1H‐NMR.‐spektroskopische Bestimmung der Enantiomerenreinheit von Allencarbonsäureestern mit optisch aktiven Europium‐Verschiebungsreagenzien

Abstract

1H‐NMR. Spectroscopic Determination of Enantiomeric Purities of Allenic Esters Using Optically Active Europium Shift ReagentsThe racemic allenic methyl esters 3‐‐7 and the racemic allenic diesters 8‐‐10 (cf. Scheme 2) in 1,1,2‐trichloro‐1,2,2‐trifluoroethane (TCFE) and CCl4 in the presence of optically active tris[3‐(heptafluorobutyryl)‐(+)‐camphorato]europium(III) (Eu(hfc)3) have induced unlike 1H‐NMR. shift differences (ΔΔδ) for the protons of the methoxycarbonyl groups of their enantiomers. In some allenic esters the shift reagent causes additional separation of resonance signals; thus, further substituents on the allenic framework may be differentiated in the racemic mixture. This finding provides a widely applicable method for the determination of absolute enantiomeric purities of allenic esters and their corresponding acids. Accordingly we found for optically pure (+)‐(S)‐2‐methyl‐2,3‐pentadienoic acid ((+)‐(S)‐13; cf. Fig. 2) a calculated [α] value of + 73.3 ± 1.8°. Finally, the substituent effects on ΔΔδ‐values (cf. Table 1‐‐3) are discussed.