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

Abstract

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...

[Arene·Me3C+] non-covalent complexes in the gas-phase (trifluoro)methylation of tert-butyl-substituted diphenylalkanes

“…For example, the CH4/CI mass spectra of tertbutyltoluenes are dominated by the loss of Me3 C+, and the radiolytic reaction of CH~-/Et + and i-Pr + results in extensive formation of proto-de-tert-butylated and alkyl-detert-butylated products [12]. Such a reactivity pattern is confirmed by the present data, regarding ionic reagents which can, however, exclusively act as Lewis acids, being unable to behave as proton donors.…”

Lifetimes of Gaseous Ion-Neutral Complexes: The Rate of Isotopic Scrambling within Ethyl Ions as an Internal Clock

“…Note that this scheme neglects the conceivable reversion of the s complex to INC2, which appears most unlikely. In fact, proto-de-ethylation does not occur to any significant extent in the pressure range typical of the radiolytic experiments, as opposed to proto-detert-butylation, [46] for instance, and the proton (deuteron) initially bound to the ipso carbon of the arenium ion is expected to migrate to other ring positions, by a low-barrier, fast intramolecular shift. [42] The major product is CF 3 C 6 X 5 , accompanied by smaller, but readily detectable amounts of CF 3 C 6 X 4 C 2 XY 4 isomers ( Table 1).…”

Nucleogenic silylium cations and their analogs

“…First attempts to accomplish electrophilic silylation of benzene with trimethylsilylium cation failed [23,40]. This fact was rationalized by assuming that Me 3 Si þ forms a p-complex (4a) with aromatics [41,42].…”