ACTIVATION OF ALPHA-BROMO KETONES BY COMPLEXATION WITH HARD AND SOFT LEWIS-ACIDS - A COMBINED X-RAY AND NMR-STUDY

Alpha-halo ketones have two heteroatoms with lone pairs where a Lewis acid can be bound, depending on the softness or hardness of the Lewis acid (HSAB principle). The crystal structures of the complexes 2.SbCl5, 3.SbCl5, 8.AgSbF6, and (2)2.AgSbF6 have been determined in order to analyze the structural and electronic changes in the bromo ketone molecules upon complexation and the different binding properties of a hard (SbCl5) and a soft Lewis acid (Ag+). The monodentate Lewis acid SbCl5 binds to the oxygen atom, whereas the silver ion is coordinated to the O and the Br atom of a bromo ketone in a chelate-like manner and to the pi-systems of phenyl rings. H-1 and C-13 NMR measurements of the complexes and of the pure bromo ketones in solution support and complete the results of the X-ray structure determinations: SbCl5 activates the C=O bond by raising the contribution of the C+-O- resonance formula, whereas Ag+ activates preferentially the C-Br bond by raising the contribution of the C+Br- resonance formula. The observations are in agreement with reactivity data from the literature.