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135 results on '"Chen, De-Zhan"'

24. Mechanistic Exploration of the Competition Relationship between a Ketone and C═C, C═N, or C═S Bond in the Rh(III)-Catalyzed Carbocyclization Reactions

Author

Xing, Yang-Yang, Liu, Jian-Biao, Sun, Chuan-Zhi, Huang, Fang, and Chen, De-Zhan

Abstract

The introduction of a C═O, C═C, C═S, or C═N bond has emerged as an effective strategy for carbocycle synthesis. A computational mechanistic study of Rh(III)-catalyzed coupling of alkynes with enaminones, sulfoxonium ylides, or α-carbonyl-nitrones was carried out. Our results uncover the roles of dual directing groups in the three substrates and confirm that the ketone acts as the role of the directing group while the C═C, C═N, or C═S bond serves as the cyclization site. By comparing the coordination of the ketone versus the C═C, C═N, or C═S bond, as well as the chemoselectivity concerning the six- versus five-membered formation, a competition relationship is revealed within the dual directing groups. Furthermore, after the alkyne insertion, instead of the originally proposed direct reductive elimination mechanism, the ketone enolization is found to be essential prior to the reductive elimination. The following C(sp2)—C(sp2) reductive elimination is more favorable than the C(sp3)—C(sp2) formation, which can be explained by the aromaticity difference in the corresponding transition states. The substituent effect on controlling the selectivity was also discussed.

Published
2024
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25. Mechanistic Investigation on the Regioselectivity of Electrochemical Co(II)-Catalyzed [2 + 2 + 2] Cycloaddition of Terminal Acetylenes

Author

Yu, Kai-Xin, Liu, Sheng-Nan, Xing, Yang-Yang, Huang, Fang, Wang, Wen-Juan, Wang, Qiong, Chen, De-Zhan, and Wang, Jin

Abstract

In this paper, the regioselectivity of electrochemical Co(II)-catalyzed [2 + 2 + 2] cycloaddition of terminal alkynes was investigated using density functional theory. We explored in detail the energy profiles for both 1,2,4- and 1,3,5-regioselectivity pathways and revealed the origin of the regioselectivity. Two kinds of conformational isomers derived from the different coordination modes of alkynes with cobaltacyclopentadiene have been found, which were formed through electrochemically mediated redox processes. The regioselectivity of the reaction depends on the two coordination modes. When the Co(II) center attacks α-C of the third alkyne, while β2-C in cyclopentadiene bonds to β-C of the alkyne, the reaction favors the formation of 1,2,4-products. In contrast, when the Co(II) center connects to β-C of the alkyne, it forms only the 1,3,5-products via [4 + 2] cycloaddition because of the steric repulsion between the bulky ligand on Co(II) and the phenyl group in the alkyne.

Published
2024
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39. A computational mechanistic study of Pd(ii)-catalyzed γ-C(sp3)–H olefination/cyclization of amines: the roles of bicarbonate and ligand effect.

Author

Liu, Jian-Biao, Tian, Ying-Ying, Zhang, Xin, Wang, Lu-Lin, and Chen, De-Zhan

Subjects
BICARBONATE ions, CHEMICAL reactions, RING formation (Chemistry)
Abstract

The detailed mechanism of palladium-catalyzed γ-C(sp3)–H olefination/cyclization of triflyl-protected amines was investigated by density functional theory (DFT) calculations. The olefinated intermediate was initially formed in the first catalytic cycle involving ligand exchange, bicarbonate-assisted C(sp3)–H bond cleavage, alkene insertion and ‘reductive β-hydride elimination’. The following syn-addition and reductive elimination furnish the aza-Wacker product. The first step of reductive elimination is the rate-determining step. The mechanism unveils the important roles of bicarbonate: aiding the C–H activation and abstracting the β-proton in the second step of reductive elimination. The parallel bridging mode in the metal-olefin intermediate facilitates the syn-addition, explaining the experimentally observed stereoselectivity. The effect of the monodentate pyridine-based ligands is also discussed. [ABSTRACT FROM AUTHOR]

Published
2018
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40. Highly sensitive and selective detection of Pd2+ ions using a ferrocene–rhodamine conjugate triple channel receptor in aqueous medium and living cells.

Author

Huang, Dan-Dan, Zhao, Mei, Lv, Xue-Xin, Xing, Yang-Yang, Chen, De-Zhan, and Guo, Dian-Shun

Subjects
FERROCENE, RHODAMINES, AQUEOUS solutions
Abstract

Herein, a novel ferrocene–rhodamine receptor conjugated with an allylimine bridge was facilely synthesized. This triple channel receptor can selectively and sensitively monitor Pd2+ ions through chromogenic, fluorogenic and electrochemical assays in aqueous medium with a low detection limit (8.46 × 10−9 M) and a fast response (＜8 min). It can be applied as a fluorescent probe for effective survey of Pd2+ ions in living cells. Moreover, a plausible recognition mode was proposed and rationalized by theoretical calculations. [ABSTRACT FROM AUTHOR]

Published
2018
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49. Understanding the mechanism of procedure-controlled enantioselectivity switch in cinchona alkaloid thiourea catalyzed [3 + 2] cycloaddition: H-bonds controlled enantioselectivity.

Author

He, Qi, Huang, Fang, Sun, Chuanzhi, Xie, Lihuan, and Chen, De-zhan

Subjects
CINCHONA alkaloids, THIOUREA, RING formation (Chemistry), DENSITY functional theory, HYDROGEN bonding, CHEMICAL reactions, CARBAMATES
Abstract

Density functional theory has been applied to the novel procedure-controlled enantioselectivity switch in the synthesis of chiral 2-oxazolidinone ( 3 ) through [3 + 2] cycloaddition between γ-hydroxy-α,β-unsaturated carbonyl compound ( 1 ) and isocyanate ( 2 ) catalyzed by a cinchona-alkaloid based aminothiourea catalyst. The study shows that three stages are involved in the whole process. In stage one, for Procedure A, 1 reacts with 2 to give carbamate intermediate. Then catalyst 4 is added. For Procedure B, catalyst 4 first activates 2 via quinuclidine N attacking isocyanate C. Then 1 is involved. In stage two, hydrogen is transferred from carbamate N to Ts O atom which is different from the quinuclidine N protonation mechanism. The last step is ring closure in which carbamate N attacks C atom of C C bond. From free energy profiles, the enantioselectivities of the reaction are determined by stage two. Further analysis shows that H-bond is the key in the procedure-controlled enantioselectivity switch. Different H-bonds (N A H A ⋯O A C and N B H B ⋯O A C in Procedure A vs N A H A ⋯O A C and N B H B ⋯O B S in Procedure B) lead to different relationships between thiourea and carbamate planes (vertical or parallel). In Procedure A, C C listing above carbamate plane is the favorable pathway to give R-3 . However, in Procedure B, C C listing behind carbamate plane is more favorable to generate S-3 . [ABSTRACT FROM AUTHOR]

Published
2015
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