Enantioselective Ti(IV) sulfoxidation catalysts-bearing C3-symmetric trialkanolamine ligands:: Solution speciation by 1H NMR and ESI-MS analysis

被引:79
作者
Bonchio, M
Licini, G
Modena, G
Bortolini, O
Moro, S
Nugent, WA
机构
[1] Univ Padua, Dipartimento Chim Organ, CMRO, CNR, I-35131 Padua, Italy
[2] Univ Padua, Dipartimento Chim Farmaceut, I-35131 Padua, Italy
[3] Univ Ferrara, Dipartimento Chim, I-44100 Ferrara, Italy
[4] Dupont Co, Cent Res & Dev, Wilmington, DE 19880 USA
关键词
D O I
10.1021/ja983569x
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
Sulfoxidation catalysts generated from titanium(IV) isopropoxide and enantiopure trialkanolamine ligands promote the enantioselective oxidation of aryl alkyl sulfides to the corresponding sulfoxides, even at low (1-2%) catalyst loadings. Electrospray ionization mass spectrometry (ESI-MS), in combination with conventional low-temperature NMR techniques, provides a powerful tool for understanding the unique nature of these catalysts. Tetradentate ligation of the titanium atom blithe trialkanolamine ligand provides a highly robust 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 species formed when the catalyst is generated in situ with a slight excess of trialkanolamine are shown to consist of discrete 2:1, 3:2, and 4:3 oligomers in which the excess trialkanolamine bridges multiple titanatrane units. In the presence of excess tert-butyl hydroperoxide, all of the precatalyst species are: cleanly converted to a mononuclear titanium(IV) peroxo complex which serves as the active sulfoxidation catalyst. Ab initio molecular orbital calculations were used to probe the structure and position of protonation of the catalytic species. Other ionization techniques (fast ion bombardment or electron impact) proved less useful than ESI-MS due to levels of fragmentation during the ionization process.
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页码:6258 / 6268
页数:11
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