Browsing by Author "Chung, 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.
A novel iodine-induced sequential cyclization reaction of norbornene derivatives leading to the formation of novel iodo-cage compounds
(1996-02-07)
Wu, HJ
;
Tsai, SH
;
Chung, WS
;
交大名義發表
;
應用化學系
Treatment of the bis-endo-thioester and acyl group substituted norbornenes 1a-d and 9a-c with iodine in aqueous tetrahydrofuran at 25 degrees C gave the novel iodo-cage compounds 2a-d and 10a-c in 80-90% yields respectively, the first example of sequential cyclization of norbornene derivatives induced by an iodine electrophile.
Calix[4]arenes with a lid in their upper rims: 1,3-dipolar cycloaddition reactions of benzonitrile oxides with 5-allyl-, 5,11-dially- and 5,17-diallylcalix[4]arenes
(2000-02-01)
Shu, CM
;
Lin, WL
;
Lee, GH
;
Peng, SM
;
Chung, WS
The upper rim substituted mono- and diisoxazolinomethylcalix[4]arenes 9-23 are synthesized in good yields from 1,3-dipolar cycloaddition reactions of para-substituted phenyl nitrileoxides with 5-allyl-, 5,11-diallyl- and 5,17-diallycalix[4]arenes 5-7; the structures of 9-23 are consistent with a 'cone' conformation and the 'vase-with-a-lid' structure of 11 was confirmed by a single-crystal x-ray analysis. Preliminary results show that the monoisoxazolinomethylcalix[4]arene can form an inclusion complex with ammonium cation.
Cycloadditions of 16-electron 1,3-dipoles with ethylene. A density functional and CCSD(T) study
(1999-09-03)
Su, MD
;
Liao, HL
;
Chung, WS
;
Chu, SY
;
應用化學系
The 1,3-dipolar cycloaddition (DC) reactions of ethylene with nitrile ylide (CNC), nitrile imine (CNN), nitrile oxide (CNO), diazomethane (NNC), azine (NNN), and nitrous oxide (NNO) in the gas phase were examined using the density functional theory and CCSD(T) calculations. All of the structures, including the precursor complexes;and the transition structures, were completely optimized at the B3LYP/6-31G* level with single-point energies evaluated at CCSD(T)/6-311G**. The theoretical results suggest that the activation energies for the DC reactions of nitrile-type molecules (CNC, CNN, and CNO) are small (5.1-11 kcal/mol) and these reactions are very exothermic (-77 to -46 kcal/mol). In contrast, the DC reactions of NNC, NNN, and NNO are less exothermic (-39 to -6.0 kcal/mol) and have larger activation barriers (13-29 kcal/mol). Moreover, this work shows that the configuration mixing (CM) model based on Press and Shaik's theory can successfully predict the relative ordering of the activation energy and reaction enthalpies of DC reactions; Combining our theoretical calculations and the CM model, the following conclusion emerges: a 16-electron 1,3-dipole reactant with more electropositive substituents at the terminal positions will possess a smaller singlet-triplet splitting. This will facilitate cycloaddition with the dipolarophile and will result in a larger exothermicity.
Density functional study of the relative reactivity in the concerted 1,3-dipolar cycloaddition of nitrile ylide to disubstituted ethylenes
(2001-07-05)
Liao, HY
;
Su, MD
;
Chung, WS
;
Chu, SY
;
應用化學系
Density functional theory was used to perform a theoretical evaluation of (E)-1,2-disubstituted ethylenes as dipolarophiles for the 1,3-dipolar cycloaddition reaction. The reactivities of electron-withdrawing and -donating substituted ethylenes were examined by estimating their activation energies. The calculated activation energies predicted that the most reactive species is (E)-1,2-C2H2(NO)(2), whereas the least reactive is (E)-2-butene. Namely, it was demonstrated that 16-electron 1,3-dipole reactants with more electropositive substituents in terminal positions and ethylenes that possess more strongly electron-withdrawing substituents facilitate 1,3-dipolar cycloaddition reactions. All of the theoretical results can be rationalized using the configuration mixing model. (C) 2001 John Wiley & Sons, Inc.
