Silylium salt

Donor-acceptor interactions between the electrophilic Si atoms of three-coordinate silicon derivatives and suitable bases lead to an effective stabilization of these usually extremely reactive systems. Thus, for example, silenes and silanimines have been shown to be stabilized by tertiary amines [1]. Extraordinary stabilizations have been achieved by intramolecular Si-N coordinations, making possible the isolation of stable silanethiones [2] and silylium salts [3]. Intramolecularly donor-stabilized silenes have been unknown till now, possibly because of the lack of suitable synthetic methods. 

Jorgensen and Helmchen introduced the first example of synthetic use of a chiral silicon Lewis acid. They reported that chiral silylium salt (75), having a B(C6p5)4 anion, catalyzes the Diels-Alder reaction of 1,3-cyclohexadiene although the enantioselectivity is rather low [120]. Ghosez and coworkers developed another type of chiral silicon Lewis acid [121]. The silicon Lewis acid (76), derived from (-)-myrtenal, achieves a moderate enantioselectivity in the reaction of methyl acrylate with 1,3-cyclopentadiene (Scheme 9.48). 

The synthetic efforts toward the isolation of silylium salts with an ideal trigonal-planar coordinated positively charged silicon atom in the cation created a series of stabilized sUyl cations in which either the interaction with the solvent, the counteranion, or intramolecular donor groups pacify the high electron demand of the silyl cation. This electron donation leads to cationic species 7 in which the silicon atom adopts a distorted tetrahedral coordination environment (Scheme 2) [9]. Examples for siliconium ions 8 in which the silicon atom has expanded its coordination number to 5 by addition of two solvent molecules are known but are less frequently observed. Intermolecular species 7 and 8 as well as intramolecular variants 9 and 10 for both modes of stabilization have been structurally characterized [9, 28, 29]. 

Recent results on the chemistry of persistent vinyl cations are summarized. / , / -Disilyl-substituted vinyl cations were synthesized by intramolecular addition of transient silylium ions to alkynes. The vinyl cations are stable at ambient temperature and were isolated in the form of their tetrakispentafluorophenylborate and hexabromocarboranate salts. The vinyl cations were characterized by IR and NMR spectroscopy and by X-ray crystallography. The experimental results for the a-alkyl- and a-aryl-substituted vinyl cations confirm their Y-shape structures, consisting of a linear dicoordinated, formally positively charged a-carbon atom and a trigonal planar coordinated /f-carbon atom. In addition, the spectroscopic data clearly indicate the consequences of, / -silyl hyperconjugation in these vinyl cations. Scope and limitations of the synthetic approach to vinyl cations via addition of silylium ions to C=C triple bonds are discussed. 

Silylium ions, which are not protected sterically or are not stabilized either electronically or by intramolecular interaction with a remote substituent do interact strongly with the solvent and/or the counteranion. The reaction of the transient silylium ion with solvents like ethers, nitriles and even aromatic hydrocarbons lead to oxonium, nitrilium and arenium ions with a tetrahedral environment for the silicon atom. These new cationic species can be clearly identified by their characteristic Si NMR chemical shifts. That is, the oxonium salt [Me3SiOEt2] TFPB is characterized by S Si = 66.9 in CD2CI2 solution at —70°C. " Similar chemical shifts are found for related silylated oxonium ions. Nitrilium ions formed by the reaction of intermediate trialkyl silylium ions with nitriles are identified by Si NMR chemical shifts S Si = 30—40 (see also Table VI for some examples). Trialkyl-substituted silylium ions generated in benzene solution yield silylated benzenium ions, which can be easily detected by a silicon NMR resonance at 8 Si = 90—100 (see Table VI). ... 

Reed and co-workers utilized the silylium carboranate salts to generate novel Bronstedt superacids based on carborane anions as conjugate bases by reaction of the salts with liquid HCl [Eq. (7)]. These carborane superacids are able to cleanly protonate Cgo and benzene at room temperature to yield and benzenium... 

The Bartlett Condon Schneider hydride transfer reaction,22 23 first employed in silicon chemistry by Corey in 1975,24 developed since then to be the most popular synthetic approach to silylium ions in the condensed phase.10 Subsequently, it was also used for the generation of germylium22,56 and stannylium compounds.4,17,26 29 This method exploits the relative weakness of the E-H bond and involves the transfer of the hydride from the element to a strong Lewis acid, in most cases to trityl cation. The easy access of trityl salts with a wide variety of weakly coordinating counteranions is a clear advantage of this method. The reaction can be applied in polar solvents such as sulfolane, ethers and nitriles but also in chlorinated... 

Until the year 2002 no experimental data existed on the structures of unperturbed R3E+ cations, the exact analogues of the carbenium ions. Computational data combined with NMR chemical shift calculations, which could be compared to experiment, were the only source of reliable structural information for silylium ions6,7,13,77,121 while for germylium, stannylium and plumbylium ions this combined approach was not attractive due to either the non-existence of the experimental data (Ge) or the complexity of the computational problem (Sn, Pb). On the other hand, a series of excellent experimental studies demonstrated, for example, the high coordination tendency of small trialkylsilylium ions either toward the counteranion38,114,127,138 or toward the solvent.36,37,67,116,127 The solid state structures of these silyl cation salts showed clear indications either of cation/anion or cation/ solvent coordination. Thus, the nature of the observed cation, i.e. the degree of silylium ion character remained disputable.10,11,13... 

Summary Bissilylated onium ions of the elements N -> Sb are formed by intramolecular addition of transient silylium ions to EPh2 groups (E = N, P, As, Sb). Solutions of the onium salts in aromatic hydrocarbons are stable at room temperature for days, with the exception of the stibonium ion, which decomposes slowly. The cations were isolated as their [B(C6F5)4] salts and were characterized by NMR spectroscopy supported by quantum mechanical calculations. 

The cations are prepared from 2,5-disilaheptanes, 3, by hydride transfer reactions. The transient silylium ion 4 undergoes an intramolecular reaction to the more stable cyclic onium ion 2. The precursor silanes, 3, are synthesized by salt metathesis reaction from compounds of the type Ph2ELi, where E is the relevant Group 15 element, and the silylchloride 5. The salts 2 [B(C6Fs)4]" are isolated after washing with pentane as white to yellow microcrystalline powders in nearly quantitative yield. Solutions of the cations 2a-c in aromatic solvents are stable at room temperature. The stibonium ion 2d decomposes in solution slowly, yielding unidentified products. 

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