7Li NMR Spectroscopy: A Tool for Determining Dimerization Constants and Averaged Dimerization Constants of the Monomer/Dimer Equilibrium of Hierarchical Helicates.

7Li nuclear magnetic resonance (NMR) spectroscopy is an ideal tool to study hierarchically assembled helicates of the form Li[Li3L6Ti2]. Internally bound and external lithium ions can be well distinguished by solution- or solid-state NMR spectroscopy and dimerization constants of the monomer/dimer equilibrium can be easily determined in solution. Averaged dimerization constants can be estimated in case of statistical mixtures of helicates formed from mixtures of ligands.

Shedding Light on the Interactions of Hydrocarbon Ester Substituents upon Formation of Dimeric Titanium(IV) Triscatecholates in DMSO Solution.

The dissociation of hierarchically formed dimeric triple lithium bridged triscatecholate titanium(IV) helicates with hydrocarbyl esters as side groups is systematically investigated in DMSO. Primary alkyl, alkenyl, alkynyl as well as benzyl esters are studied in order to minimize steric effects close to the helicate core. The 1 H NMR dimerization constants for the monomer-dimer equilibrium show some solvent dependent influence of the side chains on the dimer stability. In the dimer, the ability of the hydrocarbyl ester groups to aggregate minimizes their contacts with the solvent molecules. Due to this, most solvophobic alkyl groups show the highest dimerization tendency followed by alkenyls, alkynyls and finally benzyls. Furthermore, trends within the different groups of compounds can be observed. For example, the dimer is destabilized by internal double or triple bonds due to π-π repulsion. A strong indication for solvent supported London dispersion interaction between the ester side groups is found by observation of an even/odd alternation of dimerization constants within the series of n-alkyls, n-Ω-alkenyls or n-Ω-alkynyls. This corresponds to the interaction of the parent hydrocarbons, as documented by an even/odd melting point alternation.

Kinetic investigation of the dissociation of dinuclear hierarchically assembled titanium(iv) helicates.

Hierarchically assembled helicates consisting of lithium-bridged triscatecholate titanium(iv) complexes represent a powerful self-assembled supramolecular system with applications as e.g. molecular balances for the evaluation of weak interactions, stereoselectivity switches in asymmetric synthesis or molecular switches. Their applications or properties are based on the monomer-dimer equilibrium which can be easily observed in solution. The dimer is the only species in the crystal. After dissolution, the dimer slowly dissociates into the monomer until the equilibrium is reached. This can be observed by NMR spectroscopy to investigate the kinetics of the dissociation process. A strong steric effect can be observed by studying differently substituted ligands. Activation energies of the dissociation process can be correlated with the size of the ester substituents and the system may be described as a molecular buckle.

From One-Pot NH-Sulfoximidations of Thiophene Derivatives to Dithienylethene-Type Photoswitches.

Thiophene NH-sulfoximines have been synthesized using a one-pot NH-sulfoximidation reaction of thiophenes. The reactivity of the products was investigated, and the developed protocols were used for the synthesis of a new class of dithienylethene-type photoswitches containing a sulfoximidoyl group.

A Helicate-Based Three-State Molecular Switch.

The control of structural transformations triggered by external signals is important for the development of novel functional devices. In the present study, it is demonstrated that helicates can be designed to structurally respond to the presence of different counterions and to adopt either a compressed or an expanded structure. Reversible switching is not only possible between those two states, furthermore, the twist of the aggregate also can be controlled. Thus, three out of four possible states of a helicate (expanded/left-handed, expanded/right-handed, compressed/left-handed) based on an enantiomerically pure ester bridged dicatecholate ligand are specifically addressed by introduction, exchange, or removal of countercations. This approach is used to reversibly switch between the different states or to successively address them.

Chasing Weak Forces: Hierarchically Assembled Helicates as a Probe for the Evaluation of the Energetics of Weak Interactions.

