| Abstract
| - Sulfoxidation catalysts generated from titanium(IV) isopropoxide and enantiopure trialkanolamineligands promote the enantioselective oxidation of aryl alkyl sulfides to the corresponding sulfoxides even atlow (1−2%) catalyst loadings. Electrospray ionization mass spectrometry (ESI-MS), in combination withconventional low-temperature NMR techniques, provides a powerful tool for understanding the unique natureof these catalysts. Tetradentate ligation of the titanium atom by the trialkanolamine ligand provides a highlyrobust titanatrane core which is retained even in hydroxylic solvents and/or under acidic conditions. In contrast,the remaining apical coordination site is shown to be substitutionally labile. Previously ill-defined speciesformed when the catalyst is generated in situ with a slight excess of trialkanolamine are shown to consist ofdiscrete 2:1, 3:2, and 4:3 oligomers in which the excess trialkanolamine bridges multiple titanatrane units. Inthe presence of excess tert-butyl hydroperoxide, all of the precatalyst species are cleanly converted to amononuclear titanium(IV) peroxo complex which serves as the active sulfoxidation catalyst. Ab initio molecularorbital calculations were used to probe the structure and position of protonation of the catalytic species. Otherionization techniques (fast ion bombardment or electron impact) proved less useful than ESI-MS due to highlevels of fragmentation during the ionization process.
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