NMR elucidation of some pentacycloundecane derived ligands

The complete NMR elucidation of four pentacycloundecane (PCU) derived ligands is reported. 2D NMR techniques are used to overcome the problem of major overlapping of methine signals on the cage skeleton. One of the cage ligands is chiral and the ¹³C NMR signals of the leucinol side “arms” to the cage appear to be split into two or more peaks indicating either impurities or conformational differences. Impurities were ruled out and the only logical explanation for this unusual observation appears to be conformational effects due to different positions of the two relative bulky side chains or “arms”. The rigid cage skeleton is known for through space deshielding of signals in close proximity to oxygen atoms attached to the cage skeleton. The leucinol side chains in closer proximity to the cage ether bridge would experience a larger shielding effect causing those carbon atoms to be shifted upfield with respect to the corresponding atoms in other conformations. The intrinsic chiral nature of the cage could also play a role in this case to perhaps enhance the observed effect. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Structure elucidation of 11-amino-8-hydroxypentacyclo[5.4.0.0(2, 6).0(3, 10).0(5, 9)]undecane-8,11-lactam through selective acetylation and complete 1H and 13C NMR spectral assignment of the mono-, di- and triacetates

NMR techniques cannot unambiguously distinguish between 11-amino-8-hydroxypentacyclo[5.4.0.0(2, 6).0(3, 10).0(5, 9)]undecane-8,11-lactam and 8-amino-11-hydroxypentacyclo[5.4.0.0(2, 6).0(3, 10).0(5, 9)]undecane-8,11-lactam, both of which are possible products during the reaction of pentacyclo[5.4.0.0(2, 6).0(3, 10).0(5, 9)]undecane-8,11-dione with Strecker reagents. Treatment of 11-amino-8-hydroxy-pentacyclo[5.4.0.0(2, 6).0(3, 10).0(5, 9)]undecane-8,11-lactam with acetic anhydride at room temperature produced a monoacetate. With acetic anhydride containing sodium acetate, a triacetate was obtained at reflux temperature. Treatment with acetyl chloride and N,N-dimethylaniline produced a diacetate. High-field 1H and 13C NMR techniques were used in the structure elucidation and assignment of the different NMR resonances of these three acetylated compounds.     

Structural properties of perfluorinated linear alkanes: a 19F and 13C NMR study of perfluorononane

Liquid perfluorocarbons exhibit unique physical-chemical characteristics such as extraordinary stability, combined hydrophobia and lipophobia, low surface tension and a capacity to carry large quantities of gas. They have found widespread use in industry, medicine and biology even though the molecular origin of these properties is not fully understood. The objective of the present work was to elucidate the physical behavior of perfluorinated linear alkanes by investigating their intramolecular electronic environment using 13C and 19F NMR techniques in combination with theoretical calculations of molecular orbitals. Particular advantage was taken of 19F-19F through-space couplings, which led us to propose a molecular model in which delocalized p-electrons of the fluorines cover the entire surface of the molecule in two pairs of intertwined helices. Experimental data are presented for n-perfluorononane and supported by corresponding measurements with shorter and longer perfluorinated alkanes.     

Computational and experimental evidence of through-space NMR spectroscopic J coupling of hydrogen atoms

J coupling in NMR spectroscopy is conventionally associated with covalent bonds. A noncovalent contribution often called through-space coupling (TSC) has been observed for heavy atoms. In this study, the TSC was detected and analyzed for the more common (1)H-(1)H coupling as well. In synthesized model molecules the hydrogen positions could be well controlled. For several coupling constants the through-space mechanism was even found to be the predominant factor. The nature and magnitude of the phenomenon were also analyzed by density functional computations. Calculated carbon- and hydrogen-coupling maps and perturbed electronic densities suggest that the aromatic system strongly participates in the noncovalent contribution. Unlike covalent coupling, which is usually governed by the Fermi contact, TSC is dominated by the diamagnetic term comprising interactions of nuclei with the electron orbital angular momentum. The computations further revealed a strong distance and conformational dependence of TSC. This suggests that the through-space coupling can be explored in molecular structural studies in the same way as the covalent one.     

