A structural study of selenobenzamides: Crystal structures and dynamic13C NMR

Article abstract of DOI:10.1002/cber.19971300108

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 0 between the aromatic ring and the selenoamide group [53.3(1)° vs. 81.1(1)°] and by the shorter length of the C=Se bond [1.824(5) A vs. 1.840(3) A]. The Gibbs free energy of activation of C-N bond rotation (ΔG_rot^?) of five p-substituted (selenobenzoyl)morpholines was determined by dynamic ¹³C NMR. The activation barriers were found to range from 61.6 kJ/mol (X = NNMe₂) to 75.1 kJ/mol (X = H). The ΔG_rot^? values of the corresponding (thiobenzoyl)morpholines were found to be from 3.2 kJ/mol (X = NMe2) to 5.0 kJ/mol (X = H) lower. In both cases, AG t showed an excellent linear Hammett correlation with σ_p⁺. VCH Verlagsgesellschaft mbH.