Determination of the enantiomeric composition of chiral Δ-2-oxazolines-1,3 by 1H and 19F NMR spectroscopy using chiral lanthanide-shift reagents

The 300 MHz H-1 NMR spectra of 2-ethyl Delta(2) -oxazoline 1m have been studied in CCl4, CD3CN and C6D6 solutions, in the presence of the achiral lanthanide shift reagent (LSR), tris (6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato)-europium III, 4 known as Eu(fod)(3), (see Sch. 1). All the protons of 1 were deshielded at various extent, and the sequence observed for their Deltadelta suggested a major complexation at the basic N(3) center of the heterocycle. Then the chiral monosubstituted oxazoline 1e and the disubstituted oxazolines 1Aa-d and 1Ba, were studied in the presence of chiral LSR, tris-[D, D dicampholylmethanato] europium III Eu(dcM)(3) 5 and tris-[3-(heptafluoropropylhydroxy-methylene)-d-camphorato] praseodym III Pr(hfC)(3) 6. H-1 NMR, and eventually F-19[H-1] NMR spectra were mostly recorded in C6D6 solution. Substantial to very important enantiomeric shift difference DeltaDeltadelta values were observed i/ for proton signals concerning the diastereomeric methyl group with all the oxazolines, for the ortho aryl protons of 1Aa and 1A, and i/ for the F-19[H-1] signals of the fluorinated oxazolines 1Aa-b and 1Ba. NMR results are discussed from substrate and CLSR structure standpoints. For all tested chiral oxazolines there is at least one possibility to proceed to their enantiomeric discrimination either by H-1 or F-19 NMR using Eu(dcm)(3) or Pr(hfC)(3) as CLSR.