3(C11H21N3)], is one of the rare examples where an isolated ionic pair of the type [A]n+ [EMX3]n- (E is any non-metal, M is any transition metal and X is any halogen) could be structurally characterized. Two short N-H...Cl contacts between the two ammonium H atoms and two of the three Cl atoms of the counter-anion generate a six-membered ring. The third Cl atom is involved in a weaker intramolecular hydrogen bond to the neutral 1,3-diisopropyl-4,5-dimethyl-4-imidazolin-2-ylideneamine ligand.

Journal Article↗
PubMed2005

Grafting of [Mn(CH2tBu)2(tmeda)] on silica and comparison with its reaction with a silsesquioxane.

The reaction of [Mn(CH2tBu)2(tmeda)] (1) and a silica partially dehydroxylated at 700 degrees C (SiO(2/700)) yields a single surface species [([triple bond]SiO)Mn(CH2tBu)(tmeda)] (2a; tmeda = tetramethylethylendiamine), while a mixture of 2a and [([triple bond]SiO)2Mn(tmeda)] (2b) is obtained by using SiO(2/200), SiO(2/300), or SiO(2/500) as evidenced by mass balance analysis, and IR and EPR spectroscopy. The reaction of 1 and (c-C5H9)7Si7O12SiOH (3), a soluble silanol that is a molecular model for a silica support, generates the bis-siloxy complex 4, [[(c-C5H9)7Si7O12SiO]2Mn(tmeda)2], in a quantitative yield; compound 4 was characterized by single-crystal X-ray diffraction. These reactions exemplify the limitation of considering molecular silanol derivatives as straightforward and reliable homogeneous models for silica, and address the need for thorough characterization of surface species by the use of surface-science techniques. These studies show the possibility of preparing coordinatively and geometrically unique surface species that would be difficult to prepare by solution chemistry methods; insights into the chemical and physical properties of these surface species are also gained.

Journal Article↗
PubMed2005

Imidazolin-2-iminato titanium complexes: synthesis, structure and use in ethylene polymerization catalysis.

The Staudinger reaction of the imidazolin-2-ylidenes, 1,3-di-tert-butylimidazolin-2-ylidene (1a), 1,3-diisopropylimidazolin-2-ylidene (1b), 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene (1c), 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (1d) and 1,3-bis(2,6-diisopropylphenylimidazolin-2-ylidene (1e), with trimethylsilyl azide furnishes the corresponding N-silylated 2-iminoimidazolines 2a-e, which react with [(eta-C5H5)TiCl3] to afford half-sandwich cyclopentadienyl titanium complexes of the type [CpTi(L)Cl2] (3) (L = imidazolin-2-iminato ligand). Similarly, the reactions of 1,3-di-tert-butyl-2-(trimethylsilylimino)imidazoline (2a) with [(eta-tBuC5H4)TiCl3] results in the formation of [(eta-tBuC5H4)Ti(L)Cl2] (4) (L = 1,3-di-tert-butylimidazolin-2-imide). Bis(1,3-di-tert-butylimidazolin-2-iminato)titanium dichloride (5) is obtained from the reaction of two eq. of 2a with TiCl4. Treatment of 5 with methyllithium results in the formation of the corresponding dimethyl complex [L2Ti(CH3)2] (6), whereas [CpTi(L)(CH3)2] (7) is similarly obtained from 3a. The molecular structures of 3a, 3b, 3c, 3e x C7H8, 4 and 7 are reported revealing linearly coordinated imidazolin-2-iminato ligands together with very short Ti-N bond distances. All dichloro complexes (3a-e, 4 and 5) can be activated with methylaluminoxane (MAO) to give active catalysts for ethylene homopolymerization. In most cases, moderate to high activities are observed together with the formation of high (HMWPE) or even ultra high molecular weight polyethylene (UHMWPE).

Catalysis↗
PubMed2005

Regioselective Si-C bond activation in silicon-bridged ansa-cycloheptatrienyl-cyclopentadienyl complexes.