Face selection in addition and elimination in sterically unbiased systems
(1999-05-01)
Kaselj, M
;
Chung, WS
;
le Noble, WJ
;
應用化學系
;
Department of Applied Chemistry
Face selectivity in the 1,3-dipolar cycloaddition reactions of benzonitrile oxide with 5-substituted adamantane-2-thiones and 2-methyleneadamantanes
(1997-07-11)
Chung, WS
;
Tsai, TL
;
Ho, CC
;
Chiang, MYN
;
leNoble, WJ
The 1,3-dipolar cycloaddition reactions of benzonitrile oxide with 5-substituted adamantane-2-thiones (2-X) and 2-methyleneadamantanes (3-X) produced two geometrically isomeric Delta(2)-1,4,2-oxathiazolines (5-Xs) and two Delta(2)-isoxazolines (6-Xs), respectively. The substituent was varied from fluoro, chloro, bromo, to phenyl. X-ray single-crystal analysis confirmed the configuration of (Z)-5-F. The produce formation bias resulting from the favored attack of nitrile oxide on the zu-face is discussed in terms of transition-state hyperconjugation and frontier molecular orbital theory.
Face selectivity in the photocycloaddition reactions of acrylonitrile to 5-substituted adamantan-2-ones and pyrolysis of the products to methyleneadamantanes
(1997-03-01)
Chung, WS
;
Ho, CC
;
交大名義發表
;
應用化學系
;
National Chiao Tung University
The photocycloaddition of acrylonitrile to 5-substituted adamantan-2-ones :(l-X) produces two geometrically isomeric oxetanes in which the oxygen atom and the 5-substituent are in the anti or syn positions, The substituent was varied from fluoro, chloro, bromo, hydroxy to phenyl and the product ratios were similar (ca, 60:40) in all instances, Small portions (<5%) of the oxetanes were pyrolysed when analysed by GC at above 200 degrees C, and the products from the pyrolysis were confirmed to be the corresponding methyleneadamantanes by independent syntheses, Assignment of the configuration of the oxetanes was found to be consistent with C-13 additivity scheme and the chemical shifts are more closely predicted using known oxetanes of l-X with methacrylonitrile. The product formation bias resulting from the attack on the syn-face can be explained using-the Cieplak transition state theory.
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.
Face selectivity in the reactions of 2,4-disubstituted adamantanes.
(2000-08-20)
Chung, WS
;
Chu, JH
;
應用化學系
;
Department of Applied Chemistry
Iodine-induced cyclization reaction of endo-thioester substituted norbornenes followed by methylthio group rearrangement
(1996-11-04)
Wu, HJ
;
Tsai, SH
;
Chung, WS
;
應用化學系
;
Department of Applied Chemistry
Treatment of the endo-thioester group substituted norbornenes 3a-3d with iodine in aqueous tetrahydrofuran at 25 degrees C gave the novel methylthio group rearranged lactonization products 4a-4d in 80% yields; iodolactonization reaction of 9 was applied to the synthesis of novel diacetal trioxa-cage compound 13. Copyright (C) 1996 Elsevier Science Ltd
Photochemistry and photodissociation of benzosultine and naphthosultine: electronic relaxation of sultines and kinetics and theoretical studies of fragment o-quinodimethanes
(2005-02-28)
Tang, KC
;
Lee, SJ
;
Chi, SH
;
Lu, KL
;
Chen, WC
Fluorescence decays of benzosultine and naphthosultine excited at 263 nm are detected with time-correlated single-photon counting (TCSPC). Both molecules display biexponential decay with a rapid component (time constants 90 and 350 ps for benzosultine and naphthosultine, respectively) assigned to an electronically excited state S-n at high energy and the slow one (constants 7.5 and 9.0 ns for benzosultine and naphthosultine, respectively) to the S-1 state. Dissociation of both molecules was investigated with nanosecond laser flash photolysis combined with transient absorption to detect the intermediates and products. With excitation at 266 nm, benzosultine yields a transient with absorption maximum at lambda(max) = 370 nm; this transient has a short-lived component with lifetime around 1 mus and a long-lived component. Both components are insensitive to molecular oxygen. The short-lived component is tentatively assigned to the dissociation intermediate and the long-lived to be singlet o-benzoquinodimethane (o-BQDM). Photolysis of naphthosultine yields two transient species with absorption at; lambda(max) = 420 and 520 nm; we assign the former band to triplet-triplet absorption of naphthosultine and the latter to absorption by the product singlet o-naphthoquinodimethane. Optimal geometries, energetics, and vertical transitions of benzosultine, naphthosultine, o-benzoquinodimethane, and o-naphthoquinodimethane are calculated using methods based on density-functional theory (B3LYP/6-311++G**) and time-dependent density-functional theory (TD-DFT). The results of these calculations imply that the ground electronic state of these two o-quinodimethanes (o-QDM) is singlet with a structure of diene form. Their triplet states display a biradical structure. The energy separation between singlet and triplet states of o-benzoquinodimethane is calculated to be ca. 93.6 kJ/mol, but for o-naphthoquinodimethane, it is only ca. 27.8 kJ/mol. (C) 2004 Elsevier B.V. All rights reserved.
