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■ Hindered Rotation Around Amide OC-N Bonds: Its Energy Dependence on C-N-C Bond Angle

Tadahisa Sato, Yoshimasa Fukazawa, Yutaka Fujise, and Shô Itô*

*Department of Chemistry, Graduate School of Science, Tohoku University, Aobayama, Sendai 980-8578, Japan

Abstract

Compounds 1-5 exhibit the presence of rotamers in their PMR spectra measured at room temperature. After assignment of signals to each rotamer, temperature dependent change of the signal shape was simulated for 3-5 using line-shape method. Activation parameters deduced therefrom contain usually large free energy of activation for all of these compounds compared with the corresponding dimethylamides. This abnormality was ascribed to the additional destabilization in rotational transition states because of the unusually wide C-N-C angle in the cage system.
To support the large free energy of activations, pure rotamer 5A and nearly pure rotamer 5B were isolated by preparative TLC.