The electron-donating properties of eighteen N-heterocyclic carbenes (N,N'-bis(2,6-dimethylphenyl)imidazol)-2-ylidene and the respective dihydro ligands) with 4,4'-R substituted aryl rings (4,4'-R = NEt2, OMe, Me, H, SMe, F, Cl, Br, I) in the respective Grubbs II complexes were studied using electrochemical techniques. The nature of the 4-R substituent has a strong influence on the RuII/III redox potentials ranging between DeltaE1/2= +0.196 and +0.532 V. Three unsymmetrical Grubbs II complexes with 4-R not equal to 4-R' were also synthesized. Dynamic NMR spectroscopy revealed the restricted rotation around the (NHC)C-Ru bond (DeltaG = 89 kJ mol(-1) at 333 K) resulting in two atropisomers, respectively, with an isomer ratio close to unity. Each of the isomers, that is the two orientations of the 4-R/4-R' substituted mesityl ring with respect to the R=CHPh unit, gives rise to different redox potentials (4-R = NEt2, 4-R' = Br: DeltaE1/2= +0.232 and +0.451 V). In the oxidized Grubbs II complex (4-R = NEt2, H) and in the cathodic isomer the electron rich aryl ring is located above the Ru=CHPh unit. This orientational effect provides clear evidence for strong pi-pi through-space interactions in the RuIII complexes, assuming that the alternative through-bond transfer of electron density is equally efficient in both isomers.

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The determination of the three-dimensional structure of organic and biomolecular compounds by NMR spectroscopy usually involves the measurement of 3 J coupling constants, [1] NOEs, [2] and cross-correlated relaxation [3] to obtain information about dihedral angles, distances, and projection angles, respectively. If interconversion of conformers takes place and is fast on the NMR time scale, NMR spectroscopic parameters for flexible parts of the molecule are motionally averaged. This effect is one of the main complications in the structure determination of nonrigid molecules and often prohibits the determination of the relative configuration of organic compounds.It has been shown that residual dipolar couplings (RDCs) can yield information complementary to that obtained from 3 J coupling constants and NOE parameters also for organic compounds [4][5][6][7][8][9][10] and enable the assignment of relative configurations even in the presence of a limited degree of motion. [11][12][13][14][15] The problem of the joint treatment of an unknown configuration and conformational averaging when residual dipolar couplings are used in structure determination has scarcely been tackled. Herein two approaches are discussed, and it is shown that even conformer populations can be obtained from experimentally determined RDCs.The example chosen for illustration is a five-memberedring compound investigated recently by some of us. The amethylene-g-butyrolactone had been synthesized as single diastereoisomer, [16] the relative configuration of which (trans, denoted 1, or cis, denoted 2) was unknown and could not be determined by using conventional NMR spectroscopic parameters.[11] By using RDCs, however, it was possible to assign the relative configuration as trans (Scheme 1).[11]The number of ring conformers of 1 is restricted to two, denoted A and B in the following. These conformers are envelope conformations, one with C2 below (A) and one with C2 above (B) the almost planar arrangement of the remaining ring atoms (see also Figure SI1 in the Supporting Information).[17] In the previous study, [11] we used the transition structure between A and B as a crude approximation of the average ring conformation of 1. Additionally, we fitted the structures of the two rigid conformers to the RDC data by using one order tensor [18,19] each. By using this method we were able to assign the relative configuration as trans.[20]However, we did not attempt to extract information concerning the populations of the two conformers (p A and p B ), which was therefore one subject of the current investigation.The direct (residual) dipolar coupling D IS between spins I and S, with magnetogyric ratios g I and g S , is given by Equation (1): [9,21] ISis the dipole-dipole coupling constant (in Hz), V IS is the angle between the interspin vector and the external magnetic field, and r IS is the interspin distance (which corresponds to the bond length for directly bound nuclei). The angular brackets indicate that the RDCs are averaged over both molecular tumbling ...

Fully homodecoupled HSQC spectra can be obtained through the use of a new pulse sequence element, "perfectBIRD". By way of illustration, we show that perfectBIRD decoupling allows one-bond residual dipolar couplings (RDCs), which provide important NMR restraints for structure elucidation, to be measured with outstanding precision, even in methylene groups.

We report on a detailed NMR spectroscopic study of the catalyst-substrate interaction of a highly enantioselective oligopeptide catalyst that is used for the kinetic resolution of trans-cycloalkane-1,2-diols via monoacylation. The extraordinary selectivity has been rationalized by molecular dynamics as well as density functional theory (DFT) computations. Herein we describe the conformational analysis of the organocatalyst studied by a combination of nuclear Overhauser effect (NOE) and residual dipolar coupling (RDC)-based methods that resulted in an ensemble of four final conformers. To corroborate the proposed mechanism, we also investigated the catalyst in mixtures with both trans-cyclohexane-1,2-diol enantiomers separately, using advanced NMR methods such as T relaxation time and diffusion-ordered spectroscopy (DOSY) measurements to probe molecular aggregation. We determined intramolecular distance changes within the catalyst after diol addition from quantitative NOE data. Finally, we developed a pure shift EASY ROESY experiment using PSYCHE homodecoupling to directly observe intermolecular NOE contacts between the trans-1,2-diol and the cyclohexyl moiety of the catalyst hidden by spectral overlap in conventional spectra. All experimental NMR data support the results proposed by earlier computations including the proposed key role of dispersion interaction.

Poly(aspartic acid esters) are known to form either right-or left-handed α-helices depending on the ester group in the side chain, on solvent and/or on temperature. Polyphenethyl-l-aspartates (PPLA) exhibit a helix reversal from the right- to the left-handed form with increasing temperature. We have recently reported the application of polyphenethylaspartates as helically chiral alignment media. The thermoresponsivity observed for these polymers offers the possibility to measure different orientations of analytes before and after helix reversal of the alignment medium at 373 K. Herein we present a synthesized copolymer of phenethyl- and benzylaspartate as a new alignment medium undergoing this helix reversal at 303-313 K. Thus, the measurement of residual dipolar couplings (RDC) before and after the helix reversal is allowed for at ambient temperatures. A complete sign change of all H- C RDCs was observed, which is close to the highest possible difference in NMR spectra.

This perspective article aims to review the general methodology in the application of residual dipolar couplings (RDCs) in the structure elucidation of small molecules and give the author's view on challenges for future applications. Recent improvements in the availability of alignment media, new pulse sequences for the measurement of couplings and improvements in the analysis software have garnered widespread interest in the technique. However, further generalization is needed in order to make RDC analysis into a truly "routine" method.

The orientational properties of the two enantiomers of an example compound, namely isopinocampheol [(+)- and (-)-IPC] in the two enantiomers of a liquid crystalline phase, namely Poly-gamma-benzyl-L/D-glutamate (PBLG/PBDG) with the organic cosolvent CDCl(3), were investigated. The interactions can be either enantiomorphous, leading to equal orientations and residual dipolar couplings (RDCs), or diastereomorphous, leading to different orientations and RDCs. The difference between the two diastereomorphous orientations was determined to be rather small (5 degrees in the Euler angle beta). Furthermore, we investigated whether one of the two diastereomorphous interactions is favored.