Silyl Cations Research Articles

Cyclotrisilenylium Ion: The Persilaaromatic Compound

The highly crowded 3,3-bis(di-tert-butylmethylsilyl)-1,2-bis(tri-tert-butylsilyl)cyclotrisilene (3) was newly designed as a precursor of the cyclotrisilenylium ion and prepared by the reaction of 2 equiv of dilithiosilane (tBu2MeSi)2SiLi2 (1) with 2,2,3,3-tetrabromo-1,1,1,4,4,4-hexa-tert-butyltetrasilane. The reaction of 3 with triphenylmethylium tetraarylborate in toluene produced (di-tert-butylmethylsilyl)bis(tri-tert-butylsilyl)cyclotrisilenylium ion (4+), which was isolated in the form of the tetraarylborate salt as extremely air- and moisture-sensitive yellow crystals, representing the first isolable silicon congener of the cyclopropenylium ion. The molecular structure of 4+.TSFPB- (TSFPB- = tetrakis[4-(tert-butyldimethylsilyl)-2,3,5,6-tetrafluorophenyl]borate) was established by X-ray crystallographic analysis, showing that the three-membered ring constitutes an almost equilateral triangle with average Si-Si bond lengths of 2.216(3) A. The X-ray crystal structure and spectral data show that 4+ is not only a free silyl cation but also a 2pi electron aromatic species with delocalization of the positive charge over the three-membered skeleton.

Energies of Association of Carbenium and Silylium Cations with Oxygen-Containing Molecules

The energies of association of MenH3-n X+ cations (n = 0-3) with molecules of oxygen-containing bases (water, methanol, dimethyl ether) were calculated by the B3LYP/6-31G(d) method. These energies decrease with an increase in the number n of methyl groups at the X atom; this trend is more pronounced for X = C, which is due to a lower degree of charge transfer in the cation from the occupied orbitals of the methyl group to the vacant orbital of X in the case of X = Si. An increase in the association energy with an increase in the number of methyl substituents at the oxygen atom is due to an increase in the energy level of lone electron pairs of oxygen upon methyl substitution. As a result, the energy gap between the electronic levels of the unoccupied orbital of the cation and orbitals of the oxygen lone electron pairs becomes narrower, which makes the interaction between the unoccupied and occupied orbitals more efficient.

Lewis Base Activation of Lewis Acids: Catalytic, Enantioselective Addition of Silyl Ketene Acetals to Aldehydes

The concept of Lewis base activation of Lewis acids has been reduced to practice for catalysis of the aldol reaction of silyl ketene acetals and silyl dienol ethers with aldehydes. The weakly acidic species, silicon tetrachloride (SiCl4), can be activated by binding of a strongly Lewis basic chiral phosphoramide, leading to in situ formation of a chiral Lewis acid. This species has proven to be a competent catalyst for the aldol addition of acetate-, propanoate-, and isobutyrate-derived silyl ketene acetals to conjugated and nonconjugated aldehydes. Furthermore, vinylogous aldol reactions of silyl dienol ethers are also demonstrated. The high levels of regio-, anti diastereo-, and enantioselectivity observed in these reactions can be rationalized through consideration of an open transition structure where steric interactions between the silyl cation complex and the approaching nucleophile are dominant.

Synthesis, X-ray Structure, and Ring-Opening Polymerization of Pentacoordinate Silicon-Bridged [1]Ferrocenophane

The cationic ring-opening polymerization of cyclic pentacoodinate sila[1]ferrocenophanes was carried out in dichloromethane to give poly(ferrocenylsilanes) having pentacoordinate silicon moieties in the polymer backbone. The silyl cation gave the best results as an initiator for the polymerization, affording pentacoordinate poly(ferrocenylsilane) (Mw is 9.0 × 103) in high yields.

Generation and aromaticity of 2-silaimidazolium ion, a new π-conjugated silylium ion

A novel cationic silaaromatic, 2-silaimidazolium cation (5), was synthesized by chloride abstraction from the corresponding chlorosilane (4) with [Et3Si(benzene)]+.TPFPB- (TPFPB- = tetrakis(pentafluorophenyl)borate); cation exists as a free silylium cation in solution due to bulky substituents on the five-membered ring.

