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A combined experimental and computational investigation on the unusual molecular mechanism of the Lossen rearrangement reaction activated by carcinogenic halogenated quinones.

ABSTRACT: The classic Lossen rearrangement is a well-known reaction describing the transformation of an O-activated hydroxamic acid into the corresponding isocyanate. In this study, we found that chlorinated benzoquinones (CnBQ) serve as a new class of agents for the activation of benzohydroxamic acid (BHA), leading to Lossen rearrangement. Compared to the classic one, this new kind of CnBQ-activated Lossen rearrangement has the following unique characteristics: (1) The stability of CnBQ-activated BHA intermediates was found to depend not only on the degree but also on the position of Cl-substitution on CnBQs, which can be divided into two subgroups. (2) It is the relative energy of the anionic CnBQ-BHA intermediates that determine the rate of this CnBQ-activated rearrangement, which is the rate-limiting step, and the Cl or H ortho to the reaction site at CnBQ is crucial for the stability of the anionic intermediates. (3) A pKa-activation energy correlation was observed, which can explain why the correlation exists between the rate of the rearrangement and the acidity of the conjugate acid of the anionic leaving group, the hydroxlated quinones. These findings may have broad implications for future research on halogenated quinoid carcinogens and hydroxamate biomedical agents.

SUBMITTER: Shan GQ

PROVIDER: S-EPMC4285140 | biostudies-literature | 2015 Jan

REPOSITORIES: biostudies-literature

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A combined experimental and computational investigation on the unusual molecular mechanism of the Lossen rearrangement reaction activated by carcinogenic halogenated quinones.

Shan Guo-Qiang GQ Yu Ao A Zhao Chuan-Fang CF Huang Chun-Hua CH Zhu Ling-Yan LY Zhu Ben-Zhan BZ

The Journal of organic chemistry 20141217 1

The classic Lossen rearrangement is a well-known reaction describing the transformation of an O-activated hydroxamic acid into the corresponding isocyanate. In this study, we found that chlorinated benzoquinones (CnBQ) serve as a new class of agents for the activation of benzohydroxamic acid (BHA), leading to Lossen rearrangement. Compared to the classic one, this new kind of CnBQ-activated Lossen rearrangement has the following unique characteristics: (1) The stability of CnBQ-activated BHA int ...[more]

PMID: 25470188

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Project description:The high ethylene selectivity exhibited on the zirconia-activated-carbon bi-support catalyst is investigated by experiment and density functional theory-based (DFT) analysis. This bi-support catalyst systems prepared by the physical mixing method for the tungsten catalyst show a significant increase in ethylene selectivity up to 90% compared to the zirconia single support system (~58%) during the ethanol dehydration reaction. Besides, the optimal percent weight ratio of zirconia to activated carbon, which results in the highest ethanol conversion is 50:50. The DFT-based analysis is used to investigate high ethylene selectivity in the bi-support system. It shows that the WO5/zirconia is the most stable model for the zirconia single-support tungsten catalyst represented by the zirconia (101) facet of the tetrahedral phase. The carbon atoms were added to the WO5/zirconia to model the tungsten catalyst on the bi-support system. The Bader charge analysis is carried out to determine the electron transfer in the catalyst. The bonding between ethylene and the WO5 active site on the catalyst is weakened when the system is bi-support, where the added carbon atoms on the catalyst in the ZrO2 region decrease the ethylene adsorption energy. Thus, the desorption and the selectivity of ethylene are promoted. The decrease in adsorption energy can be explained via the analysis of the projected density of states (PDOS) profiles of atom involving the adsorption. It was found that the added carbon in the ZrO2 region induces the electron transfer from the ethylene molecule to the surface, especially to the ZrO2 region. The depletion of the electron around the ethylene molecule weakens the bonds, thus, promote desorption. Hence, the advantages of using the bi-support system in the tungsten catalyst are that the catalyst exhibit (1) high conversion due to the zirconia support and (2) high ethylene selectivity due to the added carbon promoting the desorption of ethylene via the induction of electron from an ethylene molecule to surface.

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A combined experimental and computational investigation on the unusual molecular mechanism of the Lossen rearrangement reaction activated by carcinogenic halogenated quinones.

Publications

A combined experimental and computational investigation on the unusual molecular mechanism of the Lossen rearrangement reaction activated by carcinogenic halogenated quinones.

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