1H and 13C NMR studies on 1,3,2-dioxarsolanes

The ¹H and ¹³C NMR spectra of a series of 1,3,2-dioxarsolanes have been obtained at 2.1 T and some at 9.4 T. The chemical shifts and spin-spin coupling constants have been obtained from complete spectral analyses of the ¹H and proton-coupled ¹³C spectra. The spectral data are interpreted on the basis of two rapidly interconverting half-chair conformers with a pseudoaxial substituent at arsenic. Unique assignment of syn/anti or cis/trans geometries have been made from ¹H or ¹³C NMR spectroscopy alone. The syn and trans isomers of the 4-methyl- and 4,5-dimethyl-1,3,2-dioxarsolanes, respectively, appear to be conformationally biased towards the forms with pseudoequatorial methyl groups. The general trends in the geminal and vicinal ¹H? ¹H and ¹³C? ¹H coupling constants are interpreted in terms of stereospecific, electronegativity and lone-pair effects of the oxygen heteroatoms and conformational factors. The NMR data on the 1,3,2-dioxarsolanes are discussed with reference to related 1,3-dithoxa- and 1,3-dithia- five-membered rings with As, P, S or C at the 2-position.     

13C nuclear magnetic resonance studies of saturated heterocycles: II—substituent effects on the 13C chemical shifts of methyl substituted 1,3-dithianes and their application to the determination of conformational equilibria

The ¹³C chemical shifts for 1,3-dithiane and 9 methyl substituted derivatives are reported. Only three of the methyl-1,3-dithianes were conformationally anancomeric and hence the conformational equilibria must be taken into account when deriving the values of the different substituent effects on the ¹³C chemical shifts. The best fit for each substituent effect was obtained when –ΔG^θ (5a-Me) for 5-methyl-1,3-dithiane was given the value 3.8 ± 0.3 kJ mol^?1 and when the difference between –ΔG^θ (2a-Me) and –ΔG^θ (5a-Me) for cis-2,5-dimethyl 1,3-dithiane equalled 3.4 ± 0.4 kJ mol^?1. The conformer populations chosen from our earlier paper¹ were then suitable for all the other conformational equilibria in question. The magnitude of the derived substituent effects are compared with those for cyclohexane and 1,3-dioxane.     

13C nuclear magnetic resonance and conformational analysis of 5-chloro-2-oxo-1,3,2-dioxathianes

The ¹³C NMR chemical shifts in CDCI₃ for eight cyclic sulphites, chlorinated at C-5, are reported. The α-and β-deshielding effects and the γ-shielding effects for the chlorine substituent are compared with similar effects in the 1,3-dioxane series and with the effects arising from methyl groups in the sulphite series. The study of two conformational equilibria shows that it is difficult to use ¹³C NMR spectroscopy for the conformational analysis of cyclic sulphites because of the frequent participation of twist forms, with 1,4- and 2,5-axes, to these equilibria.     

Stereochemistry and 13C NMR investigation of 9-halotetrahydro-4H-pyrido[1,2-a]pyrimidin-4-ones

It was shown that the halogen atom occupies the quasi-axial position in the predominant conformer of the 9-halo derivatives of tetrahydro-4H-pyrido[1,2-a]pyrimidin-4-ones. When R³ = Me, the conformational equilibrium is determined by the latter substituent which is always quasi-axial. The effects of the methyl group and the halogen atoms on the ¹³C chemical shifts (SCS values) were used for the identification of cis and trans isomers. Interesting non additivity of substituent effects was found in derivatives bearing quasi-axial substituents at C-6 and C-9: and this was caused by the ring flattening.     

13C chemical shifts and conformational analysis of S-methylthiolanium cations

The ¹³C NMR spectra of all cations obtained by methylation at sulphur of the mono-and dimethylthiolanes are reported. The methyl substituent on sulphur affects the shieldings of the adjacent carbons in a manner which allows easy identification of the cis and trans isomers. For most compounds the ¹³C pattern is consistent with a half-chair ring conformation with maximum staggering at C-3, C-4. Only with methyl groups at the 1,2-or 1,2,3-positions is the half-chair appreciably deformed. It is suggested that in these cases the preferred conformation is a quasi-envelope with C-3 at the top.     

Substituenteneffekte auf die NMR-Spektren von Pentafulvenen. (13C, 13C)-NMR-Kopplungskonstanten (1J(C,C))

Substituent Effects on NMR Spectra of Pentafulvenes. ¹³C, ¹³C-NMR Coupling Constants (¹J(C, C)) ¹H- and ¹³C-NMR spectra of 6-monosubstituted pentafulvenes 1 – 8 have been analysed, and ¹J(C, C) coupling constants have been determined from ID-inadequate spectra of ¹³C satellites. It turns out that ¹³C,¹³C coupling constants of the ring C-atoms, and especially J(1,2)/J(3,4) and J(2,3), reflect the extent of π delocalisation in the fulvene ring. With increasing electron-donating capacity of the substituent R, J(1,2)/J(3,4) values are decreasing, while J(2,3) (and J(1,5)/J(4,5) as well) are increasing, and linear correlations of Hammett substituent constants σ⁺ and ¹J(C,C) values are obtained.     

