A STRUCTURAL STUDY OF SELENOBENZAMIDES - CRYSTAL-STRUCTURES AND DYNAMIC C-13 NMR

The solid-state structures of (p-bromoselenobenzoyl)morpholine (2a) an d [p-(dimethylamino)selenobenzoyl]morpholine (2b) were determined by X -ray diffraction. Both molecules show a flat selenoamide group. The la rger contribution of resonance stabilization by the aromatic ring carr ying the p-dimethylamino substituent is reflected by the smaller inter planar angle Theta between the aromatic ring and the selenoamide group [53.3(1)degrees vs. 81.1(1)degrees] and by the shorter length of the C=Se bond [1.824(5) Angstrom vs. 1.840(3) Angstrom]. The Gibbs free en ergy of activation of C-N bond rotation (Delta G(rot)(not equal)) of f ive p-substituted (selenobenzoyl)morpholines was determined by dynamic C-13 NMR. The activation barriers were found to range from 61.6 kJ/mo l (X = NNMe(2)) to 75.1 kJ/mol (X = H). The Delta G(rot)(not equal) va lues of the corresponding (thiobenzoyl)morpholines were found to be fr om 3.2 kJ/mol (X = NMe(2)) to 5.0 kJ/mol (X = H) lower. In both cases, Delta G(rot)(not equal) showed an excellent Linear Hammett correlatio n with sigma(p)(+).