Photochemistry of acetone in the presence of exocyclic olefins: An unexpected competition between the photo-Conia and Paterno-Buchi reactions
(1997-02-07)
Chung, WS
;
Ho, CC
;
交大名義發表
;
應用化學系
;
National Chiao Tung University
When irradiated in the presence of several exocyclic olefins, acetone undergoes homoalkylation with the olefins to form a series of 4-cycloalkylbutan-2-ones (with quantum yields of 0.14 +/- 0.01) rather than exhibiting the expected Paterno-Buchi reaction; in contrast, the photolysis of perdeuteriated acetone gave both types of products.
Photochemistry of acetone in the presence of exocyclic olefins: An unexpected competition between the photo-Conia and Paterno-Buchi reactions
(1997-02-07)
Chung, WS
;
Ho, CC
;
應用化學系
;
Department of Applied Chemistry
When irradiated in the presence of several exocyclic olefins, acetone undergoes homoalkylation with the olefins to form a series of 4-cycloalkylbutan-2-ones (with quantum yields of 0.14 +/- 0.01) rather than exhibiting the expected Paterno-Buchi reaction; in contrast, the photolysis of perdeuteriated acetone gave both types of products.
Quinoxalino-fused sultines and their application in Diels-Alder reactions
(1997-01-21)
Chung, WS
;
Liu, JH
;
交大名義發表
;
應用化學系
;
National Chiao Tung University
The synthesis of 7,8-disubstituted quinoxalino[2,3-d]-[1,2 lambda(4)]oxathiine 2-oxides 7a-c, precursors for quinoxalino-o-quinodimethanes 3a-c, and their application in the Diels-Alder reactions are reported.
Stereoselectivity of the Diels-Alder reaction of (E)gamma-Oxo-alpha,beta-unsaturated thioesters with cyclopentadiene
(1996-06-01)
Tsai, SH
;
Chung, WS
;
Wu, HJ
;
應用化學系
;
Department of Applied Chemistry
The stereoselectivity of the Diels-Alder reaction of (E)-gamma-oxo-alpha,beta-unsaturated thioesters 3a-3d with cyclopentadiene is greatly enhanced in the presence of Lewis acids favoring the endo acyl isomers 4a-4d. In the absence of Lewis acid, Diels-Alder reaction of 3a-3d with cyclopentadiene at 25 degrees C gave two adducts 4a-4d and 5a-5d in a ratio of 1:1 respectively. In the presence of Lewis acids, Diels-Alder reaction of 3a-3d with cyclopentadiene gave 4a-4d and 5a-5d in ratios of 75 similar to 94:25 similar to 6 respectively. The stereoelectivity was enhanced to ratios of 95 similar to 98:5 similar to 2 with lowering the reaction temperature. The stereochemistry of the cycloadducts 4 and 5 was confirmed by iodocyclization. Reaction of the endo-thioester 5c with I-2 in aqueous THF at 0 degrees C gave the novel methylthio group rearranged product 6c in 80% yield, the first example of iodolactonization of endo-thioesters. Reaction of the endo-acyl isomer 4b with I-2 under the same reaction conditions gave an isomeric mixture of 7b and 8b in 1:2 ratio. The stereochemistry of the thioester group in 8b was proved by X-ray single-crystal analysis. The solvent effect on the endo selectivity of (Z)-gamma-oxo-alpha,beta-unsaturated thioester 2b was also examined.
