Stereochemicai study of 7-aryl-1,7,8,8a-tetrahydro-3(2H)indolizinones by1H and13C NMR

Stereochemistry of 7-aryl-1,7,8,8a-tetrahydro-3(2H)-indolizinones was studied by¹H and¹³C NMR. Complete assignment of¹H NMR signals and analysis of¹H-¹H coupling constants were performed using the iterative PANIC program. Values of³ J _6,7,³ J _7,8endo, and⁴ J _5,7 allow one to unambiguously identify the correspondingexo- andendo-stereoisomers. For stereoisomers with exo-orientation of H(7), complete assignment of¹³C NMR signals was performed on the basis of analysis of the¹³C-¹H coupling constants using two dimensional heteronuclear shift-correlating spectroscopy.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 591–593, March, 1996.     

Crosslinking and ageing of C labelled polyisoprene part 1: Synthesis and polymerisation of 4-C-isoprene

4-¹³C-isoprene was prepared by the Wittig reaction. All reaction steps were optimised using unlabelled compounds. By reaction with triphenyl phosphine, ¹³C labelled methyl iodide afforded labelled methyl-triphenyl phosphine iodide in 84% yield. This reacted with meth acrolein with production of 4-¹³C-isoprene in 64% yield. Labelled polyisoprene was prepared by anionic polymerisation initiated by t-butyl lithium. Based on ¹³CH₃I the overall yield is ca 30%. The polymer was characterized by ¹H and ¹³C NMR spectroscopy. The contribution of each microstructure was [cis 1-4, 72%]; [trans 1-4, 10%]; [3-4, 18%].     

A new method for determining positions of phenolic hydroxyl groups through silylation and application of H(Si)C triple-resonance NMR experiments

Two-dimensional triple-resonance H(Si)C NMR experiments have been applied at natural abundance to assign ¹³C NMR signals in silylated phenols. The method showing its great potential in determining positions of hydroxyl groups is widely applicable to signal assignment and structure elucidation of synthetic and natural phenolic compounds.     

Using LR-HSQMBC to observe long-range H–N correlations

The recently reported LR-HSQMBC experiment has been optimized for ¹H–¹⁵N long-range heteronuclear couplings. Several previously unreported four-bond correlations, consistent with the predicted by DFT calculations (0.2–0.3 Hz ⁴J_NH couplings), have been observed for strychnine using 2 Hz optimization of the LR-HSQMBC experiment. This experiment offers an advantage over accordion-optimized experiments such as IMPEACH and CIGAR for the observation of long-range ¹H–¹⁵N correlations in that the experiment is refocused and employs a CLIP pulse sequence element to bring the long-range correlations into phase, allowing broadband X-decoupling to be employed during acquisition.     

Pulsed fourier transform NMR of substituted aryltrimethyltin derivatives : III. Proton data at 100 MHz and the derived Hammett σ-constant of the trimethyltin group

Proton NMR data at 100 MHz are reported for thirteen para- and meta-substituted phenyltrimethyltin compounds, XC₆H₄Sn(CH₃)₃, where X = para-N(CH₃)₂, para-OCH₃, para-OC₂H₅, para-CH₃, meta-CH₃, -H, para-F, meta-OCH₃, para-Cl, para-Br, meta-F, meta-Cl and para-Sn(CH₃)₃. Correlation coefficients with Hammett σ-constants of greater than 0.95 are obtained with the methyltin proton chemical shifts and coupling constants to carbon [¹J(¹³C¹H)] and tin [²J(SnC¹H)]. Solvent effects and other extraneous factors invalidate comparisons of ? values in terms of the relative attenuation of the transmission of substituent effects through homologous carbon, silicon, germanium and tin systems, but coupling constant data reflect a diminution of ca. one tenthfold per bond in the order ?[C(1)Sn] > ? [SnC] > ? [CH]. Satisfactory correlations (r > 0.95) are obtained in this series of closely-related compounds among the conventionally recorded two-bond, ²J(SnC¹H) and the constituent, one-bond ¹J (Sn¹³C) and J(¹³C¹H) coupling constants, but the correlation coefficient for the comparison between the two one-bond couplings, ¹J(Sn¹³C) and ¹J(¹³C¹H) is lower (r = 0.872). Changes in the couplings at the methyltin carbon bond tin-119 atoms are interpreted in terms of isovalent hybridization; a model based upon effective nuclear charges is tested with respect to both NMR coupling constants and ¹¹⁹Sn Mössbauer Isomer shifts at tin and is invalidated. Proton and carbon-13 NMR, chemical shift and coupling constant data are used to derive a Hammett σ-constant for the para-trimethyltin group of ?0.14, and the significance of this value is discussed.     

