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Ruth M Gschwind

Publications and source records attributed to Ruth M Gschwind.

9 recordsLinked to original sources
PubMed2005

Influence of tetrahydrofuran on reactivity, aggregation, and aggregate structure of dimethylcuprates in diethyl ether.

The comprehension of factors influencing the reactivity of organocuprates is still far from enabling a rational control of their reactions. Especially the degree of aggregation and structures of organocuprates are the focus of discussion about the factors affecting their reactivity. Therefore, this study combines kinetic measurements and NMR investigations to elucidate the influence of disaggregation via addition of tetrahydrofuran (THF) on the reactivity and aggregate structure of Gilman cuprates. As model systems, Me(2)CuLi.LiI (1.LiI) and Me(2)CuLi.LiCN (1.LiCN) in diethyl ether (DEE) were chosen; as model reaction, the 1,4-addition to 4,4-dimethylcyclohex-2-enone. The kinetic data show for 1.LiI a pronounced acceleration effect upon addition of distinct amounts of THF, whereas the reactivity of 1.LiCN continuously decreases with the addition of THF. Series of NMR diffusion measurements as well as (1)H-(7)Li heteronuclear Overhauser effect (HOE), and (1)H-(1)H nuclear Overhauser effect (NOE) spectra show different structural influences of THF on 1.LiI and 1.LiCN. For 1.LiI, small salt units are separated from the cuprate aggregate by THF. In contrast to this, THF disaggregates the oligomeric structures of 1.LiCN, while the core structures remain intact with salt attached. Thus, the reactivity of 1.LiI seems to be fine-tuned through distinct amounts of salt or THF, whereas the decreasing reactivity of 1.LiCN correlates with the disaggregation of oligomers via THF. Thus, for synthetic chemists with reactivity problems in specific reactions of iododialkylcuprates, the addition of small amounts of THF might be useful to enhance the reactivity. In addition to these structure-reactivity studies, the CN(-) group is shown to be directly attached to the cuprate moiety via a combination of (1)H-(13)C HOE- and (1)H-(1)H NOEs. This represents the first direct experimental evidence in solution for the position of the CN(-) group relative to the cuprate moiety in cyano-Gilman cuprates.

Journal Article↗
PubMed2005

Controlling the rate of shuttling motions in [2]rotaxanes by electrostatic interactions: a cation as solvent-tunable brake.

A series of rotaxanes, with phenolic axle centerpieces and tetralactam macrocycles as the wheels, has been prepared in good yields. The threaded rotaxane structure is confirmed in the gas phase by tandem mass spectrometric experiments through a detailed fragmentation pattern analysis, in solution by NMR spectroscopy, and in the solid state through X-ray crystallography. A close inspection of the 1H,1H NOESY and 1H,1H ROESY NMR data reveals the wheel to travel along the axle between two degenerate diamide "stations" close to the two stoppers. By deprotonation of a phenolic OH group in the axle centerpiece with Schwesinger's P1 base, surprisingly no additional shuttling station is generated at the axle center, although the wheel could form rather strong hydrogen bonds with the phenolate. Instead, the wheel continues to travel between the two diamide stations. Experimental data from 1H,1H NOESY spectra, together with theoretical calculations, show that strong electrostatic interactions between the phenolate moiety and the P1 cation displace the wheel from the "phenolate station". The cation acts as a "brake" for the shuttling movement. Instead of suppressing the shuttling motion completely, as observed in other rotaxanes, our rotaxane is the first system in which electrostatic interactions modulate the speed of the mechanical motion between a fast and a slow motion state as a response to a reversible external stimulus. By tuning these electrostatic interactions through solvent effects, the rate of movement can be influenced significantly, when for example different amounts of DMSO are added to dichloromethane. Besides the shuttling motion, circumrotation of the wheel around the axle is observed and analyzed by variable temperature NMR spectroscopy. Force field and AM1 calculations are in good agreement with the experimental findings.

Journal Article↗
PubMed2004

NMR detection of intermolecular NH.OP hydrogen bonds between guanidinium protons and bisposphonate moieties in an artificial arginine receptor.

Hydrogen bonding plays a major role in the selective recognition of guanidinium groups by receptor molecules. The present NMR investigation provides direct experimental evidence of hydrogen bonds in an artificial arginine receptor complex consisting of alpha-N-benzoylarginine ethyl ester and a bisphosphonate tweezers molecule. trans-Hydrogen bond 2hJHP couplings between the phosphonate moieties and individual guanidinium protons as well as the amide proton have been detected by [1H,31P]-HMBC and [31P,1H]-INEPT experiments. The detected hydrogen bonding network in the investigated artificial arginine receptor shows a symmetrical end-on interaction of the guanidinium moiety, which enables concerted rotations and deviates from the structure proposed for the biological arginine fork.