Conformational dynamics of bis(BF2)‐2,2′‐bidipyrrins revealed by through‐space 13C?19F and 19F?19F couplings

The conformation of [bis‐(N,N′‐difluoroboryl)]‐3,3′‐diethyl‐4,4′,8,8′,9,9′,10,10′‐octamethyl‐2,2′‐bidipyrrin (1) in solution was studied by analyzing the ¹³C? ¹⁹F and ¹⁹F? ¹⁹F through‐space spin–spin couplings. The ¹H and ¹³C NMR spectra were assigned on the basis of nuclear Overhauser effect spectroscopy (NOESY), heteronuclear single‐quantum correlation (HSQC), and heteronuclear multiple‐bond correlation (HMBC) experiments. The ¹⁹F spectrum of 1 was compared with that of 2‐ethyl‐1,3,5,6,7‐pentamethyl‐4,4‐difluoro‐4‐bor‐3a,4a‐diaza‐s‐indacen (2). The ¹⁹F? ¹⁹F through‐space spin? spin coupling in 1 was thus assigned and the coupling constant was obtained by simulating the coupling patterns. The obtained conformation of 1 was compared with those of the known complexes [bis‐(N,N′‐difluoroboryl)]‐3,3′,8,8′,9,9′‐hexaethyl‐4,4′,10,10′‐tetramethyl‐6,6′‐(4‐methylphenyl)‐2,2′‐bidipyrrin (3)and [bis‐(N,N′‐difluoroboryl)]‐9,9′‐diethyl‐4,4′,8,8′,10,10′‐hexamethyl‐3,3′‐bis(methoxycarbonylethyl)‐2,2′‐bidipyrrin (4). The conformational dynamics of 1, 3, and 4 was surveyed by observing the temperature dependence of the through‐space coupling constants between 253 and 333 K. The ¹³C? ¹⁹F and ¹⁹F? ¹⁹F through‐space spin–spin couplings thus confirm similar conformations of different BisBODIPYs in solution in contrast to earlier findings in the solid state. Copyright © 2009 John Wiley & Sons, Ltd.     

Solution conformation of longifolene and its precursor by NMR and ab initio calculations

We describe the conformation and stereospecific 1H and 13C chemical shift assignments of longifolene 1 and its penultimate precursor 2 through the combined use of ab initio calculations and experimental NMR techniques. The predicted stable conformation for both compounds was similar and adopts a twisted chair conformation at the seven-membered ring where C4 lies on top of the exocyclic double bond. The calculated chemical shifts for the stable conformation agree well with the experimental values.     

Understanding the conformational dependence of spin-spin coupling constants: through-bond and through-space J(31P,31P) coupling in tetraphosphane-1,4-diides [M(L)x]2[P4R4

The characteristic dependence of J(31P,31P) spin-spin coupling constants of alkali metal tetraphosphane-1,4-diides on structure and composition has been analyzed by density functional methods. The computations confirm that the structure of the contact ion pairs is conserved in solution. Calculations on model systems M2P4H4, on naked P4H4(2-) anions, and on models including point charges, show that the role of the cations is mainly structural and to a smaller extent electrostatic. Three of the four J(P,P) coupling constants depend characteristically on the conformation of the anion, which in turn is determined by the substituents R and by cation-anion interactions. Several couplings exhibit a large through-space component and are thus strongly dependent on the relative orientation of nonbonding electron pairs on the phosphorus atoms involved. This is shown by visualization of coupling pathways using the recently introduced coupling energy density (CED), in combination with the electron localization function (ELF).     

Weak attractive interactions between methylthio groups and electron-deficient alkenes in peri-naphthalenes: a competition with conjugative effects

The solid-state conformations of five peri-disubstituted naphthalenes bearing a methylthio group and an electron-deficient alkene indicate a weak attractive interaction between the functional groups in four cases in which out-of-plane displacements lead to a common orientation of the MeSsp(2)-C vector to the alkene bond. In some cases the interaction is not strong enough to outweigh the tendency of the alkene to conjugate with the aromatic ring, and in one case this optimisation of conjugation alone controls the molecular conformation. The methylthio group lies close to the aromatic plane in all but one example for which the plane of the sulfide group is presented to the alkene.