Gas-Phase H/D Exchange between Arenium Ions and Selected Bases. The Site of Protonation of Simple Aromatics

Chiavarino, Barbara; Crestoni, Maria Elisa; Rienzo, Brunella Di; Fornarini, Simonetta

doi:10.1021/ja981488v

Cited by 19 publications

(24 citation statements)

References 61 publications

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“…(6a) and (6b) The presence of a base (Et 3 N typically, except in the experiments run with SO 2 , since this compound is known to give Lewis adducts with gaseous Et 3 N) [28] ensures fast deprotonation of the intermediate arenium ions, which may otherwise undergo H/D exchange with active neutral species in the radiolytic medium. [29] Mass spectral analysis of the alkylation products identifies the site of deuteration. The isomeric propylbenzenes can therefore be assigned to either mechanism with the results shown in the second part of Table 1.…”

Section: Resultsmentioning

confidence: 99%

The Protonation of Gaseous Cyclopropane

et al. 2001

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The protonation of cyclopropane by gaseous Brønsted acids of varying strength in radiolytic experiments at atmospheric pressure leads to two distinct C3H7- isomers that have been sampled by their reaction with benzene. The neutral end products, nC3H7-C6H5 and iC3H7-C6H5, arise from the electrophilic aromatic substitution reaction with the cC3H7+ and iC3H7+ ions, respectively. Their relative abundance was studied as a function of pressure, temperature, and the presence of additives in the gaseous systems; the results indicate a large extent of isomerization to the thermodynamically favored iC3H7+ from the protonation by strong acids. The presence of a kinetic barrier prevents any thermal isomerization from taking place in the time frame of 10(-8) s. In the peculiar case in which protonated benzene is the Brønsted acid, C3H7+ ions are formed in the presence of neutral benzene within the same ion - molecule complex. The ensuing reaction shows that cC3H7+ ions are formed exclusively and react in the 10(-10) s(-1) estimated lifetime of the complex. Still, such cC3H7+ ions undergo complete randomization of their hydrogen atoms; this points to a low kinetic barrier for the process. Agreement is found between the reported experimental results and updated computations of the relevant species in the C3H7+ potential energy surface.

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Section: Resultsmentioning

confidence: 99%

The Protonation of Gaseous Cyclopropane

et al. 2001

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“…58 : 6 : 36). 18 The same approach was not feasible for toluene, the prototypical alkylbenzene, due to the lack of a neat methylating agent to be used in radiolytic experiments. The present report is, therefore, the first successful attempt to reveal the preferred protonation sites for toluene in the gas phase.…”

Section: ð1þmentioning

confidence: 99%

Site-selectivity of protonation in gaseous toluene

Chiavarino

Crestoni

Rienzo

et al. 2008

Phys. Chem. Chem. Phys.

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“…Nevertheless, the preferred thermodynamic pathway is protonation, lying 11.7 kcal mol -1 ( H) and 22.3 kcal mol -1 ( G 298.15K ) lower in energy with respect to acylation, at MP4(SDTQ)/6-31G++(d,p) level. Gas phase protonation of aromatic compounds has been widely studied in the literature, at experimental [14][15][16][17][18] and theoretical levels.…”

Section: Thermochemical Data For Acylation and Protonation Of Toluenementioning

confidence: 99%

Theoretical study of the gas-phase reaction between formyl cation and aromatics

Ribeiro¹,

Mota²

2008

J. Braz. Chem. Soc.

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Cálculos teóricos da reação do cátion formila (CHO + ) com tolueno, cumeno e p-cresol mostraram que a transferência de próton é termodinamicamente preferida em relação a acilação. No caso do tolueneo, a acilação do anel aromático, para formação do complexo de Wheland, é 11,7 kcal mol -1 mais alta em energia ( H) que a protonação, em nível de cálculo MP4(SDTQ)/ 6-31++G(d,p)//MP2(full)/6-31G(d,p). Esta diferença diminui com a introdução de um grupo hidroxila no anel (p-cresol) ou substituindo o grupo metila por isopropila (cumeno). A protonação do tolueno pelo H 3 + e pelo cátion isoformila é, respectivamente, 88,6 e 84,5 kcal mol -1 mais exotérmica que a acilação em nível MP4(SDTQ)/6-31++G(d,p)//MP2(full)/6-31G(d,p). A afinidade protônica do p-cresol foi calculada como sendo 195,4 kcal mol -1 .Theoretical calculations of the reaction of formyl cation (CHO + ) with toluene, cumene and p-cresol showed that proton transfer is thermodynamically preferred over acylation. For toluene, acylation of the aromatic ring, to form the Wheland complex, is 11.7 kcal mol -1 higher in energy ( H) than protonation, at MP4(SDTQ)/6-31++G(d,p)//MP2(full)/6-31G(d,p) level of theory. This difference reduces upon introduction of a hydroxy group in the ring (p-cresol) or replacing the methyl group by an isopropyl group (cumene). Protonation of toluene by H 3 + and isoformyl cation is 88.6 and 84.5 kcal mol -1 , respectively, more exothermic than acylation, at MP4(SDTQ)/ 6-31++G(d,p)//MP2(full)/6-31G(d,p). The proton affinity of p-cresol was calculated to be 195.4 kcal mol -1 .Keywords: formyl cation, Friedel-Crafts acylation, gas phase ion reaction, proton transfer IntroductionFriedel-Crafts acylation is an important reaction in organic and industrial chemistry.1 Many important chemicals, like fragrances, pharmaceuticals and dyes are industrially produced through acylation steps. In addition, the Gatterman-Koch reaction 2 is a useful procedure to produce benzaldehydes, through the reaction of carbon monoxide and an aromatic compound, catalyzed by a strong Bronsted acid, like HF.Acylation of aromatic compounds has long been studied in the gas phase.3,4 Speranza and Sparapani showed that gas phase acetylation of alkylbenzenes does not follow the same reactivity trend found in solution, 5 suggesting that solvent, counter ion and catalyst play a major role in the reactivity of the CH 3 CO + cation in condensed phase. Strong electrophilic acylium ions, such as CF 3 CO + , are able to acylate deactivated substrates, such as trifluorobenzene, in the gas phase.6 Cerichelli et al. 7 reported that formyl cation behaves as a Bronsted acid in gas phase, exclusively transferring a proton to toluene, rather than producing formylated products. This behavior contrasts with the reactivity of C 2 H 5 + and i-C 3 H 7 + cations in gas phase, both capable of acting as Lewis (alkylation) and Bronsted (protonation) acids toward toluene. In the present study we carried out theoretical calculations on the thermochemistry of acylation (formation of the Whela...

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Gas-Phase H/D Exchange between Arenium Ions and Selected Bases. The Site of Protonation of Simple Aromatics

Cited by 19 publications

References 61 publications

The Protonation of Gaseous Cyclopropane

The Protonation of Gaseous Cyclopropane

Site-selectivity of protonation in gaseous toluene

Theoretical study of the gas-phase reaction between formyl cation and aromatics

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