Preparations of regioselectively methylated cellulose acetates and their 1H and 13C NMR spectroscopic analyses

Seven possible regioselectively methylated cellulose acetates (RS‐MCAs)—2,3,6‐tri‐O‐methyl cellulose acetate, 3,6‐di‐O‐methyl cellulose acetate, 2,6‐di‐O‐methyl cellulose acetate, 2,3‐di‐O‐methyl cellulose acetate, 6‐O‐methyl cellulose acetate, 3‐O‐methyl cellulose acetate, and 2‐O‐methyl cellulose acetate—were prepared for the first time from chemically synthesized cellulose derivatives obtained by cationic ring‐opening polymerization and then were analyzed by ¹H and ¹³C NMR spectroscopy. The chemical shifts of ring protons and carbons were influenced by substituent groups (methyl or acetyl) and clearly reflected the pattern of substituent distribution in anhydroglucose units. These data may conveniently be used for the determination of the substituent distribution of methyl cellulose. The synthesized RS‐MCAs also may be used for the elucidation of the structure–property relationship. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4167–4179, 2002     

Substituent effects on 13C?13C coupling constants and chemical shifts in 13C labelled polycyclic aromatic hydroxy compounds

The effects of an hydroxy substituent on ¹³C? ¹³C coupling constants and ¹³C chemical shifts have been measured in 1-hydroxynaphthalene-2-¹³C and 1-hydroxypyrene-1-¹³C. The changes observed in the ¹³C? ¹³C couplings show the effect of a substituent attached directly to the labelled carbon or to a carbon ortho to this. In both situations the effect is a decrease in the numerical magnitude of most of the long range ¹³C? ¹³C coupling constants.     

Vicinal 13C?13C spin–spin coupling constants of 1-butanols. Conformational and substituent effects

A series of (>90% isotopic purity) ¹³C-labeled aliphatic alcohols of the general structure C? C? C? ¹³C? OH were synthesized and studied by ¹³C n.m.r. to obtain all ¹³C? ¹³C couplings involving the labeled carbon. The ²J(CC) values were small (<0.5 Hz) and contrast with the large (up to 2.7 Hz) ²J(CC) values obtained in a previous study for 2-butanols. The ³J(CC) values, however, were strikingly similar in the two classes of compounds with respect both to magnitude and to conformational dependency. Thus, the effect of the hydroxyl substituent on ³J(CC) values is small.     

The structural dependence of two-bond 13C?13C coupling constants

Two-bond ¹³C? ¹³C coupling constants are discussed on the basis of INDO-SCPT calculations. The dependence of ²J(CC) on bond angle variation and on methyl substitution is evaluated, and it is shown that ²J(CC) depends linearly on the bond orbital s-character product of the terminal carbon atoms, whereas no systematic relationship with the hybridization of the central carbon atom was obtained. Alkyl group substituent effects are found to be additive. The coupling constants of a number of cyclobutane derivatives are discussed on the basis of these structural relationships; it is shown that the experimental findings can be interpreted quite consistently by assuming a dual-pathway coupling mechanism.     

Pteridine studies: III—13C n.m.r. data of pteridine,some of its derivatives and their covalent σ-adducts with ammonia and water

¹³C n.m.r. spectral data of pteridine and nineteen of its derivatives (containing one or more chloro, methylthio, methyl, t-butyl or phenyl substituents) are reported. The ¹³C n.m.r. spectrum of the title compound has been assigned conclusively. ¹³C n.m.r. substituent effects are shown to be very useful in discerning between 6- and 7-substituted pteridines. Additionally, the ¹³C n.m.r. spectra of several covalent amination products, i.e. the 3,4-dihydro-4- amino- and the 5,6,7,8-tetrahydro-6,7-diaminopteridine derivatives, formed by dissolving the appropriate pteridine in liquid ammonia, have been recorded. The ¹³C n.m.r. spectra of the corresponding covalent hydrates are also reported.     

13C NMR study of some polychloro-isobutane and -isobutene compounds

The ¹³C chemical shifts and the carbon–proton coupling constants have been determined for some chlorinated isobutane and isobutene compounds. The one-bond coupling constants in isobutane derivatives showed a regular increase with an increasing number of γ-chlorine substituents. The three-bond coupling constant of the methyl carbon decreased from 4.2 to 2.0 Hz as the number of chlorine substituents in the γ-position increased. In the isobutene compounds, the vicinal coupling of C-1 was larger to protons in a group that is trans with respect to a chlorine substituent on C-1 than to those in the corresponding group cis to the chlorine. The vicinal coupling constants between atoms in geminal groups (on C-2) seem to be affected by the orientation of the chlorine substituent on C-1.     