Dihydrogen and Methane Elimination from Adducts Formed by the Interaction of Carbenium and Silylium Cations with Nucleophiles

Stationary points for reactions R'R' 'HX(+) + YH --> [R'R' 'X-Y](+) + H(2) (I) and R'(CH(3))HX(+) + YH -->[R'HX-Y](+) + CH(4) (II) (R', R' ' = CH(3), H; X = C, Si; Y = CH(3)O, (CH(3))(2)N, and C(6)H(5)) are located and optimized by the B3LYP/aug-cc-pVDZ method. A similar mechanism was found to be operative for both types of reactions with X = C and X = Si. Formation of the intermediate (adduct) results in the transfer of electron density from the electron-rich bases to the X atoms and in the growth of a positive charge on a hydrogen atom attached to Y. This mobile proton may shift from Y to X, and the relative energies of transition states for elimination reactions (Delta) depend on the ability of the X atom to retain this proton. Therefore, Deltagrows on going from Si to C and with increasing numbers of methyl substituents. For X = C, the Deltavalue for both reactions correlates well with the population of the valence orbitals of X in a wide range from -44 kcal/mol (methyl cation/benzene) to 31 kcal/mol (isopropyl cation/methanol). For X = Si this range is more narrow (from -19 to -5.0 kcal/mol), but all Delta values are negative with the exclusion of silylium ion/benzene systems, adducts of which are pi- rather than sigma-complexes. The energy minima for product complexes for H(2) elimination are very shallow, and several are dissociative. However, complexes with methane which exhibit bonding between X and the methane hydrogen are substantially stronger, especially for systems with X = Si. The latter association energy may reach 8 kcal/mol (Si...H distance is 2 A).

Activation of Silylphosphines by Diethyl Azodicarboxylate: Novel Silylation of Alcohols.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Activation of silylphosphines by diethyl azodicarboxylate: novel silylation of alcohols

A novel activation mode of silylphosphines and an application of that to silylation of alcohols were described. Silylphosphines were found to be instantly activated by means of DEAD and PPTS to form reactive silyl cation equivalents. By using the activated species, silylation of alcohols successfully proceeded under mild acidic condition.

A metal-free catalytic intramolecular hydrosilylation

A metal-free catalytic intramolecular hydrosilylation reaction is reported. The key intermediates in the catalytic cycle are silyl cationic species. The reaction is used to produce 2-silanorbornanes 2 from 3-cyclopentenemethylsilanes 1 under mild conditions and in high isolated yields.Key words: hydrosilylation, catalysis, silaheterocycles, silyl cations.

Donor-Stabilized Silyl Cations. 5. Comparison between Mono- and Binuclear Siliconium Chelates1

Binuclear hexacoordinate silicon chelates have been prepared and shown to have octahedral structure by X-ray crystallography. Their ionization in CD2Cl2 solution has been studied by 29Si NMR spectroscopy. Only one Si−Cl bond in 5a−c ionizes at low temperature to form the monosiliconium bis-chelates 11a−c. Use of a more acidic solvent, CHFCl2, facilitated the second ionization step to the disiliconium dichloride 12c. Replacement of the chloro ligands by better leaving groups (triflate, bromide, or iodide) caused complete ionization (7a−c, 8c, 9c) at room temperature. Crystal structure analyses of the binuclear siliconium triflates 7a,b show a square-pyramid geometry around the silicon atoms, with the ethylene bridge at the apex.

Gas-phase ion chemistry of silyl cations obtained from hexamethyldisilazane

The gas-phase ion-molecule reactions of the Me3SiN(H)SiMe2+ ion, obtained by electron ionization from Me3SiN(H)SiMe3, have been studied in a Fourier transform ion cyclotron resonance spectrometer in order to understand the mechanistic details of an important chemical system presently used in film formation. This silyl cation has been observed to undergo addition reactions at electron rich centers to form stable adducts that may undergo further methane elimination in the case of alcohols and amines. The most important feature of these reactions is the fact that a metathesis type reaction can be observed in the presence of H2O, and other hydrogen labile substrates like alcohols, leading to the formation of the corresponding oxygen-containing ion, i.e. Me3SiOSiMe2+. For alcohols (ROH), facile formation of a tertiary product ion, presumably corresponding to an Me3Si-O-Si(Me)=O+-R structure with elimination of an amine reveals the strong tendency of these nitrogen-containing ions to undergo metathesis type reactions with oxygen containing substrates.

Crystallographic evidence for a free silylium ion.

