Stereochemical aspects of proton chemical shifts. VI—increments due to geminal hydroxyl,methyl substitution

In geminal methyl, hydroxyl substitution an axial hydroxyl, equatorial methyl arrangement has a downfield effect on a syn axial hydrogen atom which is larger by 0.3 ppm than an axial methyl, equatorial hydroxyl arrangement. It is proposed that these observations may constitute a basis for configurational assignment.     

Stereochemical aspects of proton chemical shifts. X—The slow exchange 1H NMR spectrum of cis-transoid-cis-perhydroanthracene and of cis-decalin

The slow exchange 360 MHz ¹H NMR spectra of cis-transoid-cis-perhydroanthracene and of cis-decalin have been assigned. Shift increments due to the cis-annelation of a tetramethylene bridge to cyclohexane are presented.     

Stereochemical aspects of proton chemical shifts—V: The cumulative nature of the proton shift by vicinal substitution

It is shown that the proton chemical shift may be predicted with fairly good precision for a proton in an ethano fragment carrying a (long) vicinal substituent. Caution must be used however in calculating geminal effects.     

Stereochemical aspects of proton chemical shifts. VII. The γ effects of hydroxyl,methoxyl and methyl substituents

The ¹H NMR chemical shifts of some hydroxy, methoxy or methyl substituted trans-decalins, trans-1, 3-dioxadecalins and cyclohexanes are reported. It is concluded that the replacement in a g⁺g⁺ H? C? C? C? H fragment of one hydrogen by hydroxy, methoxy or methyl results in a modest (0.1 ppm) upfield shift of the other hydrogen atom. Experimental limitations to the transferability of shift increments from one molecular environment to another are demonstrated. The syntheses of 1α,5β-dimethoxy- and 1β,5α-dimethoxy-trans-decalin are given.     

Stereochemical aspects of proton chemical shifts—IV: The cumulative nature of the geminal proton shift influenced by long chain substituents

Substituent shift effects on geminal protons are consistent with cumulative α-, β- and γ-effects of a long chain substituent, e.g. whereby not only the first, but also the subsequent β- and γ-atoms of the sidechain should be taken into consideration. These shift contributions depend on rotational conformation in a way that lends itself to calculation where the necessary data are available. Therefore the shifts of the unsubstituted parent compound, the individual increment-contributions of the β and γ groupings and the rotameric distribution must be known. This would enable, conversely, a semi-quantitative prediction of one of these parameters if the others are accessible.     

Lanthanide-induced shifts in proton NMR spectra—IV: Praseodymium-induced shifts to lower applied fields and obscured chemical shifts from lanthanide-induced shift (LIS) data

Praseodymium(III) tris(dipivalomethanato) [Pr(DPM)₃] reduces the complex overlapping aromatic absorptions in the proton NMR spectrum of a monosubstituted naphthalene, cis-3-(1-naphthyl)-1,3,5,5-tetramethlcyclohexan-1-ol( 1 ) to a virtually first-order pattern, whereas Eu(DPM)₃ at the same and higher molar concentrations does not completely resolve the aromatic proton signals. Plots of the Pr(DPM)₃-induced shifts measured in carbon tetrachloride solution versus molar equivalents of Pr(DPM)₃ added may be extrapolated to provide accurate chemical shifts in the absence of added lanthanide shift reagent (LSR). The qualitative conformational conclusions from the Pr(DPM)₃ shifts agree with those from the Eu(DPM)₃-induced shifts, but there are detailed differences in the relative lanthanide-induced shifts (LIS) for different proton types in the molecule.     

Configurational assignments in some dihydrobenzo[c]thiophenes by NMR solvent effects. Correlation of the relative chemical shifts of α-methyl groups with sulphoxide stereochemistry

The configuration of isomeric 3,3-dimethyl-1-phenyl-1,3-dihydrobenzo[c]thiophene 2-oxides was assigned on the basis of selective benzene- and TFA-induced NMR shifts. The same method was employed for assigning the α-methyl resonances of these sulphoxides. Examination of NMR data for a variety of cyclic sulphoxides revealed that in CCl₄ or CDCl₃ solution, α-hydrogen cis to S? O often resonates at higher magnetic field, while α-methyl group cis to S? O characteristically resonates at lower field, than the same hydrogen or methyl group trans to S? O, respectively.     

Carbon-13 NMR studies of substituted naphthalenes. I—complete assignments of the 13C chemical shifts with the aid of deuterated derivatives

Proton-coupled as well as noise-decoupled ¹³C NMR spectra of several substituted naphthalenes have been studied. Complete assignments of the ¹³C signals based on selectively deuterated derivatives and on the ¹³C? ¹H coupling pattern have been made. For the methoxynaphthalenes, acetylnaphthalenes and naphthaldehydes the dominant conformations of the substituents have been deduced from the ¹³C chemical shifts.     

The proton resonance spectra of heterocycles—VI: The correlation of substituent effects on chemical shifts in bicyclic compounds

The proton chemical shifts are reported for monosubstituted naphthalenes, quinolines and quinoxalines. Together with literature data, these chemical shifts are compared with the parent compounds and substituent effects evaluated statistically. The effect of substituents parallels that in benzenes, but is modified by bond fixation, steric hindrance and other effects which can, at least qualitatively be understood. The treatment enables estimation of likely chemical shifts for ABC spectra in fused aromatic systems which should facilitate the solution of such spectra.     

