Cationic aromatic silylation in the gas phase

Cacace, Fulvio; Crestoni, Maria Elisa; Fornarini, Simonetta; Gabrielli, Roberto

doi:10.1016/0168-1176(88)83024-6

Cited by 39 publications

(12 citation statements)

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“…When alkylation occurs meta to the ipso carbon, the basicity of the latter is not appreciably enhanced by the i-C3H7 group, which makes formation of 5 a energetically unfavourable. This depresses the rate of detertbutylation [5], allowing alkylation [6] to occur, but, significantly, only from the meta to the t-Bu group (Table 4). Such interpretation is supported by the pressure dependence of the products composition.…”

Section: Alkylation-promoted Detertbutylation and Desilylationmentioning

confidence: 99%

A comparative study of gas phase aromatic desilylation and detertbutylation by charged electrophiles

Cacace¹,

Crestoni²,

Petris³

et al. 1988

Can. J. Chem.

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Chem. 66, 3099 (1988).Aromatic desilylation and detertbutylation by CH5+, CzH5+, i-C3H7+, and t-C4H9+ ions have been studied in the gas phase by mass spectrometric and radiolytic techniques. The higher rate of desilylation than of dealkylation is traced to the step involving the formation of ipso arenium ions, protonated at the ring carbon bearing the SiMe, or the CMe3 substituent. The latter has been shown by a b initio calculations at the SCF STO-3G level to selectively depress the basicity of the ipsoposition, hence the stability of the corresponding arenium ion relative to its protomers, e.g. the basicity of the ipso position of PhCMe, is lower by ca. 1 1 kcal mol-' than of the para position. The %Me3 group has no comparable effect, very close basicities being computed for the para and the ipso position of PhSiMe,. The different effects of the CMe3 and SiMe3 groups are traced to electronic and steric factors.FULVIO CACACE, M. ELISA CRESTONI, GIULIA DE PETRIS, SIMONETTA FORNARINI et FELICE GRANDINETTI. Can. J. Chem. 66, 3099 (1988).Operant en phase gazeuse et faisant appel 2 des techniques de spectromttrie de masse et de radiolyse, on a ttudiC les rCactions de dt-tert-butylation et de dCsilylation aromatiques par les ions CH5+, C2H5+, i-C3H7+, et tert-C4H9+. Le fait que la vitesse de dCsilylation est plus rapide que celle de ladealkylation est attribue a l'Ctape impliquant la formation des ions ipso arCnium qui sont protones au niveau du cycle carbone portant les substituants SiMe, ou CMe3. Faisant appel a des calculs ab initio, en champ auto-cohdrent, au niveau STO-3G, on a dCmontre que, par rapport A ses protomeres, le dernier abaisse la basicitt de la position ipso et, par constquent, la stabilitC de l'ion arCnium correspondant; par exemple, la basicitt de la position ipso du PhCMe, est environ 11 kcal mol-' plus faible que celle de la position para. Le groupement SiMe3 n'a pas d'effet semblable et les calculs fournissent des valeurs trks comparables pour les positions para et ipso du PhSiMe3. On attribue les effets difftrents des groupements CMe3 et SiMe, a des facteurs Clectronique et sttrique. [Traduit par la revue] IntroductionElectrophilic cleavage of aryl-silicon bonds, observed as early as in 1901 (I), has been characterized by the systematic work of Eaborn (2, 3) as a typical aromatic substitution, whose rate-determining step is the formation of the ipso u complex.We report here extension of the work to the gas phase by an integrated approach based on the coordinated application of mass spectrometric, radiolytic, and computational techniques (4). The same approach has recently allowed us to demonstrate the first example of the inverse process, the direct aromatic substitution by SiMe3+ ( 5 , 6), unprecedented in solution, nor previously characterized in the gas phase.In order to provide a proper perspective, and in view of the close correlations between carbon and silicon chemistry, a parallel investigation has been carried out of gas-phase aromatic detertb~t~lation, the subject of earlier detaile...

