Electronic effects in some 3-substituted-2-cyclohexenones: A study by carbon-13 n.m.r. spectroscopy

The carbon-13 n.m.r. spectra of some 3-substituted-2-cyclohexenones show very wide variations in chemical shift at C-3 that correlate with Pauling electronegativity χ, even wider variations at C-2 that correlate with the Taft resonance parameter σ_Rº and almost no variation at C-1. These very different effects are shown to be consistent with the electronic distribution for these and related compounds revealed by other spectroscopic techniques. Reasons for the insensitivity of carbon-13 n.m.r. to configurational and conformational variation in simple enamino-ketones are discussed.     

Investigation of the torsional angle at the glycosidic bond in 5′-amino-5′-deoxythymidine derivatives by carbon-13 n.m.r. spectroscopy

The conformational preference of the thymine base ring with respect to the sugar ring in β,β,β,-trichloroethyl 5′amino-5′-deoxythymidine-5′-phosphate has been studied by ¹³C n.m.r. spectroscopy. The magnitude of the three bond vicinal coupling constant, J(C-2, H-1′), for β,β,β-trichloroethyl 5′-amino-5′-deoxythymidine-5′-phosphate and the similarity between the chemical shifts for the furanose carbons C-1′, C-2′, and C-3′ in β,β,β-trichloroethyl 5-′-amino-5′-deoxythymidine-5′-phosphate and in β,β,β-trichloroethyl thymidine 5′-phosphate indicate that the amino analogue exists in aqueous solution predominantly in the anti conformation, as is the case with natural nucleotides.     

Substituent effect of the nitro group on alkanes in carbon-13 n.m.r. spectroscopy

Linear correlation coefficients were obtained for the substituent effect of the nitro group in primary nitroalkanes. The solvent effect of chloroform is also discussed, as well as the influence of the scalar relaxation on the half-width of α-carbons in ¹³C n.m.r. spectroscopy.     

Carbon-13 n.m.r. spectroscopy of some Strychnos alkaloids

¹³C N.m.r. spectra have been determined for strychnine and a series of fourteen derivatives. The assignments are based on previously established data and on off-resonance decoupled spectra. The shifts resulting from the alterations in molecular structure are discussed and explained, in part, as a consequence of conformational changes. This detailed study indicates that changes are required in some previously published assignments.     

Application of carbon-13 n.m.r. Spectroscopy to the structural investigation of ezomycins

Ezomycins A₁, A₂, B₁, B₂, C₁, C₂, D₁ and D₂, antifungal antibiotics, are anhydrooctose uronic acid nucleosides. Their novel structures were confirmed by studying their natural abundance ¹³C n.m.r. spectra in detail from gated proton-decoupled, long-range selective proton-decoupled and selective proton-decoupled spectra. The J(CH) value of the anomeric carbon signal (C-1″) supports the β-configuration.     

A carbon-13 n.m.r. study of sulfur-donor ligand ortho-metalated iron carbonyl complexes

Carbon-13 n.m.r. data are reported for a series of sulfur-donor ligand ortho-metalated iron carbonyl complexes derived from thiobenzophenones. The chemical shift δ153·5–161·4 for the aromatic carbon σ-bonded to iron is readily identified, and occurs in the same region as the signal for the corresponding carbon in a related nitrogen-donor ligand complex. The benzylic carbon gives a signal at δ62·8–63·3, subject to the nature of the complex.     

Carbon-13 n.m.r. studies of flavanone 5,3′,4′-trihydroxy-7β-glucoside from Juelia subterranea (Balanophoraceae)

The complete structure and stereochemical elucidation of flavanone 5,3′,4′-trihydroxy-7β-glucoside has been achieved by the aid of ¹³C n.m.r.     

Silicon-29 and carbon-13 n.m.r. studies of organosilicon chemistry. VII—trimethylsilylated silicates

Silicon-29 n.m.r. was used to study the products from trimethylsilylation of minerals. Chemical shifts are reported for four model compounds, viz. the monomer, dimer, linear trimer (from natrolite) and cyclic tetramer (from laumontite). Information is then obtained from the products of trimethylsilylation of the anhydrous calcium silicates wollastonite and pseudo-wollastonite. In the latter case it is shown that the principal products are the four model compounds, and percentage yields are obtained for these.     

On the dimerization of α-mercaptoacetone: An n.m.r. study

¹H n.m.r. is used to show that α-mercaptoacetone is formed as a mixture of monomer and a dimer. No evidence is found to support the previously claimed isolation of two dimeric species. The dimer dissociates to the monomer on standing as a solution in chloroform, but the monomer is dimerized by grinding. 1-Mercapto-3-phenylpropan-2-one behaves similarly.     

A comment on the interpretation of 13C n.m.r. δ-syn-axial substituent effects in cyclic molecules

An alternative explanation for δ-syn-axial substituent effects of ¹³C chemical shifts is given using the concept that the δ-syn-axial interaction in the substituted compound replaces the γ-gauche interaction in the parent molecule. The application of this principle to substituted norbornanes and steroids leads to the conclusion that the ‘net steric’ δ-syn-axial shift is upfield rather than downfield.     

13C n.m.r. chemical shifts in aliphatic alcohols and the γ-shift caused by hydroxyl and methyl groups

Empirical correlations for the prediction of the chemical shifts of the carbon atoms in aliphatic alcohols are presented. Structural and conformational effects contributing to the ¹³C chemical shifts of γ-carbons are discussed.     