Transmission of substituent effects through unsaturated systems. I—relative magnitudes of proton and carbon substituent chemical shifts in ethylenes,butadienes, α-enones and benzenes

The carbon-13 shifts of C-1, C-2 and C-3 are determined in a series of 1-cyclohexen-3-ones substituted in position 1. Linear relationships are demonstrated between the substituent chemical shifts of corresponding carbons in substituted ethylenes, butadienes, α-enones and benzenes. The substituent chemical shifts of proton H-2 are also reported and correlated with those of corresponding protons in ethylenes and benzenes. The slopes of the lines for the carbons directly linked to the substituent are close to unity, showing a relative independence of the substituent effect for this nucleus from the variation of the unsaturated framework. In contrast to this, the transmission of the substituent effect through one double bond (nuclei β to the substituents) decreases as the number of conjugated π bonds in the whole structure increases. This relationship is interpreted as being due to the ability of an unsaturated system to spread the variation of π electron density induced by the substituent.     

Stereochemical effects in mass spectrometry. 3—Detection of chirality by chemical ionization mass spectrometry

It was found that the chemical ionization mass spectra of the enantiomers of some α-amino acids and α-hydroxy acids could be definitely distinguished when a chiral compound, l-amyl alcohol, was used as a component of the reagent gases. The relative abundances of some characteristic ions produced through ion-molecule reactions in the gas phase were much higher in the D isomers than in the L isomers. The results were highly reproducible. This approach proved to be a convenient way for the detection of the chirality of these compounds by chemical ionization mass spectrometry.     

Stereochemical studies—VIII Unequivocal determination of the absolute configuration in biaryl systems

According to the reaction sequences shown in Figs 1-5, the absolute configuration of optically active biaryl derivatives such as biphenyl, binaphthyl, bianthryl and bianthroaquinonyl have been clearly established, based on the standard compound (R)-(+)-I, in the binaphthyl system, whose absolute configuration was unequivocally determined by the anomalous X-ray diffraction method.     

Magnetic circular dichroism of cyclic π-Electron systems. 24. Polarizations and assignments in some pentacyclic and hexacyclic benzenoid hydrocarbons

Measurements of magnetic circular dichroism and of linear dichroism in stretched polyethylene permitted the assignment of a large number electronic transitions in dibenz[a,c]anthracene, dibenza[a,h]anthracene, dibenz[a,j]anthracene, picene, and benzo[rst]pentaphene. The results are in satisfactory agreement with π-electron calculations. Implications for the analytical chemistry of these hydrocarbons and their simple derivatives are discussed.     

An NMR study of 2-alkoxy-1,3,2-dioxaphospholanes. I—13C chemical shifts

Measurements of the ¹³C chemical shifts of 2-alkoxy-1,3,2-dioxaphospholanes have allowed the determination of the contribution of the substituent to the α-, β- and γ-carbon chemical shifts of attached alkyl groups. The preliminary assignments of the signals were made using the following information; relative intensities, variations in coupling constants J(³¹P¹³C) and the existence of linear correlations between the shifts of carbon atoms in the P-alkoxy groups and the degree of substitution of the observed carbon or of its neighbours.     

Distribution of the electron density and proton chemical shifts in a series of nitromethane derivatives

Shifting of the PMR signal of the CH proton to the strong field with an increase in the positive charge on the proton was found in nitroalkanes and nitrocarboxylic acid esters. This shift is due to the presence of steric interactions between the substituents and the CH proton, which are manifested by a change in the paramagnetic contribution to the magnetic shielding constant.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1769–1773, August, 1989.     

NMR study of organosilicon compounds. III—Silicon-29, nitrogen-14, carbon-13 and proton NMR spectra of silylakylamines

²⁹Si, ¹⁴N ¹³C and ¹H NMR data are presented for a series of homologous (methylethoxysilyl)alkylamines of the type (CH₃)_3?n(C₂H₅O)_nSi(CH₂)_mNH₂(n=o to 3; m = 1 to 4). The measured ¹³C and ¹H chemical shifts correlate with the total net charges Q_A on the corressponding atoms, estimated by the Del Re method. ¹⁴N and ²⁹Si chemical shifts which do not show simple linear relationships to the charges are found to correlate with the relative basicities of the compounds. The influence of the remote substituent (? NH₂ and others) on the ₂₉Si chemical shifts is shown to depend on the number and nature of substituents directly on the silicon atom. Argyments for d-orbital participation in the Si? O bounds are given. The chemical shifts of ²⁹Si, ¹⁴N and ₁₃C nuclei are not consistent with the fromation of intramolecular ‘long bonds’ between the solicon and nitrogen atoms in aliphatic silymethylamines.     

A unified correlation of substituent effects with carbon,proton and fluorine chemical shifts in aromatic and olefinic systems

At the present time no completely satisfactory quantum mechanical calculations exist for carbon, proton or fluorine chemical shifts in various substituted aromatic or olefinic systems. However, the chemical shifts in such systems have been shown to be well correlated by a linear multiple regression analysis with the Swain and Lupton field and resonance para meters ? and ?, and the semiempirical parameter Q. The utility of Q in testing substituent stereochemistry has been exemplified previously. Here the applications of the complete regression analysis to a wide variety of different systems for the three nuclei are given. The correlation is also shown to apply to selenium in substituted selenophenes. The ¹³C chemical shifts for a series of ortho substituted toluenes are presented and comparisons made with other ortho disubstituted benzenes.