Silylium ion chemistry

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. ... 

This type of a-elimination with the generation of a singly coordinated four-valence-electron compound is rather unusual in silylium ion chemistry, only two... 

Anion Sb2Fn (30), which is often used in carbenium ion investigations, represents an example for a counterion with delocalized negative charge. [87,97] However, 30 cannot be used in silylium ion chemistry since trialkylsilylhalides do not dissociate into R3Si+X" ion-pairs in superacid media. [18] Corey proposed the perchlorate anion (31) as a suitable counterion [85] and Lambert used it in many of his investigations. [80-84] However, Olah and co-workers showed that... 

Fig. 1 Weakly coordinating anions used in silylium ion chemistry...

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. 

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." ... 

George Olah published a perspective in which he provided an overview of the research of his group over five decades.1 Christopher Reed surveyed the chemistry of carborane acids, the strongest pure Brpnsted acids known, with a conjugate base that engages in little chemistry.2 These acids are employed to prepare highly acidic carbocations, including protonated benzene, protonated C6o, tertiary carbocations, vinyl cations, and silylium ions. The synthesis and properties of unusual... 

Silylium ion intermediates were also observed in the chemistry involving the hydrosilation of a,(3-unsaturated ketones.250 For most substrates, the H-SiR3 reagent is... 

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... 

We use the term silylium ion for R3Si+, rather than silicenium ion, silylenium ion, or silyl cation, thus following IUPAC recommendations. See Nomenclature of Inorganic Chemistry G.J. Leigh, Ed. Blackwell Oxford, U.K., 1990, p. 106. 

Over the last decade, the synthesis and characterization of trivalent silyl cations (silylium ions), including silatropy-lium ion 1 (Figure 1), have remained as a major challenge in organosilicon chemistry, and, therefore, was the subject of intensive studies. The earlier suggestion that silatropylium ion 1 is more stable than silabenzyl cation 2 and may... 

Our ultimate goal, the control of the reactivity of silylium ions is very ambitious, and we are still far away from a chemistry of cationic silicon in solution, in particular from a controlled chemistry. The benefits of our model system 1, however, are already obvious. The two silicon substituents confer to the cyclic cations 1 a high thermodynamic stability. An example is shown for the arenium ions 9 and the vinyl cations 11 and 12 and can be easily extended to every substituent X acting as a Tt-donor, such as alkenes, nitriles, allenes or azides. In addition the very weakly nucleophilic conditions applied in our reactions allow the generation of only weakly stabilized cations with very uncommon structural features, as for example the hydrogen bridged ion 16. 

Reed, C. A. The Silylium Ion Problem, RjSP. Bridging Organic and Inorganic Chemistry, Acc. Chem. Res. 1998, 31, 325-332. A review by a major contributor to the field. 

Carbocations (carbenium ions) are important intermediates in organic chemistry, and have been intensely studied since as long ago as 1900. The analogy between carbon and silicon is often stressed, but the isolation of a silylium ion, RaSi, took much longer than for carbon chemistry, although as silicon is more electropositive than carbon it should form cations more easily. But it was not until 1986 that Lambert first reported the synthesis of a free silylium cation, by the following reaction [52] ... 

Abstract The chemistry of silylium ions and related silyl cations is reviewed. The discussion is focused on work since 2005 with recourse on investigation since the early 1990s. Stmctural and spectroscopic aspects of silylium ions are emphasized. Included is also survey of methods for their preparation and of their applications in organic synthesis and catalysis. 

Keywords Bond activation Silicon NMR Silylium ion Lewis acid XRD Low coordinated compounds Main group chemistry Organosilicon chemistry Hydrosilylation Silicon cation Small-molecule activation Silyliumylidene... 

The chemistry of silylium ions has been repeatedly reviewed over the past few decades, often in connection with the possibility of their existence in the condensed matter [1-7] and in relation of historical aspects of their synthesis [8]. Only in the last years reviews focused on the spectroscopic, structural and reactivity aspects of silylium ions [9-15]. This overview is a sequel to these previous reports and will... 

The major problem encountered during the synthesis of silylium ions is their inherent high electrophilicity and reactivity, which leads such species to interact in solution or the solid state with solvents or counteranions that are found to be innocent in other areas of chemistry. The ability of silicon to expand its coordination... 

Si NMR spectroscopy and X-ray diffraction (XRD) methods are of major importance in the development of silicon catirm chemistry and are the most significant source for structural information [6,7, 9,10]. Theoretical investigatiOTis in particular in combinati(Hi with Si NMR spectroscopic results played a dominant role in silicon cation chemistry in the 1990s [5], In particular, the contributions of theory to the debate on the existence of silylium ions in the condensed phase proved to be fruitful and decisive. This has been comprehensively illustrated by several authors [5,15]. In this chapter emphasis is laid on experimental structural data which results from XRD measurements or is deduced from Si NMR spectroscopy. Theoretical results are only quoted as supplement to the experimental data. Similarly H, NMR data or results from IR spectroscopy are only provided when significant structural insights were obtained from these data. 

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