Evidence for a three-coordinate silyl cation is provided by the crystal structure of [(Mes)3Si][H-CB11Me5Br6].C6H6 (where Mes is 2,4,6-trimethylphenyl). Free (Mes)3Si+ cations are well separated from the carborane anions and benzene solvate molecules. Ortho-methyl groups of the mesityl substituents shield the silicon atom from the close approach of nucleophiles, while remaining innocent as significant ligands themselves. The silicon center is three-coordinate and planar. The downfield 29Si nuclear magnetic resonance chemical shift in the solid state (226.7 parts per million) is almost identical to that in benzene solution and in "gas phase" calculations, indicating that three-coordination can be preserved in all phases.

Donor-Stabilized Silyl Cations. 3. Ionic Dissociation of Hexacoordinate Silicon Complexes to Pentacoordinate Siliconium Salts Driven by Ion Solvation1

Neutral hexacoordinate silicon complexes 1 undergo an equilibrium dissociation to ionic siliconium chlorides 6 at low temperatures in polar solvents: chloroform, dichloromethane, and fluorodichloromethane. The extent of dissociation increases with decreasing temperature, despite the formation of two ions from each molecule. The reaction enthalpies and entropies for the ionizations are negative, and their absolute values increase with increasing solvent polarity, indicating that solvation of the ions drives the dissociation process. The position of the equilibrium is readily controlled by variation of solvent polarity, temperature, replacement of the chloro ligand by better leaving groups (triflate, bromide, or tetrachloroaluminate), and variation of substituents (R) or ligands (X). The results are supported by crystal structures of the siliconium salts: 8a,c,d and 14d.

Silylation of an OH-terminated self-assembled monolayer surface through low-energy collisions of ions: a novel route to synthesis and patterning of surfaces.

Using a multi-sector ion-surface scattering mass spectrometer, reagent ions of the general form SiR(3) (+) were mass and energy selected and then made to collide with a hydroxy-terminated self-assembled monolayer (HO-SAM) surface at energies of approximately 15 eV. These ion-surface interactions result in covalent transformation of the terminal hydroxy groups at the surface into the corresponding silyl ethers due to Si--O bond formation. The modified surface was characterized in situ by chemical sputtering, a low-energy ion-surface scattering experiment. These data indicate that the ion-surface reactions have high yields (i.e. surface reactants converted to products). Surface reactions with Si(OCH(3))(3) (+), followed by chemical sputtering using CF(3) (+), yielded the reagent ion, Si(OCH(3))(3) (+), and several of its fragments. Other sputtered ions, namely SiH(OCH(3))(2)OH(2) (+) and SiH(2)(OCH(3))OH(2) (+), contain the newly formed Si--O bond and provide direct evidence for the covalent modification reaction. Chemical sputtering of modified surfaces, performed using CF(3) (+), was evaluated over a range of collision energies. The results showed that the energy transferred to the sputtered ions, as measured by their extent of fragmentation in the scattered ion mass spectra, was essentially independent of the collision energy of the projectile, thus pointing to the occurrence of reactive sputtering.A set of silyl cations, including SiBr(3) (+), Si(C(2)H(3))(3) (+) and Si(CH(3))(2)F(+), were similarly used to modify the HO-SAM surface at low collision energies. A reaction mechanism consisting of direct electrophilic attack by the cationic projectiles is supported by evidence of increased reactivity for these reagent ions with increases in the calculated positive charge at the electron-deficient silicon atom of each of these cations. In a sequential set of reactions, 12 eV deuterated trimethylsilyl cations, Si(CD(3))(3) (+), were used first as the reagent ions to modify covalently a HO-SAM surface. Subsequently, 70 eV SiCl(3) (+) ions were used to modify the surface further. In addition to yielding sputtered ions of the modified surface, SiCl(3) (+) reacted with both modified and unmodified groups on the surface, giving rise not only to such scattered product ions as SiCl(2)OH(+) and SiCl(2)H(+), but also to SiCl(2)CD(3) (+) and SiCl(2)D(+). This result demonstrates that selective, multi-step reactions can be performed at a surface through low-energy ionic collisions. Such processes are potentially useful for the construction of novel surfaces from a monolayer substrate and for chemical patterning of surfaces with functional groups.

Intramolecular acceleration effect of a tosylamide group on the electrochemical oxidation of N-α-silylalkyl amides

In electrochemical oxidation, N-alkyl-N-α-silylalkyl (N-tosyl)amino acid amides react faster than N-alkyl-N-α-silylalkyl aliphatic amides. This phenomenon is due to the two roles of the tosylamide moiety. One is the assistance in release of the silyl cation by the tosylamide moiety to transform the cation radical, which possesses a higher oxidation potential, to the radical which possesses a lower oxidation potential. Another is the stabilization of the cation radical by the coordination of the tosylamide moiety to the positively charged silyl atom of the cation radical to shift the equilibrium of the first one-electron oxidation to the more stable side.

