Conformational analysis. 15—Lanthanide-induced shift investigation of 2-exo-norborneol using tailored models of lanthanide complexation

✍ Scribed by Raymond J. Abraham; A. E. Rowan

• Publisher: John Wiley and Sons
• Year: 1988
• Tongue: English
• Weight: 758 KB
• Volume: 26
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1260261117

✦ Synopsis

The complete assignment of the proton chemical shifts for kxo-norborneol was obtained from a COSY experiment at 250 MHz. Using these assignments the conformation of this molecule in solution was studied using the lanthanide-induced shift technique, applying a new three-site lanthanide complexation model (LIRAS4) reflecting the coordination geometry at the hydroxyl oxygen atom.

The substrate geometry which was found to give the best correlation between the calculated and observed shifts was not in agreement with the optimized geometry calculated using ub initio methods, the CCO angle being optimized as 108" in the former compared with 110.80" in the latter. The lanthanide complex geometry found indicated that one of the sites (truns to H-2n) was occupied by the hydroxyl proton with the shift reagent bound to two other sites. This was supported by the increase in the vicinal J(HC0H) coupling observed on addition of the shift reagent. This LIRAS4 soltuion was then refined using a more complex model, HARDER, which treats each site as being independent of each other. The final two-site solution obtained reflects the symmetry about the binding site in this molecule, and gave an excellent correlation between the observed and calculated shifts