Arginine↗
PubMed2004

Gs-HSQC-NOESY versus gs-NOESY-HSQC experiments: signal attenuation due to diffusion; application to symmetrical molecules.

Gradient-selected (gs) HSQC-NOESY type experiments are often applied in order to obtain NOE cross peaks in symmetrical molecules or in complex organic molecules, e.g. carbohydrates. Since the coherence-selecting gradients in these pulse sequences are separated by several delays, including the mixing time, the gs-HSQC-NOESY spectra exhibit severe signal attenuation due to diffusion effects. The respective NOE information unaffected by diffusion effects can be obtained by X-half-filtered gs-NOESY-HSQC spectra. In this paper, an X-half-filtered version of the gs-NOESY-HSQC experiment is presented and the NOE cross peak integrals of its spectra are compared with those of a standard gs-HSQC-NOESY experiment. With phenanthrene as an example of symmetrical molecules, it is shown that the relative signal attenuation of gs-HSQC-NOESY cross peaks versus gs-NOESY-HSQC cross peaks is caused by diffusion effects and follows the Stejskal-Tanner equation. In contrast, the X-half-filtered gs-NOESY-HSQC experiment provides NOE cross peaks with reasonable signal-to-noise ratios even for long-range interactions in the presence of quadrupolar relaxation. This is demonstrated by the spectra of lithium dimethylcuprate.

Journal Article↗
PubMed2003

Dimethyl- and bis[(trimethylsilyl)methyl]cuprates show aggregates higher than dimers in diethyl ether: molecular diffusion studies by PFG NMR and aggregation-reactivity correlations.

The molecular sizes of higher aggregates of dimethylcuprates (Me(2)CuLi (1), 1.LiI, and 1.LiCN) and bis[(trimethylsilyl)methyl]cuprates ((Me(3)SiCH(2))(2)CuLi (2), 2.LiI, and 2.LiCN) in diethyl ether (Et(2)O) were determined by pulsed field gradient (PFG) NMR diffusion measurements. The obtained diffusion coefficients show molecular sizes larger than those of dimers for all systems. In these higher aggregates, steric hindrance and dilution reduce aggregation, whereas LiCN increases it. The molecular sizes were first determined by a spherical model-free approach and then refined by structure models of higher aggregates. These models were built by a combination of diffusion results, known NMR studies, and crystal structures. Thus, polymeric chains with homodimeric cores connected by solvent (salt-free case) or solvent and salt (salt-containing case) were proposed. These models were confirmed by a solvation analysis, whereby the number of solvent molecules attached to the aggregates was determined by a weighted average study. On the basis of these structure models, the number of repetition units (length index) was determined to be between 1.3 and 5.2, with the general trends in aggregation independent of the structure model used. A combined analysis of the determined length indices and known relative reactivities led for the first time to a correlation between higher aggregation and reactivity of dimethylcuprates in the addition reaction with enones: aggregates higher than dimers reduce the reactivity. Consequently, despite their consistent homodimeric core structures, for the first time the remaining reactivity differences between iodo- and cyanodimethylcuprates in Et(2)O are explained by the difference in their aggregation.

Journal Article↗
PubMed2002

1,8-bis(tetramethylguanidino)naphthalene (TMGN): a new, superbasic and kinetically active "proton sponge".

1,8-Bis(tetramethylguanidino)naphthalene (TMGN, 1) is a new, readily accessible, and stable "proton sponge" with an experimental pK(BH(+)) value of 25.1 in MeCN, which is nearly seven orders of magnitude higher in basicity than the classical proton sponge 1,8-bis(dimethylamino)-naphthalene (DMAN). Because of the sterically less crowded character of the proton-accepting sp(2)-nitrogen atoms, TMGN also has a higher kinetic basicity than DMAN, which is shown by time-resolved proton self-exchange reactions. TMGN is more resistant to hydrolysis and is a weaker nucleophile towards the alkylating agent EtI in comparison to the commercially available guanidine 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD). Crystal structures of the free base, of the mono- and bisprotonated base were determined. The dynamic behavior of all three species in solution was investigated by variable-temperature (1)H NMR experiments. DeltaG (++) values obtained by spectra simulation reveal a concerted mechanism of rotation about the C-N bonds of the protonated forms of TMGN.

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