Dependence of the γ carbon-13 shielding effect on electronegativity

Three γ effects on ¹³C shielding in 3,3-dimethylheteracyclohexanes as a function of the hetero-atom X have been examined. The γ-anti effect on the equatorial 3-methyl group is small in absolute magnitude but strongly dependent on the polar properties of X. The plot of the ¹³C shielding of this carbon vs the electronegativity of X is linear, with a slope of ?5.8 ppm/electronegativity unit. The γ-gauche effects on the axial 3-methyl group and on the 4-carbon are large in absolute magnitude but have quite different dependences on the polar properties of X. Whereas the shielding of the 4-carbon exhibits a linear dependence on electronegativity (slope ?3.5), the axial 3-methyl group shows little dependence (slope crudely ?0.7), even though the geometric relationship between X and either carbon is almost the same. Neither gauche carbon shielding appears to be related to the steric properties of X. The polar component of both the γ-anti effect and the γ-gauche effect is interpreted as arising from overlap of appropriately positioned parallel orbitals. For the anti case, the pathway is the familiar zigzag arrangement of bonds. For the gauche case, the pathway may be either through space (the orbitals would be only on X and C-α; for the 4-carbon, this interaction would be through the center of the ring) or through bonds (there are parallel axial orbitals on all four atoms). The absence of a significant polar effect for the axial 3-methyl group suggests that the gauche interaction requires a rigid pathway. The polar component of the general γ-gauche effect is superimposed upon a larger contribution that is essentially independent of the nature of X and may be associated with the removal of the hydrogen on the β-carbon and replacement with the γ-X group.     

Investigation of substituent effects of chalcones by 13C n.m.r. spectroscopy

¹³C chemical shifts for 23 para- and meta-substituted chalcones of the types 1 and 2 have been determined. The aromatic shieldings are compared with previous results for other aromatic derivatives. Correlations of the ¹³C chemical shifts of vinyl carbons and carbonyl carbons as well as ring carbons with Hammett σ parameters, π electron densities and the reactivity parameters of Swain and Lupton provide a consistent picture of electronic effects transmitted through the carbon framework of the compounds studied.     

13C n.m.r. spectra of 2-substituted pyrimidines

¹³C n.m.r. chemical shifts and carbon-proton coupling constants of 2-substituted pyrimidines are reported. The carbon chemical shifts are correlated with π-electron densities. Substituents which cause deshielding at the directly bound carbon (e.g. NH₂, OCH₃ and F) exert a more powerful effect in the benzene series than in the pyridine or pyrimidine series. The carbon-proton coupling constants do not correlate with the electronegativity of the substituents. Carbon-proton coupling constants and proton-proton coupling constants over the same number of bonds do not obey the Karabatsos relationship. The changes in the carbon-proton coupling constants in 2(1H)-pyrimidinone and 2(1H)-pyrimidinethione which accompany anion and cation formation are reported.     

The structure of 1,2-dithioles studied by 13C n.m.r. spectroscopy

The charge separation between the dithiole ring and oxygen in 3-(1′,2′-dithiole-3′-ylidene)-6-methyl-2,3-dihydropyran-2,4-diones and 3-(1′,2′-dithiole-3′-ylidene)-2,3-dihydrobenzo[b] pyran-2,4-diones has been determined by ¹³C and ¹⁹F n.m.r. spectroscopy. By both methods it is found that these compounds are relatively polar. A correlation is established between the ¹³C chemical shift of some carbon atoms and the S2p binding energies measured by e.s.c.a. in the 1,2-dithiole derivatives. This correlation shows that these ¹³C shifts depend on the positive charge taken by the 1,2-dithiole ring and can then constitute a convenient evaluation of this charge.     

The structure of α-(1,2-dithiole-3-ylidene) ketones and α-(1,2-dithiole-3-ylidene) aldehydes studied by means of n.m.r. spectroscopy

1H and ¹³C n.m.r. studies of a series of twelve 1,2-dithiole-3-ylidene ketones and aldehydes have shown that the geometry of the carbon backbone is the same as found in 1,6,6aλ⁴-trithiapentalenes. No evidence has been found which favours a bicyclic structure for the system. A linear correlation of observed ¹³C chemical shifts with calculated charge densities is found to be valid. The observations are in agreement with a structure which is a hybrid between a true ketonic structure and a true mesoionic structure. By using the difference in the ¹³C chemical shifts of ortho and meta carbon atoms in substituent phenyl groups it is possible to qualify the degree of coplanarity of the phenyl groups with the backbone of the molecule.     

13C n.m.r. spectra of steroids —a survey and commentary

The literature on ¹³C n.m.r. examinations of steroids has been reviewed and the shielding data for over 400 examples are tubulated. The assignments for each compound have been considered and, where necessary, revised in the light of more recent evidence and for consistency throughout each series. The methods available for assignments are reviewed and, in many cases, illustrated with specific examples. The major practical features concerning ¹³C studies of steroids are discussed as a guide to the use of the technique. From the main body of shielding data, an extensive set of substituent effects has been generated to aid the examination of new systems. The utility and the limitations of these effects are described.     

CIDNP assisted assignment of carbon-13 NMR lines in α-tetralone and α-indanone

The reversible abstraction of hydrogen from benzene-1,3,5-triol by photoexcited α-tetralone or α-indanone in solutions of methanol leads to CIDNP in the ¹³C NMR spectra of these compounds. The information gained from the phase of the polarization in the CIDNP spectra is used to resolve the ambiguities in the assignment of the ¹³C NMR lines for α-indanone and α-tetralone, arising from conflicting literature data. Chemical shifts for both compounds in methanol are reported. 2′-Methylacetophenone does not exhibit CIDNP in the ¹³C spectrum under similar conditions.