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Title: Mechanism of the Sharpless Epoxidation Reaction: A DFT Study
The Sharpless reaction is an enantioselective epoxidation of prochiral allylic alcohols that employs a Ti(IV) catalyst formed from titanium tetra(isopropoxide), Ti(O-i-Pr)4, diethyl tartrate (DET) and the oxidizing agent tert-butyl hydroperoxide. The M06-2X DFT functional with the 6-311+G(d,p) basis set has been employed to model the structures and energetics of the Sharpless epoxidation reaction. The monomeric tetracoordinate titanium(IV) diethyltartrate is thermodynamically strongly favored to dimerize, producing pentacoordinate catalyst, [Ti(DET)(O-i-Pr)2]2, that is a more reactive chiral epoxidation catalyst. The rapid ligand exchange reactions needed to generate the “loaded” catalyst and to repeat the overall catalytic cycle have been examined and are found to have activation energies that are much lower than the epoxidation barriers. The transition structures and activation energies for the enantioselective epoxidation of allyl alcohol, trans-methyl-allyl alcohol and trans-tert-butyl-allyl alcohol with the “loaded” Sharpless catalyst, [Ti2(DET)2 (O-i-Pr)2-(OAllyl)-(OOt-Bu)], are presented. The effect of the C=O•••Ti interactions on the activation energies and the significance of the O-C-C=C dihedral angle on the enantioselectivity of the epoxidation reaction are discussed.  more » « less
Award ID(s):
1856437
PAR ID:
10500150
Author(s) / Creator(s):
;
Publisher / Repository:
American Chemical Society
Date Published:
Journal Name:
The Journal of Physical Chemistry A
Volume:
128
Issue:
11
ISSN:
1089-5639
Page Range / eLocation ID:
2072 to 2091
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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