Jakub Grajewski

17O NMR studies of substituted 1,3,4-oxadiazoles.

Three series of substituted 1,3,4‐oxadiazoles were studied by 17O NMR spectroscopy. Chemical shifts values were correlated with empirical Hammett parameters as well as calculated bond lengths and chemical shielding values. Copyright

Extending the applications of circular dichroism in structure elucidation: aqueous environment breaks the symmetry of tartrate dianion

Electronic circular dichroism and quantum mechanical studies show that in aqueous solution tartrate dianions are non-symmetrically hydrated and their expected C2 symmetry is reduced to C1 while preserving the conformation of the carbon skeleton. Our results indicate that inclusion of explicit solvent (water) molecules in quantum mechanical calculations is instrumental in detailed studies on molecular structure in solution. Moreover, the presented combined experimental and theoretical approach is a unique demonstration of the capability of ECD spectroscopy to detect subtle structural changes of molecules in solution. In addition to the n-π* Cotton effect at ca. 212 nm, the CD spectra of (R,R)-tartrates in aqueous solution reveal the presence of a Cotton effect at ca. 193 nm which is, at least partly, the result of n-σ* excitation.

Intramolecular Interactions of Trityl Groups

Trityl group, Tr, is a molecular dynamic rotor of which the conformation and helicity depend on other groups in the close vicinity. Interactions with another covalently linked Tr group and with other substituents are analyzed in terms of transfer of chirality to the trityl group. Two trityl groups in a molecule can mutually interact at a distance of two, three, or five bonds. Despite its size, a Tr group attached to a cyclohexane or cyclopentane ring through an oxygen or nitrogen atom adopts either an axial or equatorial position, depending on additional stabilizing interactions, such as hydrogen bonding.

Benzhydryl ethers of tartaric acid derivatives: stereochemical response of dynamically chiral propeller

The benzhydryl (diphenylmethyl) group is a molecular propeller that can act as a chirality reporter if it is introduced nearby a stereogenic center by making an ether bond. The hydrophobic character of the benzhydryl group allows transformation of insoluble natural tartaric acid derivatives into soluble entities in a nonpolar environment. Electronic circular dichroism spectra, recorded within the short-wavelength region of the phenyl 1 B transitions (190-200 nm) shows strong bisignate Cotton effects. The signs and magnitudes of these Cotton effects are a function of absolute configuration and conformation of the molecule and do not primarily arise from exciton coupling of chiral benzhydryl chromophores. In crystals, the main-chain conformation is stabilized by intramolecular hydrogen bonds and CH-CO dipolar interactions. The number of the donor NH groups has a pronounced effect on the preferred conformations and inclusion properties of benzhydryl-(R,R)-tartaric acid diamides. Evidence is shown for the solvent dependency of the conformations of NH amides of tartaric acid diphenylmethyl ethers.

Folding of aromatic oligoimides of trans-1,2-diaminocyclohexane

Chiral oligomeric diimides prepared from pyromellitic dianhydride, (R,R)-1,2-diaminocyclohexane and phthalic anhydride fold into M or P helical conformers; trimer 1 folds into the P conformer in the crystal but the M conformer dominates in solution; longer chain oligomers 2 and 3 form preferentially P conformers in solution, as a result of intermolecular interactions.

One-step Construction of the Shape Persistent, Chiral But Symmetrical Polyimine Macrocycles

A unique combination of structural flexibility, shape persistency and functionality, makes macrocycles and molecular cages as essential molecular entities that have displayed applications that go beyond chemistry. Among macrocycles, the selectively obtained symmetrical (poly)cyclic polyimines have shown great utility in the design of molecules varied in shape and properties. The reversible and thermodynamically controlled cycloimination reaction is governed by configurational and conformational constraints imposed on the intermediate products, ensures a sufficiently high level of preorganization. The high geometrical control over the macrocycle structure has profound effect on their assembly mode. In this Account, we were interested in showing how the structure of small building blocks affects the structure of macrocyclic product and further, how influenced the association mode of the given macromolecule. The latter is of primarily importance in supramolecular and in material chemistry.

Multinuclear magnetic resonance studies of fluoronitroanilines

Ten fluoronitroanilines have been synthesized and the 1H, 13C, 15N and, 19F NMR spectra of these compounds have been recorded and fully assigned. Density functional theory(DFT) calculations have been performed for all compounds studied. Experimental and theoretical results are compared and the structure and atom character influence on the accuracy of the calculation discussed. Copyright