On treatment with [Pt(PEt3)3], silicon-bridged ansa-cycloheptatrienyl-cyclopentadienyl Ti and V complexes, [1]silatroticenophane and [1]silatrovacenophane, undergo oxidative addition and regioselective insertion of a Pt(PEt3)2 moiety into the silicon-carbon bond to the seven-membered ring; the resulting complexes, [2]platinasilatroticenophane and [2]platinasilatrovacenophane, can be partly used as single-source catalysts for the ring-opening polymerization (ROP) of the original highly strained sandwich molecules.

Journal Article↗
PubMed2004

Solvent-ligated manganese(II) complexes for the homopolymerization of isobutene and the copolymerization of isobutene and isoprene.

Polyisobutenes with a high content of terminal olefinic groups can be synthesized by using manganese(II) initiators in homogeneous solution. These easily accessible complexes initiate the polymerization at room temperature and above, and afford highly reactive, gel-free polyisobutenes with high viscosities. Furthermore, the initiators were successfully used for the copolymerization of isobutene with isoprene. The high activities of the Mn(II) initiators seem to be related to their weakly coordinating nitrile ligands, which are easily displaced by substrate molecules. Replacing the nitrile ligands by other more strongly coordinating ligands such as water reduces the initiator activity significantly. The Mn(II) initiators are surprisingly resistant to temperature.

Journal Article↗
PubMed2004

Nonclassical vs classical metal...H3C-C interactions: accurate characterization of a 14-electron ruthenium(II) system by neutron diffraction, database analysis, solution dynamics, and DFT studies.

A neutron diffraction study of the complex RuCl(2)[PPh(2)(2,6-Me(2)C(6)H(3))](2) (1) defines the precise nature of the delta agostic interactions between the unsaturated metal center and two o-methyl groups of the xylyl substituents. The CH(3) carbon atoms lie in the RuP(2) equatorial plane with Ru...C distances of 2.637(7) and 2.668(6) A, whereas four short Ru.H distances (from 2.113(11) to 2.507(11) A) indicate that each methyl group interacts with two C-H bonds. A survey of the X-ray structures with beta, gamma, delta, and epsilon M...H(3)C-C moieties (no neutron data have been previously reported) shows a linear correlation between the angle M.C-C and the torsion of the methyl group about the C-C bond. Thus, the agostic interactions span the range between the classical (M...eta(2)-HC) and the nonclassical (M...eta(3)-H(2)C) types. A solution study of 1 shows intramolecular rearrangement of each xylyl substituent that equilibrates the environments of its two ortho CH(3) groups. Activation parameters, evaluated from the analysis of (1)H NMR line shape as a function of temperature, are Delta H(++) = 9.6 +/- 0.2 kcal mol(-1) with Delta S(++) = -15.4 +/- 0.7 eu (CDCl(3)). The related 14-electron complexes RuX(2)[PPh(2)(2,6-Me(2)C(6)H(3))](2) (X = I, 2; NCO, 3), prepared from 1 and NaX, show a similar dynamic process in solution, with the iodo derivative displaying the most hindered rotation of the xylyl group. A DFT optimization of the complex RuCl(2)[PH(2)(2,6-Me(2)C(6)H(3))](2) (1a) reproduces well the nonclassical Ru...eta(3)-H(2)C agostic mode, whereas the classical Ru...eta(2)-HC one corresponds to a transition state 1b, destabilized by 3.4 kcal mol(-1). A similar barrier (ca. 3.8 kcal mol(-1)) is calculated for the xylyl rotation in the further simplified model RuCl(2)[PH(2)(2,6-Me(2)C(6)H(3))][PH(2)CH[double bond]CHCH(3)] (1c), the absence of bulky phenyl substituents being largely responsible for the difference with respect to the experimental value. Finally, the MO analysis addresses the intrinsic stability of the 14-electron complex RuCl(2)(PH(3))(2) and, in agostic complexes, accounts for the different interactions between the methyl group and the metal atom in relation to the length of their interconnecting chain.

