A Structural Study of Selenobenzamides: Crystal Structures and Dynamic 13 C NMR

The solid‐state structures of ( p ‐bromoselenobenzoyl)morpholine ( 2a ) and [ p ‐(dimethylamino)selenobenzoyl]morpholine ( 2b ) were determined by X‐ray diffraction. Both molecules show a flat selenoamide group. The larger contribution of resonance stabilization by the aromatic ring carrying the p ‐dimethylamino substituent is reflected by the smaller interplanar angle Θ between the aromatic ring and the selenoamide group [53.3(1)° vs. 81.1(1)°] and by the shorter length of the CSe bond [1.824(5) Å vs. 1.840(3) Å]. The Gibbs free energy of activation of C‐N bond rotation (Δ G ≠ rot ) of five p ‐substituted (selenobenzoyl)morpholines was determined by dynamic 13 C NMR. The activation barriers were found to range from 61.6 kJ/mol (X = NNMe 2 ) to 75.1 kJ/mol (X = H). The Δ G ≠ rot values of the corresponding (thiobenzoyl)morpholines were found to be from 3.2 kJ/mol (X = NMe 2 ) to 5.0 kJ/mol (X = H) lower. In both cases, Δ G ≠ rot showed an excellent linear Hammett correlation with s̀ + p .