Silylium

The stable (f-Bu2MeSi)3Si radical reacts with lithium in hexane at room temperature affording the silyllithium 7, with the central anionic silicon atom almost planar, whereas the one-electron oxidation with tiiphenylmethyl ion produces the corresponding silylium ion 8 (Scheme... 

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. 

Klare HEX, Bergander K, Oestreich M (2009) Taming the silylium ion for low-temperature... 

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. ... 

When the counterion is complex, for example metal-halogen anions such as BF4-, the most electronegative portion of the counterion becomes attached to the silicon center. Because of this attachment, it is natural to consider the intermediacy of a silicenium cation (silylium or silylenium ion) intermediate in such reactions (Eq. 4). Bond energies derived from electron impact studies indicate that Eq. 4 is exothermic in the gas phase by about 8 kcal/mol.26,29 There seems little doubt that trivalent silicon-centered cationic species do exist in the gas phase30,31 or that processes similar to that shown in Eq. 4 do occur there.32,33... 

Bu3Si(OH2)] [H-CBuMesBBrg] 106, which can be rationalized in two canonical forms, namely a Lewis acid-Lewis base adduct of the silylium ion and water, and, alternatively, a protonated silanol (Scheme 18).264... 

Unlike silyl radicals and silylium ions that have been elusive for a long time, silyl anions (silanides) are well known since the pioneering work of Gilman etal. in the 1960s. Two literature reviews on silyl anions by Lickiss and Smith as well as Tamao appeared in 1995. Many structural elucidations and reactivity studies of silyl anions have been... 

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. 

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). 

This indicates that the deviations are due to systematic errors, for example deficiencies of the applied methods and basis sets. DFT-based methods, such as GIAO/DFT calculations are known to overestimate paramagnetic contributions to the chemical shielding. This results, for critical cases with small HOMO/LUMO separations, in overly deshielded competed chemical shifts. Notorious examples for these deficiencies are 29Si or 13C NMR chemical shift computations of silylenes, silylium ions or dienyl cation .(5/-54) Taking into account the deficiencies of the applied method, and bearing in mind very reasonable correlations shown in Figures 4 and 5, the computational results do support the structural characterization of the synthesized vinyl cations. 

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. 

In the case of silylium ions the formation of Wheland-type intermediates is shown by appreciable solvent effects on the 29Si NMR chemical shifts. 

Several reviews appeared on the heavier congeners of the carbenium ions. Clearly, the silylium ion problem has received the most attention, and both theoretical as well as experimental aspects have been reviewed. The chemistry of cationic germanium, tin and lead is covered by a recent review by Zharov and Michl. We will concentrate in this review on the description of the progress made during the last 4 years and will try to give an account on the synthesis, the properties and the structure of organosubstituted three-coordinated, tiivalent group 14 element cations and closely related species in the condensed phase. 

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 cleavage of a silicon-methyl bond by the silylated benzenium ion [EtsSi/ CgHg] and formation of the silylium ion 14 is the first step in the unexpected synthesis of the homoaromatic silyl cation 3 from trisilacyclopropene 15 (Scheme 6). ... 

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... 

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