Journal Article↗
PubMed2004

Enantioselective [4+2]-cycloaddition reaction of a photochemically generated o-quinodimethane: mechanistic details, association studies, and pressure effects.

1,2,3,4-Tetrahydro-2-oxoquinoline-5-aldehyde (2) was prepared from m-aminobenzoic acid and 3-ethoxyacryloyl chloride (4) in 19 % overall yield. Compound 2 underwent a photochemically induced [4+2]-cycloaddition reaction with various dienophiles upon irradiation in toluene solution. The exo product 10 a was obtained with acrylonitrile (9 a) as the dienophile, whereas methyl acrylate (9 b) and dimethyl fumarate (9 c) furnished the endo products 11 b and 11 c (69-77 % yield). The reactions proceeded at -60 degrees C in the presence of the chiral complexing agent 1 (1.2 equiv) with excellent enantioselectivity (91-94 % ee). The enantiomeric excess increases in the course of the photocycloaddition as a result of the lower product association to 1. The intermediate (E)-dienol 8 was spectroscopically detected at -196 degrees C in an EPA (diethyl ether/isopentane/ethanol) glass matrix. The association of the substrate 2 to the complexing agent 1 was studied by circular dichroism (CD) titration. The measured association constant (K(A)) was 589 M(-1) at room temperature (25 degrees C) and normal pressure (0.1 MPa). An increase in pressure led to an increased association. At 400 MPa the measured value of K(A) was 703 M(-1). Despite the stronger association the enantioselectivity of the reaction decreased with increasing pressure. At 25 degrees C the enantiomeric excess for the enantioselective reaction 2 + 9 a-->10 a decreased from 68 % ee at 0.1 MPa to 58 % ee at 350 MPa. This surprising behavior is explained by different activation volumes for the diastereomeric transition states leading to 10 a and ent-10 a.

Journal Article↗
PubMed2004

Titanium complexes with imidazolin-2-iminato ligands.

The reaction of 1,3-di-tert-butylimidazolin-2-ylidene with trimethylsilyl azide furnishes the corresponding N-silylated 2-iminoimidazoline which reacts with titanium(IV) chlorides to yield imidazolin-2-iminato titanium complexes.

Journal Article↗
PubMed2004

Highly substituted pyrrolidinones and pyridones by 4-CR/2-CR sequence.

[reaction: see text] By combining a Ugi four-component reaction of isocyanides, phosphonoacetic acids, primary amines, and glyoxals or alternatively 3-keto aldehydes with a subsequent Wittig ring-closing reaction (using the Horner/Wadsworth/Emmons variant (HWE)), highly substituted 5-oxo-2,5-dihydro-1H-pyrrole-2-carboxylic acid amides and 6-oxo-1,2,3,6-tetrahydro-pyridine-2-carboxylic acid amides can be assembled, respectively. The corresponding tandem of a Passerini reaction on 3-keto aldehydes and subsequent Wittig ring closure does not afford the expected six-membered 6-oxo-3,6-dihydro-2H-pyran-2-carboxylic acid amides but instead leads to the formation of 4-oxo-pent-2-enoic acid amides via an elimination route.

Journal Article↗
PubMed2003

Massive parallel catalyst screening: toward asymmetric MCRs.

[reaction: see text]. Hundreds of Lewis acid/ligand combinations have been screened for stereochemical induction in the Passerini multicomponent reaction. The combination of titan tetraisopropylate and (4S,5S)-4,5-bis(diphenylhydroxymethyl)-2,2-dimethyldioxolane was found to give enantiomeric excesses between 32% and 42% in several examples. The absolute stereo induction of one example was determined chemically and by means of X-ray crystallography. This comprises the first asymmetric Passerini reaction and the first example of a stereochemical induction in an isocyanide based multicomponent reaction by a chiral Lewis acid.

Journal Article↗