28927-31-3Relevant academic research and scientific papers
Kinetics of hydride transfer reactions from hydrosilanes to carbenium ions. Substituent effects in silicenium ions
Mayr, Herbert,Basso, Nib,Hagen, Gisela
, p. 3060 - 3066 (2007/10/02)
Rates of hydride transfer from hydrosilanes HSiR1R2R3 with widely varying substitution to para-substituted diarylcarbenium ions have been measured in dichloromethane solution. Generally the reactions follow a second-order rate law, -d[Ar2CH+]/dt = k2[Ar2CH+][HSiR1R2R3], and k2 is independent of the degree of ion-pairing and the nature of the counterion (exceptions are reported). The reaction rates are almost independent of solvent polarity. Kinetic isotope effects exclude an SET-type mechanism and are in accord with a polar mechanism with rate-determining formation of silicenium ions. The reactivities of para-substituted aryldimethylsilanes are linearly correlated with σp (ρ = -2.46), not with σp+. In the series H3SiHex, H2SiHex2, HSiHex3, the relative reactivities are 1.00:155:7890, and in the corresponding phenyl series the reactivity increase is much smaller (H3SiPh:H2SiPh2:HSiPh3 = 1.00:17.2:119). As a consequence, trihexylsilane is approximately two orders of magnitude more reactive than triphenylsilane though hexylsilane and phenylsilane show similar reactivities. Tris(trimethylsilyl)silane is just slightly more reactive than trimethylsilane. Replacement of hydrogen by chlorine reduces the reactivity by one order of magnitude. Variation of the electrophilicities of the hydride abstractors does not affect the relative reactivities of the silanes, i.e., constant selectivity (Ritchie-type) relationships are encountered. Correlation equations are given, which permit the calculation of hydride transfer rates from hydrosilanes to any carbenium ion on the basis of pkR+ values or the ethanolysis rate constants of the corresponding alkyl chlorides.
Mechanism of CH2.+ Transfer from Distonic Ions X-CH2.+ (X=CH2O, CH2CH2) to ?- and n-Electron Bases in the Gas Phase. A Fourier Transform Ion Cyclotron Resonance (FTICR) Study Supplemented by ab initio MO Calculations
Rusli, Ridwan D.,Schwarz, Helmut
, p. 535 - 540 (2007/10/02)
The reactions of acetonitrile, propyne, acetylene, trimethylsilylacetylene, and tetramethylsilane, with distonic ions CH2XCH2.+ are studied in the gas phase using Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometry.In line with previous studies, CH2.+ is transferred to the electron lone-pair of the nitrogen atom of CH3CN to generate CH3CN-CH2.+ (4); upon collisional excitation, this ion undergoes competitive losses of H. and CH..While both neutrals originate from the "methylene" unit of 4, detailed studies employing labeledsubstrates and using various types of collision experiments reveal an intriguing dissociation pattern in that the dissociations are preceded by two intramolecular hydrogen migrations giving rise to CH3C(H)=NCH.+ (6) and CH3C=N(H)CH.+ (7).While 6 serves as intermediate en route to loss of H. from the "CH" moiety, 7 is the actual precursor to generate, by loss of CH., protonated acetonitrile, CH3CNH+ (12) (Scheme 5).In addition, 12 is formed by bimolecular proton transfer.In this reaction, translationally excited CX3CN-CY2.+* (X, Y = H, D) transfers X+ to neutral CX3CN to generate CX3CNX+ (Scheme 4).The bimolecular proton transfer as well as the intramolecular isomerizations of 4 to 6 and 7 are subject to very large kinetic isotope effects.In the transfer of CH2.+ to CH3CCH two products are formed .+ (16) and CH2=C=CHCH3.+ (17) presumably via intermediate 18 (Scheme 6)>; the latter is formed by addition of CH2.+ to the less hindered carbon atom of HCCCH3 reflecting the higher stability of the so-formed intermediate compared with addition to C-2.Reactions of 2 and 3 with HCCH do not result in the formation of a detectable CH2.+-transfer product.When using CH2CH2CH2.+ (2) the reaction is prohibited by the endothermicity to generate the initial complex (structurally related to 18).On the other hand, when CH2OCH2.+ (3) is employed, the intermediate of CH2.+ transfer is formed with sufficient energy to split off a hydrogen atom.Preliminary experiments with silicon-containing molecules, like Si(CH3)4 or HCCSi(CH3)3, demonstrate that the favored processes of these neutrals with 2 and 3 are due to charge transfer (in the form of an electron or an anion like CH3- or C2H-) from the silicon-containing molecule to the distonic ions.The experimental results obtained for CH3CN/CH2.+ system are supported by ab initio MO calculations (3-21G/3-21G + ZPVE).
Precise Determination of Stabilities of Primary, Secondary, and Tertiary Silicenium Ions from Kinetics and Equilibria of Hydride-Transfer Reactions in the Gas Phase. A Quantitative Comparison of the Stabilities of Silicenium and Carbonium Ions in the Gas Phase
Shin, Seung Koo,Beauchamp, J. L.
, p. 900 - 906 (2007/10/02)
Fourier transform ion cyclotron resonance spectroscopy has been used to examine kinetics and equilibria of hydride-transfer reactions of methyl-substituted silanes with various hydrocarbons having well-established gas-phase hydride affinities.The derived hydride affinities, D(R3Si(1+)-H(1-)), for the silicenium ions SiMeH2(1+), SiMe2H(1+), and SiMe3(1+) are 245.9, 230.1, and 220.5 kcal/mol, respectively, to be compared with the values of 270.5, 251.5, and 233.6 kcal/mol for the corresponding carbonium ions.This indicates that the silicenium ions are significantly more stable than the corresponding carbonium ions in the gas phase with H(1-) as a reference base.