SCHWEFELDIIMIDE MIT SILYL-, GERMYL-, STANNYL- UND PHOSPHINYL-HETEROSUBSTITUENTEN. SYNTHESE UND NMR-SPEKTROSKOPISCHE CHARAKTERISIERUNG

Abstract

Reactions of the salts K₂SN₂ and K[(NSN)R] (R = ′Bu, SiMe₃ and P′Bu₂) with organoelement chlorides R′R′ěl have been used to prepare four series of model sulfur diimides: R′R″E(NSN)ER″R′, ′Bu(NSN)ER″R′, Me₃Si(NSN)E″R′ and ^tBu₂P(NSN)ER″R′, respectively (E = C, Si, Ge, Sn; R′ and R″ = alkyl or aryl group). All compounds have been characterized by ′H and ¹³C NMR and—if possible—by ³¹P, ²⁹Si and ¹¹⁹Sn NMR spectroscopy. The configuration (Z or E) of the substituents R and E″R′ has been assigned in several cases using ^tBu(NSN)^tBu (1) as a reference. The E,Z assignment of ¹H, ¹³C and ¹⁵N nuclei in 1 is based on selectively ¹H-decoupled refocused INEPT ¹⁵N NMR and two-dimensional (2D) ¹³C/¹H heteronuclear shift correlations. The sulfur diimides under study are in general fluxional in solution.     

Deuterium-isotopieeffekte auf die 13C-chemischen verschiebungen und kohlenstoff-deuterium kopplungskonstanten in deuterierten verbindungen

The isotope effects on the ¹³C NMR chemical shifts and coupling constants (¹³C¹H and ¹³C¹H) have been determined by pulse Fourier transform ¹³C NMR investigation at 22·63 MHz for more than 30 common deuterated and protonated solvents. The observed isotope effects correlate with hybridization and electron withdrawal at the coupling carbon within the series of comparable compounds. In agreement with MO-theoretical calculations a linear correlation between the J_CD values of CD_x groups and the J_CH values of the corresponding CH_x groups was found. The experimentally determined J_CD values show an average deviation from the calculated line J_CD = (γ_D/γ_H)J_CH = 0·154 ×J_CH on the order of± 1 Hz.     

Analysis of F and C NMR spectra of tetrafluorophthalic anhydride and its derivatives

¹⁹F and ¹³C NMR spectra of perfluorinated compounds (i.e., tetrafluorophthalic anhydride, its hydroxyl- and amino-derivatives, N-pentafluorophenyltetrafluorophthalimide, and hexafluoroindan-1,3-dione) were analysed. Different signals in NMR spectra were assigned based on the analysis of spin-spin coupling constants. All assignments made were further confirmed by density functional theory (DFT) calculations of ¹³C chemical shifts and J_C,F coupling constants.     

Hochauflösungs-13c-nmr-spektroskopie—III: Strukturabhängigkeit der vicinalen methyl-13c,h-kopplungen in methylazulenen,methylaceheptylenen und anderen methyl-sub stituierten polycyclen mit fünf- und siebengliedrigen ringen

The vicinal couplings of ring hydrogens to methyl C atoms (³J_CH3H) in 22 methyl substituted non-benzenoid polycyclic conjugated hydrocarbons have been determined from the undecoupled ¹³C NMR spectra and have been correlated with bond lengths as well as with the corresponding vicinal H,H couplings, which are taken partly from own ¹H NMR analysis and partly from literature. As a result the (³J_CH3H) couplings of sterically unperturbated methyl groups in 7-membered rings are proportional to the corresponding ³J_HH values which is indicative of comparable influences, but both types of vicinal couplings are not dependent on bond lengths only. Moreover they are to a large extent determined by the CCH bond angles θ and θ', which show a significant variation in condensed 7-membered rings so that this twofold dependence has to be taken into account for structure determinations.     

The effect of silica nanoparticles on the morphology, mechanical properties and thermal degradation kinetics of PMMA

Silica-PMMA nanocomposites with different silica quantities were prepared by a melt compounding method. The effect of silica amount, in the range 1-5 wt.%, on the morphology, mechanical properties and thermal degradation kinetics of PMMA was investigated by means of transmission electron microscopy (TEM), X-ray diffractometry (XRD), dynamic mechanical analysis (DMA), thermogravimetric analyses (TGA), Fourier-transform infrared spectroscopy (FTIR), ¹³C cross-polarization magic-angle spinning nuclear magnetic resonance spectroscopy (¹³C{¹H} CP-MAS NMR) and measures of proton spin-lattice relaxation time in the rotating frame (T_1ρ(H)), in the laboratory frame (T₁(H)) and cross-polarization times (T_CH). Results showed that silica nanoparticles are well dispersed in the polymeric matrix whose structure remains amorphous. The degradation of the polymer occurs at higher temperature in the presence of silica because of the interaction between the two components.     