The use of trimethylsilyl iodide as a synthon in coordination chemistry

Trimethylsilyl iodide is shown to be an efficient metathetical reagent for preparing transition-metal iodides from the corresponding chlorides, though often complications can cause problems. These include reduction of the starting metal chloride when its oxidation state is high, due to the reaction of iodide, and even oxidation of low-oxidation-state compounds, presumably by incipient silyl cations. Finally, some very inert chlorides, such as of iridium(III), react too slowly with the iodide under the experimental conditions, and simple reaction with solvent becomes predominant.

The Nature of Protonated Decamethylsilicocene, (Me5C5)2Si+H

Structure and bonding in several stereoisomers of the recently synthesized silyl cation (Me5C5)2Si+H (2) and its hydrogen-substituted analogue (C5H5)2Si+H (3) has been investigated by quantum-mechanical methods. DFT calculations at the B3LYP/6-311G(2d,p)//B3LYP/6-31G(d)+ΔZPVE level as well as coupled cluster calculations at the CCSD(T)/6-311G(d) level for the parent silyl cation 3 reveal that the η1.5:η1.5-coordinated isomer 3c of C2 symmetry is the most stable isomer. Two isomers of 3, the η2:η3-bonded 3f (Cs symmetry) and the η2:η2-coordinated 3e (C2v symmetry), are, however, very close in energy (ΔE < 1 kcal mol-1). This implies that 3 is a highly fluxional molecule which undergoes a series of effectively barrierless haptotropic shifts leading to a circumambulatory migration of the SiH group about the cyclopentadienyl ring. The positive charge in the silyl cation 3 is effectively transferred to the attached cyclopentadienyl substituent, leading to an exceptional thermodynamic stability: the secondary cation 3 is more stable than Me3Si+ by 18.7 kcal mol-1 (at B3LYP/6-311G(2d,p)//B3LYP/6-31G(d)+ΔZPVE). Similarly, for the experimentally observed permethylated silyl cation 2, three low-energy isomers with different hapticities for the silicon have been found to be very close in energy (ΔE < 1.5 kcal mol-1 at B3LYP/6-311G(2d,p)//B3LYP/6-31G(d)), with the η2:η3 isomer 2f only marginally favored. This clearly indicates that also 2 is a highly fluxional molecule. 29Si NMR chemical shift calculations at the GIAO/B3LYP/6-311G(2d,p)//B3LYP/6-31G(d) level suggest that the experimentally observed cation adopts the η3:η2-bonded structure 2f. The unique intramolecular stabilization of the silyl cation 2 by the pentamethylcyclopentadienyl substituents and the additional steric protection of 2 by the methyl groups results in very low association energies AE between the cation and solvent molecules (benzene, AE = −3.2 kcal mol-1; dimethyl ether, AE = −6.4 kcal mol-1). Although these interaction energies must be corrected by ca. −2 kcal mol-1 due to the neglect of intermolecular dispersion energy contributions by the applied DFT method, it can be concluded that 2 is best regarded as a free silyl cation even in solvents as nucleophilic as THF.

The tridurylsilylium and tridurylstannylium cations: free and not so free.

Allyltridurylsilane has been prepared and converted to the tridurylsilylium cation by treatment with electrophiles. Allyltridurylstannane has been prepared and converted to the tridurylstannylium cation under similar conditions. The (29)Si chemical shift of the silylium cation indicates that it is free and tricoordinate, whereas the (119)Sn chemical shift of the stannylium cation indicates that it maintains loose fourth coordination with the solvent or the anion.

ChemInform Abstract: Preparation of the First Tricoordinate Silyl Cation

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A Silyl Cation with a Three-Center Si-H-Si Bond

Die außergewöhnlich hohe kinetische Stabilität des Silylkations 1 ist auf das Vorliegen einer Dreizentren-Zweielektronen-Si-H-Si-Bindung zurückzuführen. NMR-Daten (1H, 2H, 13C, 29Si) sind mit der gezeigten statischen, symmetrischen Struktur in Einklang, die auch durch die Ergebnisse von Dichtefunktionalrechnungen gestützt wird.