Abstract
The temperature dependence of several 7‐membered 5‐substituted‐5,11‐dihydrodibenz[b,e] [1,4]oxazepines (Z = O) and thiazepines (Z = S), as well as of 8‐membered 12‐substituted ‐6,11‐dihydro‐12__H__‐dibenz[b,f] [1,4]oxazocines (Z = O) and thiazocines (Z = S), has been studied by NMR spectroscopy. The average Δ__F__* (kcal/mol) values at coalescence temperatures are 14·0 (M = CHO), 18·2 (M = COCH~3~) for oxazepines, 17·3 (M = CHO), >21 (COCH~3~) for thiazepines, and 13·0 for oxazocines and 13·3 for thiazocines, respectively. These data show that replacement of oxygen by a sulfur atom raises the barrier height of ring inversion, but that this effect is greater with the 7‐membered than with 8‐membered heterocycles. Thus the 8‐membered rings have a greater degree of flexibility than do the 7‐membered ones. With the oxazepines and the thiazepines, it was also possible to measure the thermodynamics (Δ__F__*, Δ__H__* and Δ__S__*) of the hindered rotation around the NCO bond by studying the temperature dependence of the formyl and acetyl proton resonances. The results suggest that the processes of conformational inversion and hindered rotation in the same molecule are independent of each other. The rate of hindered rotation is slower than the rate of conformational inversion. The conformational preferences of the compounds are also discussed.