Homonuclear J-couplings and rotationally induced sideband enhancements in NMR spectra of rotating solids

We demonstrate direct detection of ¹³C-¹³C J-couplings in magic angle sample spinning NMR spectra despite the presence of the much larger homogeneous broadening caused by the homonuclear dipolar couplings. Carbon-carbon J-couplings were observed in doubly ¹³C-enriched samples of sodium acetate, glycine and glucose. The resolved J-coupling permits carbon-carbon connectivities to be established with standard two-dimensional techniques. Interesting spectral features are observed when the rotational sidebands of the coupled spins overlap: when a sideband from a dipolar-coupled pair approaches the centerband of its partner, a significant enhancement in sideband intensity is observed as well as small shifts in the resonance frequency.     

Direct Identification of the Carbon Skeleton of Organic Compounds using Double Quantum Coherence 13C-NMR Spectroscopy. The INADEQUATE Pulse Sequence

NMR spectroscopy plays an important part in the determination of the structures of organic compounds. The parameters of importance here are the chemical shifts of the ¹H and ¹³C nuclei and the spin-spin interactions both between ¹H nuclei and between ¹H and ¹³C nuclei. Couplings between ¹³C nuclei were almost completely neglected until a few years ago, since they were extremely difficult to observe because of the low natural abundance of ¹³C. However, it is these couplings which afford information directly on the carbon-carbon connectivities in the molecule. It is now possible to use a special NMR pulse sequence to make these couplings more readily visible: the result of using this sequence is a ¹³C-NMR spectrum from which the carbon skeleton concerned can be directly read off. Two-dimensional spectra in particular are very easy to evaluate. The pulse sequence involved, which bears the somewhat puzzling name INADEQUATE, produces double-quantum coherences from which the NMR signals of the coupled carbon nuclei can be obtained. In this article the principle of double-quantum coherence is described and a number of examples for the application of the INADEQUATE pulse sequences to problems in synthetic organic chemistry, biosynthesis and natural products chemistry are presented; in addition, the possibility of applying the INADEQUATE method to other nuclei is considered.     

A N NMR study of tautomerism in dimethyl dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate

Dimethyl dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate can exist either in 1,2- or 1,4-dihydro tautomeric forms. The ¹⁵N NMR spectra of dimethyl dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate were measured at the ¹⁵N natural abundance level as well as in ¹⁵N doubly labelled selectively and in ¹⁵N completely labelled compounds (20% ¹⁵N). The J(¹⁵N,¹⁵N) value was determined in ¹⁵N completely labelled compounds (20% ¹⁵N) using 1D ¹⁵N INADEQUATE and was found to be 12.2 ± 0.2 Hz in deuteriochloroform, acetonitrile-d₃, DMSO-d₆ and CD₃OH. Very similar ¹⁵N chemical shifts and ¹J(¹⁵N,¹H) values were also observed in all the solvents. This indicates that compound 1 exists completely in the 1,4-dihydro tautomeric form (i.e., as dimethyl 1,4-dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate) in all the solvents tested.     

Two-Dimensional NMR Studies of Polyene Systems. I. All-trans-Retinal

Two-dimensional (2-D) NMR results are presented for all-trans-retinal. 2-D J-resolved ¹H-NMR separated the multiplets of the olefinic protons and accurately determined their chemical shifts. 2-D shift-correlated ¹H-NMR gave the connectivities between scalar coupled protons. From the observed H,H long-range couplings the assignment of the methyl resonances was possible. 2-D J-resolved ¹³C-NMR separated overlapping C,H-multiplets and allowed analysis of the C,H long-range couplings, 2-D shift-correlated ¹³C-NMR related each directly bonded C,H-pair in this molecule. The potential of 2-D NMR in resolving and identifying individual resonances in polyene spectra is discussed.     

Additional pitfalls of using 1,1‐ADEQUATE for structure elucidation

1,1‐ADEQUATE is a powerful and robust NMR experiment to establish carbon–carbon connectivities using modest sample quantities when cryogenic probe technology is available. Yet potential pitfalls of applying this method are not widely appreciated, such as weak or missing ¹J_CC correlations in strongly coupled ¹³C‐¹³C AB spin systems and unusually large multi‐bond (ⁿJ_CC) correlations associated with particular functional groups. These large ⁿJ_CC correlations observed in 1,1‐ADEQUATE spectra could be mistaken for ¹J_CC correlations. Copyright © 2016 John Wiley & Sons, Ltd.     

