Lanthanide-induced shifts of diastereotopic groups

The chemical shift differences of diastereotopic protons in aliphatic acyclic alcohols, mostly of the type R? CH₂? CR′R″? (CH₂)_nOH, have been investigated. Fairly small amounts of Eu(dpm)₃ cause the spectra of these materials to simplify dramatically; indeed, even diastereotopic protons rather far removed form the hydroxyl group give discrete signals in the presence of the shift reagent. Large shift differences were realized in the γ-protons (n = 1) and the δ-protons (n = 2), particularly if R is bulky and R' and R″ have different steric requirements. Semi-quantitative conformational preferences can be determined from the data obtained.     

Lanthanide-induced shifts in proton NMR spectra of cyclohexanones

The lanthanide-induced shifts (LIS) of the lanthanide shift reagent (LSR) Eu(FOD)₃ are reported for a large number of cyclohexanones, especially those which are highly substituted. The following compounds were synthesized and characterized: 3-(aryl)-3,5,5-trimethylcyclohexanones, in which aryl = 1-naphthyl, phenyl, o-anisyl, m-anisyl, p-anisyl and p-chlorophenyl. Some analogous compounds were also studied: 3,5,5-trimethylcyclohexanone, 4-tert-butyl-cyclohexanone, 3,3,5,5-tetramethylcyclohexanone 3-(1-naphthyl)5,5-dimethylcyclohexanone and para-tert-butyl-anisole. A method for the regression analysis of the concentration dependence of the LIS of these substrates is developed and reported, and used to derive the limiting incremental LIS (Δ₂) for the LS₂ complex and to evaluate the proton chemical shifts (δ₀) in the absence of LSR. An ‘incremental dilution’ technique was found to be most appropriate to insure constant substrate concentrations, needed to extract both Δ₂ and δ₀. The conformations of the 3-(aryl)-type systems and analogous compounds were studied via LIS and found to conform to:—(i) an axially disposed aryl substituent in the 3,3,5,5-tetra-substituted cyclohexanones and (ii) a flattened chair form of the cyclohexanone ring with distortions in this chair form being an increase in the syn-diaxial (C-3, C-5) substituent distance (C-3 and C-5 substituents still eclipsed). The LIS were fully compatible with these structural assumptions.     

Substituent effect on ring proton chemical shifts in PMR spectra of monosubstituted pyridines

The relative chemical shifts of ring protons in PMR spectra of 2-, 3-, and 4-monosubstituted pyridines (in DMSO-D₆) are correlated with the F- and R-constants of the substitutents by using a two-parameter approach. Transmission factors characterizing the transfer of inductive and resonance effects in pyridine, pyrimidine, and s-triazine rings are analyzed.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1520–1523, November, 1989.     

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.     

31P chemical shifts in N-aryliminotriphenylphosphoranes

³¹P chemical shifts in N-aryliminotriphenylphosphoranes and the corresponding N-methylated phosphonium salts are presented. They can be correlated with those theories of the bonding in phosphorus-nitrogen ylides which involve partial double bonding as a result of p^π–d^π overlap.     

15N chemical shifts in aziridines

For a number of 1-substituted aziridines and also some 1,2-disubstituted aziridines it has been shown that electron-donating substituents on the nitrogen atom produce a downfield shift of the ¹⁵N resonance. The ¹⁵N chemical shifts of aziridines correlate with the ¹⁵N shifts in N,N-dimethylamines and primary amines as well as with the ¹⁷O shifts in oxiranes. A correlation is also observed between the ¹⁵N chemical shifts and the electronegativity of the substituents on the nitrogen atom.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, 1336–1339, October, 1988.     

Effect of the pH of the medium on the chemical shifts in the PMR spectra and on the distribution of the π-electron density

The dependence of the chemical shifts of the ring protons on the pH of the medium for a number of 4-hydroxyquinoline derivatives was studied by means of PMR spectroscopy. The dipolar and uncharged hydroxy forms exist in equilibrium in aqueous solutions. The effect of intramolecular hydrogen bonding on the character of the dependence of the chemical shift on the pH of the medium in the case of 3-piperidinomethyl-4-hydroxyisoquinoline was investigated. The possibility of the separate protonation of the ring nitrogen and the side-chain nitrogen in 3-piperidinomethyl-4-hydroxyisoquinoline was established. The distribution of the -electron density in 4-hydroxyisoquinoline is in good agreement with its chemical behavior.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1546–1549, November, 1971     

Effect of the pH of the medium on the chemical shifts in the PMR spectra and on the distribution of the π-electron density

The dependence of the chemical shift of the ring protons on the pH of the medium for a number of 3-hydroxyquinoline derivatives was studied by PMR spectroscopy. The regions of ionization of the molecules, which correspond to the ranges of acidic, neutral, and alkaline media, were found. In D₂O, 3-hydroxyquinoline is present only in the uncharged hydroxy form. The presence of intramolecular hydrogen bonding in 4-dimethylaminomethyl-3-hydroxyquinoline has a substantial effect on the character of the dependence of the chemical shift on the pH of the medium; the existence of separate protonation of the nitrogen atoms of the ring and the side chain of 4-dimethylaminomethyl-3-hydroxyquinoline was established. The distribution of the -electron density in the 3-hydroxyquinoline molecule is in good agreement with its chemical behavior during electrophilic substitution.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1540–1545, November, 1971.     

Tautomerism in monothio-β-diketones studied by PMR and quantum chemical calculations

It is shown by proton NMR and quantum chemical PCILO calculations that monothio-β-ketones

in solution exist as different isomers depending on substituents, solvent and temperature. In solvents with low dielectric constants the hydrogen-bridged cis enol form predominates, and is in fast equilibrium (in the NMR scale) with nonchelated trans enol forms down to ?100°C at least. Transition enthalpies are of the order of 7–15 kJ mol^?1. In compounds with R₂ = Me the rotation of the Me -CS group around the neighbouring C-C- bond can be detected and the thermodynamic and kinetic parameters estimated. The reason for the slowing down of this rotation, inferred from the line broadening in NMR, is probably the stabilization of a trans isomer by interaction between the protons of the methyl group and the lone pairs of the oxygen atom. There are no indications for the presence of chelated and non-chelated enethiol isomers from the PMR studies and quantum chemical calculations performed.