13C, 13C coupling constants and 13C chemical shifts of aromatic carbonyl compounds. Effects of ortho- and peri-interactions involving the carbonyl substituent

Carbon-13 n.m.r. data have been determined for a series of 26 aromatic carbonyl compounds including benzoyl, naphthoyl and pyrenoyl derivatives ¹³C labelled in the carbonyl group. Doubly labelled anthraquinone has also been included. The compounds investigated comprise non-hindered molecules and molecules in which the carbonyl substituent is subject to ortho- or peri-interactions affecting conjugation of the carbonyl group with the aromatic ring. The dependence of long range ¹³C,¹³C coupling constants involving the carbonyl carbon, in particular ²J and ³J, on steric conditions is discussed, as is the possibility of deciding on the orientation of the carbonyl bond. The following results have emerged. ²J(s-t)>²J(s-c) for ketones and aldehydes, and the reverse is valid for acids and acid derivatives. (s-t and s-c refer to the orientation of the C?O group relative to the aromatic bond in question with respect to the connecting single bond). For ketones ³J(t,s-c)<³J(t,s-t), and both of these ³J(t) values decrease with increasing angle of twist, θ, about the single bond, whereas ³J(c,s-c) increases with θ. For acids and acid derivatives no similar regularity was found. (The initial t and c refer to the geometry of the three-bond coupling path). Generally it is found that ³J(t)>³J(c) and ³J(t)>²J, confirming earlier results. Theoretical calculations on a few model compounds are qualitatively in accordance with the experimental results. Some sign determinations for coupling constants are presented. A short discussion is given of substituent effects on chemical shifts. Observed trends are consistent with earlier results.     

13C n.m.r. spectra of lysergic acid derivatives: II—dihydrolysergamides

The conformations of dihydrolysergamides and 10-methoxydihydrolysergamides have been inferred from the study of the chemical shifts of the amide hydrogens and their temperature dependence. The ¹³C chemical shifts have been shown to be sensitive to conformational changes of the piperidine ring. The results indicate that the conformation of the latter depends both on the substituent in position 10 and on the solvent.     

On the stereochemistry of 1,4-diheterocyclanes. IV—carbon-13 NMR spectra and structural properties of 1,4-dioxane-2,3-diols and their methyl-substituted derivatives

The 1,4-dioxane-2,3-diols and a number of their methyl-substituted derivatives were synthesized and their ¹³C NMR spectra measured. The configurational properties were studied on the basis of the spectral data and chemical equilibration. For the parent compound the trans configuration is slightly more stable. The hydroxyl groups are predominantly axial. Introduction of a methyl group at the 2 position causes the cis configuration of the hydroxyl substituents to become more stable. The substituent effects on the chemical shifts were calculated and used to test the conformational homogeneity of the compounds. The results indicate a biased (chair) conformation for all the methyl-substituted derivatives which were studied. Methyl groups have strongly predominating equatorial orientation in each case studied.     

Carbon-13 NMR spectra of some epoxides derived from dicyclopentadiene

The ¹³C NMR spectra of a series of epoxides derived from endo- and exo-dicyclopentadiene and their partially hydrogenated compounds were determined to examine the substituent effects arising from the introduction of the oxirane ring in comparison with those found in other ring systems. The ¹³C signals of some epoxides were assigned by using lanthanide shift reagents. Characteristic substituent effects exerted by an oxirane ring were demonstrated. Marked steric γ-effects of ?8—13 ppm were observed at the bridge carbon signal in the bicyclo[2.2.1]heptane skeleton. Differences were found in the substituent effects between endo- and exo-dicyclopentadiene epoxides, and have been discussed in relation to the molecular geometry.     

13C NMR: Substituent effects on one-bond 15N?13C coupling constants in anilines and anilinium ions

¹J(¹⁵N¹³C) values obtained from FT ¹³C NMR spectra were measured for a number of ¹⁵N-enriched aniline derivatives and are found to exhibit varying degrees of dependence on the nature of the ring substituent. Theoretical calculations of ¹J(¹⁵N¹³C) values for representative members of the systems examined were made using INDO parameters and a ‘sum-over-states’ perturbation approach. The calculated coupling constants are generally in fair agreement with experimental values when the integral products S_N²(o)S_C²(o) and (r^?3)_N(r^?3)_C have values of 34.437 au^?6 and 2.770 au^?6, respectively.     

Methyl substituent effects in the 13C spectra of cyclohexanone O-methyl oximes and N,N-dimethylhydrazones

The ¹³C NMR spectra of a series of 4-t-butylcyclohexane-O-methyl oximes and N,N-dimethylhydrazones containing α-methyl substituents have been measured. The effects of α-methyl substitution (syn-axial, anti-axial and anti-equatorial) on the carbons α, β and γ to the substitution site are similar to those previously found in cyclohexanones. Use has been made of these substituent parameters to determine the conformational equilibria of the mobile 2-methylcyclohexanone derivatives.     

1H, 13C and 77Se NMR spectra of substituted selenoanisoles

The ¹H, ¹³C and ⁷⁷Se chemical shifts and the ¹J[C(Me)H(Me)], ^1.2J(SeC) and ²J(SeH) coupling constants in 14 para- or meta-substituted selenoanisoles, R? C₆H₄? Se? CH₃, have been measured and the dependence of these parameters on the electronic effects of the substituent R is discussed. A significant (up to 6 ppm) deviation from additivity of the substituent influence on the shielding of the ¹³C ring carbons has been found.