Highly regioselective synthesis of trifluoromethyl derivatives of pyrazolo[1,5-a]pyrimidines bearing fused cycloalkane rings using (2-ethoxycycloalkenyl)-2,2,2-trifluoroethanones

8-Trifluoromethyl-6,7-dihydro-5H-1,4,8a-triaza-s-indacene and 9-trifluoromethyl-5,6,7,8-tetrahydropyrazolo[5,1-b]quinazolines were efficiently generated by condensation of 5(3)-aminopyrazoles with (2-ethoxycycloalkenyl)-2,2,2-trifluoroethanones and isolated in excellent yields. The regiochemistry of the prepared compounds was established by ¹H, ¹³C and ¹⁹F NMR spectroscopy and X-ray diffraction analysis.     

Gold(III)pentafluorophenylarylazoimidazole: Synthesis and spectral (H, C, COSY, HMQC NMR) characterisation

Reaction of [Au^III(C₆F₅)₃(tht)] with RaaiR′ in dichloromethane medium leads to [Au^III(C₆F₅)₃ (RaaiR′)] [RaaiR′=p-R-C₆H₄-N=N-C₃H₂-NN-l-R′, (1-3), R = H (a), Me (b), Cl (c) and R′= Me (1), CH₂CH₃ (2), CH₂Ph (3), tht is tetrahydrothiophen]. The nine new complexes are characterised by ES/MS as well as FAB, IR and multinuclear NMR (¹H,¹³C,¹⁹F) spectroscopic studies. In addition to dimensional NMR studies as¹H,¹H COSY and¹H¹³C HMQC permit complete assignment of the complexes in the solution phase.     

Comparison of 1H–19F two‐dimensional NMR scalar coupling correlation pulse sequences

The effectiveness of hetero‐COSY, HETCOR, HMQC, and HSQC two‐dimensional NMR pulse sequences for detection of ¹⁹F–¹H correlations by scalar coupling was evaluated on monofluorinated and polyfluorinated test compounds. All four of these sequences were effective in observing ¹H–¹⁹F correlations, using either ¹⁹F or ¹H as the observe nucleus. All four sequences were amenable, to some degree, to adjustment to observe larger or smaller couplings preferentially. A 1/2J echo filter was effectively applied to remove artifacts from ²J_FF strong coupling. The HETCOR experiments afforded the best overall combination of sensitivity, resolution and selectivity for J_HF. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantitative analysis of deuterium using the isotopic effect on quaternary C NMR chemical shifts

Quantitative analysis of specifically deuterated compounds can be achieved by a number of conventional methods, such as mass spectroscopy, or by quantifying the residual ¹H NMR signals compared to signals from internal standards. However, site specific quantification using these methods becomes challenging when dealing with non-specifically or randomly deuterated compounds that are produced by metal catalyzed hydrothermal reactions in D₂O, one of the most convenient deuteration methods. In this study, deuterium-induced NMR isotope shifts of quaternary ¹³C resonances neighboring deuterated sites have been utilized to quantify the degree of isotope labeling of molecular sites in non-specifically deuterated molecules. By probing ¹³C NMR signals while decoupling both proton and deuterium nuclei, it is possible to resolve ¹³C resonances of the different isotopologues based on the isotopic shifts and the degree of deuteration of the carbon atoms. We demonstrate that in different isotopologues, the same quaternary carbon, neighboring partially deuterated carbon atoms, are affected to an equal extent by relaxation. Decoupling both nuclei (¹H, ²H) resolves closely separated quaternary ¹³C signals of the different isotopologues, and allows their accurate integration and quantification under short relaxation delays (D1 = 1 s) and hence fast accumulative spectral acquisition. We have performed a number of approaches to quantify the deuterium content at different specific sites to demonstrate a convenient and generic analysis method for use in randomly deuterated molecules, or in cases of specifically deuterated molecules where back-exchange processes may take place during work up.     

New pentacoordinated Schiff-base diorganotin(IV) complexes derived from nonpolar side chain α-amino acids

In this work we report the synthesis and spectroscopic characterization of twenty new pentacoordinated diorganotin(IV) compounds. These compounds have been prepared in good yields by multicomponent reactions (MCRs) of α-amino acids (isoleucine, leucine, methionine, phenylalanine and aminophenylacetic acid), 2,4-dihydroxybenzaldehyde, 2-hydroxy-4-methoxybenzaldehyde and either di-n-butyltin(IV) oxide or diphenyltin(IV) oxide. All compounds were characterized by IR spectroscopy, ¹H, ¹¹⁹Sn and ¹³C NMR spectroscopy and mass spectrometry. Each compound has a coordinative N→Sn bond and shows the expected ¹¹⁹Sn NMR chemical shift indicative of a pentacoordinated or hexacoordinated tin atom in CDCl₃ and DMSO-d₆, respectively. These compounds were also tested in tumoral cell lines, HeLa, HCT-15 and MCF-7, in order to evaluate the antiproliferative activity and to obtain the medial inhibitory concentrations (IC₅₀) values.