Browsing by Author "Li, WS"

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A computational study of regioselectivity in a cyclodextrin-mediated Diels-Alder reaction: Revelation of site selectivity and the importance of shallow binding and multiple binding modes
(2003-02-17)
Li, WS
;
Chung, WS
;
Chao, I
;
應用化學系
;
Department of Applied Chemistry
The use of a cyclodextrin . Diels-Alder transition structure complex (CD . TS) as a model in molecular dynamics simulations has enabled us to gain insight into the controlling factors in the cyclodextrin-mediated Diels-Alder reaction of methyl-p-benzoquinone with isoprene. MD Simulations were carried out with multiple binding configurations of the CD . TS (TS = meta-TS, para-TS) complexes at the top and bottom rims of beta-CD. We discovered that i) only shallow binding with the CD is necessary for the regioselectivity, and multiple binding geometries are possible: ii) the narrow bottom rim. with the primary hydroxyl groups, of the CD binds both regio-TSs better than at the wider top rim (secondary hydroxyl groups), which was unexpected from the perspective of shape complementarity that governs the stability of most CD . guest complexes. Overall, the bottom rim of the CD exhibits higher regioisomer discrimination for the meta-TS; iii) structural clustering analyses of the CD . TS configurations (sampled during MD simulations) have enabled us to evaluate the binding energies of the different binding configurations. The result indicates that there is a direct correlation between meta-product selectivity and a higher number of binding configurations favoring the formation of the CD . meta-TS complex, The main forces of stabilization in the CD . TS complexes are the van der Waals interactions when the TS is bound at the top rim. At the bottom rim, closer contacts between polar functional groups of the TS and CD have increased the importance of electrostatic interactions, We found that van der Waals, solvation, and torsional forces are less favorable for complexation at the bottom rime however, this is compensated by large favorable electrostatic interactions. With insights obtained from the study of CD . TS complexes and MD simulations of the modified heptakis-[6-O-(2-hydroxyl)propyl]-beta-CD, we were able to explain why a low selectivity was observed when the Diels-Alder reaction was carried out in this modified CD. Two types of search method [Monte Carlo and multiple minimum (MCMM) and molecular dynamics (MD)] to explore and evaluate the different possible binding geometries of the TS within beta-CD, were discussed.
Face selectivity in the reactions of 2,4-disubstituted adamantanes and their modification by inclusion in beta-cyclodextrin solutions
(2004-10-11)
Chu, JH
;
Li, WS
;
Chao, I
;
Chung, WS
;
應用化學系
Sodium borohydride reduction reactions on 4-X-adamantan-2-ones (where X=ethyleneketal 11, ethylenethioketal 12, and methylene 15) were studied, which gave Z-alcohols 16 and 17 (from en-face attack) as the predominant products for ketones 11 and 12, but gave 1: 1 mixture of Z- and E-18 alcohols for ketone 15. The en/zu face selectivity of 15 in sodium borohydride reduction was enhanced to 32/68 in beta-CD solution. Both 1,3-dipolar addition and dichlorocarbene addition reactions on 4-ethyleneketal-2-methyleneadamantane 13 underwent again predominant en-face attack to give products in an E/Z ratio of > 99:1 and 92:8, respectively. The exceptional high zu-face selectivity on the dichlorocarbene addition reaction of 15 may be explained by a temporal complexation between the carbene and the C-4-oxo group. In the epoxidation reaction of 13 and 15 the zu-face attack products were favored despite their steric congestions suggesting that hydrogen bonding interaction between the peroxide reagent and the C-4-oxo or 4-ethyleneketal is involved. (C) 2004 Elsevier Ltd. All rights reserved.