Mesoporous SBA-15 immobilized Proline-derived ligands with linkers of various lengths in asymmetric diethylzinc addition to benzaldehyde
Date Issued
2008
Date
2008
Author(s)
Chung, Hsin-chieh
Abstract
The main advantages of heterogeneous catalysts are the easy separation of the catalyst from the reaction mixture, allowing the recovery and reuse of the catalyst, and the possibility of continuous-flow operation. However, covalent anchoring a highly enantioselective catalyst to a solid support usually results in a significant loss of its activity and asymmetric induction capability in comparison to that of the homogeneous phase, attributed to the sterical hindrances of the catalytic site and the interaction between the active sites and the surface of the support. In the present study, chiral proline derivatives with linkers of various lengths were anchored to mesoporous silica SBA-15. Thiol-functional mesoporous SBA-15 (SH-SBA-15) was prepared by one-pot co-precipitation method. With the aid of NaCl salt, the silicate condensation was more complete and the residual silanol groups on the silica surface were reduced. Besides, the addition of NaCl salt in the initial mixture greatly enhanced the ordering of the mesostructure and the stabilities of the mesoporous materials. Vinyl-substituted chiral proline derivatives were then attached to SH-SBA-15 through AIBN-mediated radical addition of SH groups to C=C bonds. The resultant materials were examined as the catalysts in asymmetric diethylzinc addition to benzaldehyde reaction. The catalytic activity and chiral selectivity indeed increased progressively with the length of the tether linking the ligand to the silica surface. Moreover, the e.e. values and the conversions were obviously better over the mesoporous materials synthesized with NaCl than that without NaCl. The highest enantioselectivity of 74% and conversion of 82% was achieved using catalyst 20%Cl(1.5)-t-cata, attributed to the highest content of active sites. If comparison is based on the TON value, catalyst 10%SH(1.5)-t-8Ccata has the best performance (25.4/day), 68% ee and 75% conversion can be achieved.
Subjects
SBA15
Proline-derived ligands
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-97-R95223033-1.pdf
Size
23.32 KB
Format
Adobe PDF
Checksum
(MD5):277187665f47bb6d9410cb85fff9626e