The nature of the Katsuki-Sharpless asymmetric epoxidation catalyst

The ternary complexes formed by reactions of Ti2DIPT2(OiPr)4 (H2DIPT = (2R,3R)-diisopropyl tartrate) with N-benzoyl-N-phenylhydroxylamine, triethylamine, and diisopropylamine (3-5) were examined by NMR spectroscopy in order to link solid-state with solution-state structures and to obtain NMR data for chelating DIPT units lacking ester coordination. The chemical shift differences and the coupling constants between the tartrate skeletal methines in these three complexes were significantly different from those in tartrate complexes previously examined. A linear relation was found between the chemical shift differences at methine positions in various tartrate ester-Ti(IV) alkoxide complexes (i.e. Katsuki-Sharpless catalysts), and the coupling constants (3J(HH)) between them. The H-C-C-H dihedral angles among the 2:2 complexes were calculated to span about 30-degrees. Parallel changes in the C-13-NMR positions and in the 1J(HC) and 2J(HC) values indicated that as the 3J(HH) values increase, the methines become more and more similar. Further, shielding of one OCH by metal-bound carbonyl was deduced to be at the origin of the H-1-NMR chemical shift changes accompanying the angle changes. Along with supporting IR, kinetic, and other evidence, it is argued that these trends reflect a transition between chelating and nonchelating modes of diolate ligation, the latter being stabilized by stronger carbonyl coordination and pi donation, and served to confirm that the parent catalyst, Ti2DIPT2(OiPr)4, is best represented by an open, monocyclic structure (A). The pentacoordination implied in A can explain much of the reactivity of Ti2DIPT2(OiPr)4 compared to that of the hexacoordinate complexes of non-tartrate diols. It is argued that the various ester-alkoxide combinations will equilibrate and catalyze epoxidations by the same mechanisms and via the same open structure. Explanations are provided for the success in epoxidation with tartrates, for the lack of success with non-tartrates, and for the epoxidation behavior with two tartrate homologues.