Addition–Elimination versus Direct Substitution Mechanisms for Arene Chlorination

Galabov, Boris; Koleva, Gergana; Kong, Jing; Schaefer, Henry F.; Schleyer, Paul von Ragué

doi:10.1002/ejoc.201402765

Cited by 18 publications

(20 citation statements)

References 58 publications

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“… Catalysis by HBr produced by the reaction. (The autocatalytic effect of HCl formed as one of the reaction products was observed experimentally for anisole chlorination and studied theoretically for benzene chlorination: However, the presence of formed HBr does not influence the benzene bromination rate during the whole time course of the reaction [see e.g., Supporting Information, pp. S6–S7]). Catalysis by light. Catalysis by triplet oxygen. …”

Section: Experimental Results For Reaction Between Bromine and Benzenementioning

confidence: 99%

Noncatalytic bromination of benzene: A combined computational and experimental study

et al. 2015

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The non-catalytic bromination of benzene is shown experimentally to require high 5-14M concentrations of bromine in order to proceed at ambient temperatures to form predominantly bromobenzene, along with detectable (<2%) amounts of addition products such as tetra and hexabromocyclohexanes. The kinetic order in bromine at these high concentrations is 4.8 ± 0.06 at 298K and 5.6 ± 0.11 at 273K with a small measured inverse deuterium isotope effect using D 6 -benzene of 0.97 ± 0.03 at 298K. These results are rationalized using computed transition states models at the B3LYP+D3/6-311++G(2d,2p) level with an essential continuum solvent field for benzene applied. The model with the lowest predicted activation free energies agrees with the high experimental kinetic order in bromine and involves formation of an ionic, concerted and asynchronous transition state involving a Br 7 -cluster resembling the structure of the known Br 9 -. This cluster plays three roles;; as a Br + donor, as a proton base and as a stabilizing arm forming weak interactions with two adjacent benzene C-H hydrogens, these aspects together combining to overcome the lack of reactivity of benzene induced by its aromaticity. The computed inverse kinetic isotope effect of 0.95 agrees with experiment, and arises because CBr bond formation is essentially complete whereas C-H cleavage has not yet commenced. The computed free energy barriers for the reaction with 4Br 2 and 5Br 2 for a standard state of 14.3M in bromine are reasonable for an ambient temperature reaction, unlike previously reported theoretical models involving only one or two bromines.

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Section: Experimental Results For Reaction Between Bromine and Benzenementioning

confidence: 99%

Noncatalytic bromination of benzene: A combined computational and experimental study

et al. 2015

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“…However, this was only possible when AlCl 3 was included as a Lewis acid catalyst in the static calculations . Nevertheless, multiple researchers have contested this classic mechanism and an alternative mechanism has been proposed, which does not include the formation of a Wheland‐type intermediate …”

Section: Introductionmentioning

confidence: 99%

“…Galabov, Schleyer and co‐workers studied the possibility of two alternative mechanisms for the electrophilic chlorination of arenes in gas phase and in a nonpolar solvent: the addition–elimination mechanism and the direct substitution mechanism (Scheme ) . Both these mechanisms do not involve the formation of a σ‐complex.…”

Section: Introductionmentioning

confidence: 99%

“…The direct substitution mechanism consists of a single concerted reaction step, in which the hydrogen transfer from HCl to Cl 2 and from the aromatic compound to HCl takes place. The addition–elimination mechanism, as proposed by Galabov, Schleyer and co‐workers, involves three steps. The first step is an addition of Cl 2 to the aromatic ring.…”

Section: Introductionmentioning

confidence: 99%

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Solvent and Autocatalytic Effects on the Stabilisation of the σ‐Complex during Electrophilic Aromatic Chlorination

Lommel

Moors

Proft

2018

Chemistry A European J

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The solvent and autocatalytic effects of the electrophilic aromatic chlorination of benzene are studied using a combined approach of static calculations and ab initio metadynamics simulations. Different possible reaction pathways are investigated and the influence of the solvents (CCl , acetonitrile and acetic acid) is thoroughly assessed. Our results show that the stability and lifetime of a charged σ-complex is increased by electrostatic stabilisation effects of the environment, which can originate from catalytic HCl, solvating effects of polar solvents (acetonitrile), or specific hydrogen bonding interactions with the solvent (acetic acid). Metadynamics simulations reveal a new chlorine addition mechanism explaining the autocatalytic effects of the reaction. The strength of combining static calculations and metadynamics simulations is highlighted, which provide complementary insight into chemical reactions in solvent.

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“…One carbonyl oxygen of 11 serves as base to deprotonate anisole which is found already aromatic in 18. Concerted S E Ar mechanisms for arene sulfonation 23 and chlorination 24 have been computed and refute the traditional text-book perspective of Friedel-Crafts reactions systematically involving Wheland σ-complexes. However, the possibility of concerted Friedel-Crafts alkylation involving a three-centered transition state is unknown to us (Scheme 9).…”

Section: Ts 14-11mentioning

confidence: 77%

Non-Innocent Behavior of Substrate Backbone Esters in Metal-Catalyzed Carbocyclizations and Friedel–Crafts Reactions of Enynes and Arenynes

et al. 2015

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On the basis of DFT computations and experimental results, we show that the presence of the ester group in the backbone of organic substrates can influence the mechanism of metal-catalyzed carbocyclization reactions. The non-innocent role of the ester functionality in lowering the activation barrier of the key step of the gallium- and indium-catalyzed cycloisomerization of 1,6-enynes is revealed. In the case of the gallium-catalyzed hydroarylation of arenynes, the esters in the tether can deprotonate the Wheland intermediate, thus avoiding more energetically demanding [1,3]- or [1,2]/[1,2]-H shifts. As for the gallium-catalyzed Friedel-Crafts alkylation, an unusual concerted SEAr mechanism involving the esters has been calculated. Lastly, computations evidence that the ester group of methyl propiolates enables a divergent mechanism in the platinum-catalyzed intramolecular hydroarylation.

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Addition–Elimination versus Direct Substitution Mechanisms for Arene Chlorination

Cited by 18 publications

References 58 publications

Noncatalytic bromination of benzene: A combined computational and experimental study

Noncatalytic bromination of benzene: A combined computational and experimental study

Solvent and Autocatalytic Effects on the Stabilisation of the σ‐Complex during Electrophilic Aromatic Chlorination

Non-Innocent Behavior of Substrate Backbone Esters in Metal-Catalyzed Carbocyclizations and Friedel–Crafts Reactions of Enynes and Arenynes

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