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.