Stereoselectivity of the Diels-Alder reaction of (E)gamma-Oxo-alpha,beta-unsaturated thioesters with cyclopentadiene
(1996-06-01)
Tsai, SH
;
Chung, WS
;
Wu, HJ
;
應用化學系
;
Department of Applied Chemistry
The stereoselectivity of the Diels-Alder reaction of (E)-gamma-oxo-alpha,beta-unsaturated thioesters 3a-3d with cyclopentadiene is greatly enhanced in the presence of Lewis acids favoring the endo acyl isomers 4a-4d. In the absence of Lewis acid, Diels-Alder reaction of 3a-3d with cyclopentadiene at 25 degrees C gave two adducts 4a-4d and 5a-5d in a ratio of 1:1 respectively. In the presence of Lewis acids, Diels-Alder reaction of 3a-3d with cyclopentadiene gave 4a-4d and 5a-5d in ratios of 75 similar to 94:25 similar to 6 respectively. The stereoelectivity was enhanced to ratios of 95 similar to 98:5 similar to 2 with lowering the reaction temperature. The stereochemistry of the cycloadducts 4 and 5 was confirmed by iodocyclization. Reaction of the endo-thioester 5c with I-2 in aqueous THF at 0 degrees C gave the novel methylthio group rearranged product 6c in 80% yield, the first example of iodolactonization of endo-thioesters. Reaction of the endo-acyl isomer 4b with I-2 under the same reaction conditions gave an isomeric mixture of 7b and 8b in 1:2 ratio. The stereochemistry of the thioester group in 8b was proved by X-ray single-crystal analysis. The solvent effect on the endo selectivity of (Z)-gamma-oxo-alpha,beta-unsaturated thioester 2b was also examined.
Synthesis of 2,5-disubstituted thienosultines and their thermal reactions with dienophiles and nucleophiles
(2002-12-27)
Liu, WD
;
Chi, CC
;
Pai, IF
;
Wu, AT
;
Chung, WS
The 2,5-disubstituted thienosultines (5,7-disubstituted 1,4-dihydro-1H-3lambda(4)-thieno[3,4-d][2,3]oxathiin-3-oxides) 5a-d were prepared from the corresponding dichlorides 4a-d with the commercially available Rongalite (sodium formaldehyde sulfoxylate) in 17-60% yields. When heated in the presence of electron-poor dienophiles, sultines 5a-d underwent elimination of SO2, and the resulting non-Kekule biradicals 7a-d were intercepted as the 1:1 adducts 8-12 in good to excellent yields. The pyrolysis of sultines and sulfolenes with different concentrations of dienophiles revealed that either a preequilibrium between starting reagents and biradical species or Diels-Alder and retroDiels-Alder reaction mechanisms may be involved; however, more work is necessary to establish the proposed mechanisms. Reaction of sultine 5b with nBuLi was found to undergo a nucleophilic ring-opening reaction to give sulfinyl alcohol 17 after H2O workup. When sultine 5a was heated in benzene in a sealed tube in the presence of methanol, methanol-d4, or 2-mercaptoethanol, the respective 1:1 trapping adducts 19-21 as well as the rearranged sulfolene 6a were isolated in similar amounts. The isolation of adducts 19-21 may be explained by the involvement of either biradical or ionic intermediates during the pyrolysis.
Synthesis of 3,5,7-trioxapentacyclo[7.2.1.0(2,8).0(4,11).0(6,10)]dodecane. A novel diacetal trioxa-cage
(1996-10-01)
Tsai, SH
;
Wu, HJ
;
Chung, WS
;
交大名義發表
;
應用化學系
3,5,7-Trioxapentacyclo[7.2.1.0(2,8).0(4,11).0(6,10)] dodecane, the parent compound of novel diacetal trioxa-cages, was synthesized from maleic anhydride cyclopentadiene adduct 1 by a four-step sequence. Attempts for the synthesis of monoaza dioxa-cage 12 failed. Ozonolysis of compound 9 in CH2Cl2-EtOH (1:1) at -78 degrees C followed by reduction with Me(2)S gave 13 in 65% yield.
Synthesis of calix[4]arenes with four different "lower rim" substituents
(1999-04-16)
Shu, CM
;
Chung, WS
;
Wu, SH
;
Ho, ZC
;
Lin, LG
Calix[4]arenes with four different "lower rim" substituents were synthesized following six-step sequences. The first alkyl group (R-1) was introduced using R1X and CH3ONa, and then reaction with R2X and K2CO3 led to the introduction of a second alkyl group (R-2) at the 3-position. Benzoate was introduced as a protecting group at the 1,3-O-disubstituent stage and was removed under basic conditions after the incorporation of the third alkyl group (R-3) With R3X and NaH, The final alkyl group (R-4) was introduced with R4X and NaH to give the chiral title compounds.