Silylium ions, reactions

Recently, this concept could be transferred to the homologous silylium ions 3-Si (Si in cx-position to the ferrocene core) [18, 19], which were found to be potent catalysts for Diels-Alder reactions at low temperature [19]. The electron-rich ferrocene core buffers the Lewis acidity, thus avoiding the irreversible formation of Lewis pairs. 

An important contribution to silylium ion chemistry has been made by the group of Muller, who very recently published a series of papers describing the synthesis of intramolecularly stabilized silylium ions as well as silyl-substituted vinyl cations and arenium ions by the classical hydride transfer reactions with PhjC TPEPB in benzene. Thus, the transient 7-silanorbornadien-7-ylium ion 8 was stabilized and isolated in the form of its nitrile complex [8(N=C-CD3)]+ TPFPB (Scheme 2.15), whereas the free 8 was unstable and possibly rearranged at room temperature into the highly reactive [PhSi /tetraphenylnaphthalene] complex. ... 

On the basis of these results we embarked on a systematic study on the synthesis of vinyl cations by intramolecular addition of transient silylium ions to C=C-triple bonds using alkynyl substituted disila alkanes 6 as precursors.(35-37) In a hydride transfer reaction with trityl cation the alkynes 6 are transformed into the reactive silylium ions 7. Under essentially nonHnucleophilic reaction conditions, i.e. in the presence of only weakly coordinating anions and using aromatic hydrocarbons as solvents, the preferred reaction channel for cations 7 is the intramolecular addition of the positively charged silicon atom to the C=C triple bond which results in the formation of vinyl cations 8-10 (Scheme 1). 

Interestingly, reaction of alkynyl disilane 14 with trityl cation did not result in the formation of stable vinyl cations. Obviously, the formation of the four-membered disilacyclobutane ring is unfavorable. Similarly, treatment of alkyne 15 with the pre-formed triethylsilylarenium ion 1 derived from toluene did not give the expected intramolecular transfer of the silylium ion to the triple bond. Instead, only a complex product mixture was obtained. 

The Bartlett Condon-Schneider hydride transfer reaction, first employed in silicon chemistry by Corey in 1975, " developed since then to be the most popular synthetic approach to silylium ions in the condensed phase. Subsequently, it was also used for the generation of germylium and stannylium compounds." ... 

The intermediate formation of the ferrocenyl-substituted silylium ion 16 by protonation of the ansa-ferrocenyl silane 17 can be regarded as a special case of electrophilic cleavage of an activated C-Si bond (see Scheme 7). The driving force for this reaction is the release of a strain by formation of the silyl cation. In a... 

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). ... 

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... 

During the 80s and early 90s, Lambert and co-workers reported a number of experiments, which, according to the authors, suggested the existence of free silylium ions in solution. [80-84] The alleged silylium ions were prepared by hydride abstraction from an alkylsilane with triphenylcarbenium perchlorate according to reaction (2) first suggested by Corey. [85]... 

A solvent-free but delocalized silylium ion has been prepared recently. Interestingly, the silylium ion undergoes reversible redox reaction to give the corresponding radical and anion (equation 62). The silylium ion is indicated to be a homotrisilacyclopropenium ion. In fact, 1,3-Iength of the silylium ion is much shorter than that of the corresponding radical. ... 

Silylium ions stabilized by Si-X-Si three-center bonds (X = halogen or hydrogen) have been synthesized by the reaction of the corresponding l-tris(trimethylsilyl)methyl-2,3-bis(trimethylsilyl)-l-silacyclopropene derivative with triethylsilylbenzenium tetrakis(pentaflourophenyl)borate <2004CL530>. 

So far, the first approach, i.e. the enantioselective Sakurai reaction using chiral Lewis acid catalysts has been accomplished by Yamamoto s chiral (acyloxy)borjme (CAB) catalysis [40] and others that are not included in this chapter. Progress in developing chiral silicon Lewis acids for the reaction is now awaited but a recent publication by Jprgensen and Helmchen on a chiral silylium ion appears in a later section on ring construction [41]. Thus, other two approaches are shown herein. 

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