London dispersion forces are the weakest interactions between molecules. Because of this, their influence on chemical processes is often low, but can definitely not be ignored, and even becomes important in cases of molecules with large contact surfaces. Hierarchically assembled dinuclear titanium(IV) helicates represent a rare example in which the direct observation of London dispersion forces is possible in solution even in the presence of strong cohesive solvent effects. Hereby, the dispersion forces do not unlimitedly support the formation of the dimeric complexes. Although they have some favorable enthalpic contribution to the dimerization of the monomeric complex units, large flexible substituents become conformationally restricted by the interactions leading to an entropic disadvantage. The dimeric helicates are entropically destabilized.

Palladium/Copper-Cocatalyzed Oxidative Amidobrominations of Alkenes.

In the presence of LiBr, a palladium/copper combination catalyzes dehydrogenative amidobrominations of acrylates with NH-sulfoximines, leading to N-vinylated products by dual NH/CH coupling, followed by oxidative enamide bromination. Mechanistically, the domino process is proposed to involve palladium(II) species as key intermediates. First synthetic applications of the products have been demonstrated.

Copper- and Vanadium-Catalyzed Oxidative Cleavage of Lignin using Dioxygen.

Transition-metal-containing hydrotalcites (HTc) and V(acac)3 /Cu(NO3 )2 ⋅3 H2 O (acac=acetylacetonate) mixtures were tested for their catalytic activity in the cleavage of the lignin model compound erythro-1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-l,3-propanediol (1) with molecular oxygen as oxidant. Both catalytic systems displayed high activity and good selectivity and afforded veratric acid as the main product. The catalyst behavior was studied by EPR spectroscopy, XRD, and Raman spectroscopy. After the catalysts were established for the model system, lignin depolymerization studies were performed with various organsolv and kraft lignin sources. The oxidative depolymerization and lignin bond cleavage were monitored by gel permeation chromatography (GPC), MALDI MS, and 2D-NMR (HSQC). Irrespective of the lignin pretreatment, both HTc-Cu-V and V(acac)3 /Cu(NO3 )2 ⋅3 H2 O were able to cleave the ß-O-4 linkages and the resinol structures to form dimeric and trimeric products.

Solvent-dependent enthalpic versus entropic anion binding by biaryl substituted quinoline based anion receptors.

Anion receptors based on an 8-thiourea substituted quinoline with pentafluorinated (1a) or nonfluorinated (1b) biarylamide groups in the 2-position show similar binding of halide anions with somewhat higher association constants for the more acidic fluorinated derivative. Surprisingly, binding affinities for the halides in the case of the nonfluorinated 1b are similar in nonpolar chloroform or polar DMSO as solvent. Thorough thermodynamic investigations based on NMR van't Hoff analysis show that anion binding in chloroform is mainly enthalpically driven. In DMSO, entropy is the driving force for the binding of the ions with replacement of attached solvent.

Saturation transfer difference NMR studies of the interaction of the proteinkinase CK2 with peptides.

Saturation Transfer Difference NMR (STD NMR) is used for the detection of the binding constant of the decapeptide RRRDDDSDDD with CK2α, the catalytic subunit of the proteinkinase CK2. For this work a valid irradiation frequency of the CK2α had to be found ensuring that no peptide resonance is affected by the irradiation. This is the principle problem for investigations of protein peptide interactions by STD NMR due to the similarity of protein and peptide resonances. It is shown that by careful selection of the irradiation point a KD value averaging to 1 mM can be found. In addition, preferred binding sites of the peptide are detected and it is shown that the side chains of serine and aspartate are closest to the protein surface.

13C-NMR detection of STD spectra.

We have investigated the use of (13)C for the detection of saturation transfer difference (STD) NMR spectra. By detecting the STD spectrum in the (13)C channel it is possible to eliminate the residual water signal in the STD-NMR spectrum. We have employed an INEPT transfer in order to shift the magnetization from the proton channel to (13)C. As a sample system to check our method we have used human serum albumin and phenylalanine. We have shown that such a transfer can be accomplished and gives reasonable signal intensities.