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Section: Alkylation-promoted Detertbutylation and Desilylationmentioning

confidence: 99%

A comparative study of gas phase aromatic desilylation and detertbutylation by charged electrophiles

Cacace¹,

Crestoni²,

Petris³

et al. 1988

Can. J. Chem.

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“…The (CH,),S~+ adducts of benzene, benzene-d,, toluene and toluene-a,a,a-d,, and mesitylene were readily formed in both the HP source and the EI/CI source by addition of (cH,),s~+ to the aromatic via reactions [ l ] and [2]. Higher relative intensities of the adducts were obtained using the HP source because of more efficient third-body stabilization but the observed metastable and CAD spectra obtained were independent of the source employed.…”

Section: Resultsmentioning

confidence: 99%

“…y-Radiolysis of CH,/(CH,),Si/ArH mixtures (ArH = aromatic = benzene, toluene, xylene) at pressures of around 1 atmosphere (1 atm = 101.3 kPa) leads to the formation of the silylated aromatics (CH,),SiAr (1,2). The suggested mechanism has the u-bonded structure I as an intermediate.…”

Section: Introductionmentioning

confidence: 99%

“…Adducts were formed by the addition of either (CH3),Si+ or (CH3),C+ to perdeuterated benzene and toluene and (C2H,),N was used as a deprotonating agent. The appearance of (C,H,),ND+ was taken as evidence that deprotonation of an arenium ion had occurred according to reaction [2]. The disappearance of (cH,),CC&,+, for example, was accompanied by an almost equal increase in (C2HS),ND+, which characterizes (CH,),C+ adducts as arenium ions.…”

Section: Introductionmentioning

confidence: 99%

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A CAD study of isobaric (CH₃)₃SiArH⁺ ions (ArH = benzene, toluene) formed by association and protonation

Li¹,

Stone²

1992

Can. J. Chem.

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“…Among the bases used at comparable concentrations, pyridine is most effective in promoting alkylation products, whereas alkyldesilylation is the dominant channel when (MeO) 3 [12,16] in the presence of 1.5 Torr of (MeO) 3 PO failed, in spite of the estimated comparable basicity of (MeO) 3 PO and p-TSDPP that could have ensured the neutralization of the intermediate […”

Section: Radiolytic Experimentsmentioning

confidence: 99%

The Gas-Phase Reactivity ofp-Me3Si-Substituted 1,3-Diphenylpropane Towards Charged Electrophiles: Intra- and Interannular Hydrogen Migrations

Crestoni

1998

Chem. Eur. J.

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Cationic aromatic silylation in the gas phase

Cited by 39 publications

References 12 publications

A comparative study of gas phase aromatic desilylation and detertbutylation by charged electrophiles

A comparative study of gas phase aromatic desilylation and detertbutylation by charged electrophiles

A CAD study of isobaric (CH₃)₃SiArH⁺ ions (ArH = benzene, toluene) formed by association and protonation

The Gas-Phase Reactivity ofp-Me3Si-Substituted 1,3-Diphenylpropane Towards Charged Electrophiles: Intra- and Interannular Hydrogen Migrations

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Cationic aromatic silylation in the gas phase

Cited by 39 publications

References 12 publications

A comparative study of gas phase aromatic desilylation and detertbutylation by charged electrophiles

A comparative study of gas phase aromatic desilylation and detertbutylation by charged electrophiles

A CAD study of isobaric (CH3)3SiArH+ ions (ArH = benzene, toluene) formed by association and protonation

The Gas-Phase Reactivity ofp-Me3Si-Substituted 1,3-Diphenylpropane Towards Charged Electrophiles: Intra- and Interannular Hydrogen Migrations

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A CAD study of isobaric (CH₃)₃SiArH⁺ ions (ArH = benzene, toluene) formed by association and protonation