The Resolution of Chiral Dialkylarylsufonium Ions by 1H NMR Spectroscopy

The goal of this research was to design a rapid Nuclear Magnetic Resonance method to measure enantiomeric excess of chiral sulfonium ions by using chiral lanthanide shift reagents. The sulfonium ions studied were dialkylarylsulfonium ions. The shift reagent that was employed was Europium (III) surrounded by chiral ligands of 3-trifluoroacetyl-d-camphor (tfc), which is derived from (+)-camphor. Ethylmcthylphenylsulfonium ion was synthesized for study as a racemic and enantiomerically enriched mixture. The sulfonium ion was then ion-paired with the shift reagent for resolution of R and S enantiomers. The best resolution was obtained with the Eu(fod)4- shift reagent, with nearly baseline resolution of the aromatic ortho hydrogens of the phenyl ring. An analysis of the R-enriched mixture allowed assignments of the peaks as either R or S isomer. Heating of this R-enriched ion in the presence of the shift to 60 C allowed the observation of pyramidal inversion of the sulfonium ion. Other sulfonium ions were studied, including benzyhnethylphenylsulfonium ion. The diastereotopic hydrogens of this ion were baseline resolved into R and S isomers