Application of NMR spectroscopy of chiral association complexes. 8—Rotation about the C(sp2)–C(aryl) Bond in 2,6–disubstituted benzamides

✍ Scribed by Miroslav Holik; Albrecht Mannschreck

• Book ID: 102951692
• Publisher: John Wiley and Sons
• Year: 1979
• Tongue: English
• Weight: 554 KB
• Volume: 12
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1270120411

✦ Synopsis

Abstract

The barrier to rotation about the C(sp^2^)C(aryl) single bond in non‐planar benzoyl compounds was investigated using N,N‐dimethyl, N,N‐tetramethylene and N,N‐diisopropyl derivatives of 2,4,6‐trimethylbenzamide and 2,6‐dimethoxybenzamide and N,N‐dimethyl derivatives of the corresponding thiobenzamides. Their ^1^H and ^13^C NMR spectra were determined and assigned and the splittings in the ^1^H spectra due to the addition of the optically active shift reagent (+)‐Eu(hfbc)~3~, are discussed. The free enthalpy of activation was calculated from the coalescence temperature of the ortho‐methyl or ‐methoxyl signals and from a line‐shape analysis. An exact equation for the determination of the rate constant at coalescence and a valid approximation which includes the line‐width are presented. A distinct effect of the size of the N‐substituent on the barrier to rotation in 2,6‐dimethoxybenzamides was observed, while the π values for the 2,4,6‐trimethylbenzamides studied are practically identical.