4.4.43 Product Subclass 43: Silylium Ions and Stabilized Silylium Ions

T. Müller

General Introduction

Silylium ions [R3Si+] are tricoordinate silicon species with a positively charged silicon atom. The central silicon atom adopts a trigonal planar coordination environment with R—Si—R bond angles close to the ideal value of 120°. The vacant p-type molecular orbital at the central silicon atom is oriented perpendicular to the R3Si plane. Therefore, these six-valence electron species are isostructural and isolobal to boranes [R3B] and they are the silicon analogues of classical carbenium ions [R3C+]. The history of their synthesis and chemistry has been repeatedly reviewed over the past few decades, describing both transient and stable representatives.[1–9] Carbenium ions are frequent intermediates in many chemical transformations in carbon chemistry, and their existence was recognized and firmly established in 1901.[10,11] In contrast, in organosilicon chemistry, there are only a few examples of reactions with silylium ions as established intermediates, and the first conclusive evidence for a silylium ion in condensed media was only found in 1997.[12,13] This striking difference between these two so closely related species is not a result of the thermodynamic instability of the silicon compound. On the contrary, the silicon cation [R3Si+] is, for most of the synthetically important substituents R, more stable than the corresponding carbenium ion [R3C+].[8] The major obstacle to the formation of silylium ions in the condensed phase is their high electrophilicity and, as a consequence, their high reactivity toward any nucleophile. The lower electronegativity of silicon compared to carbon leads to an accumulation of positive charge at the central silicon atom for every organic substituent R. This accretion of positive charge is not dispersed by π-conjugation and/or hyperconjugative effects due to less-effective orbital overlap between the silicon atom and the carbon substituents. The larger size of the silicon atom means that steric protection by bulky substituents is less effective than for carbocations, and it allows extension of the coordination sphere at silicon to coordination numbers larger than 4. For these reasons many reactions in organic chemistry that proceed by a dissociative SN1-type mechanism will in organosilicon chemistry follow an associative course via pentacoordinated transition states or even intermediates. Furthermore, these are also factors that severely hamper the synthesis of silylium ions unfettered by interactions with solvent or counteranion. Silylium salts are prepared by synthetic approaches that are different from those well established in carbon chemistry (see ▶ Sections 4.4.43.1–4.4.43.6). The use of unusual solvents for ionic compounds, such as aromatic hydrocarbons, or silanes paired with the application of very robust and extremely weakly coordinating anions {e.g., fluorinated tetraphenylborates of the type [B(C6F5–nR1n)4]– (R1 = SiR23, H; n = 0, 1), or halogenated closo-carborates [HCB11H11–nXn]– (X = Cl, Br, I; n = 6, 11) or -borates [B12X12]2–(X = Cl, Br)} are absolute prerequisites for their successful generation and isolation.[14,15] As a consequence, only a small number of silylium ions have been synthesized that are fully consistent with the textbook definition of a trigonal planar coordinated silylium ion [R3Si+].[12,16–18] These cations 1 are substituted with three bulky aryl groups to prevent reactions with nucleophiles and only one alkyldiarylsilylium ion 2 has been prepared (▶ Scheme 1). In addition, several silylium ions 3–6 in which the positively charged silicon atom is part of a delocalized π-system are known (▶ Scheme 1).[19–23] From the viewpoint of synthetic chemistry, the enormous Lewis acidity, in particular of the aryl-substituted ions 1 and 2, is interesting, but the bulky substituents, essential for their successful synthesis, and their high reactivity severely hamper their application in organic synthesis (see, however, ▶ Section 4.4.43.10). Silylium ions are easily identified spectroscopically by their 29Si NMR resonance at very low field. For cation 2 a resonance at δ29Si = 245 is reported and the triarylsilylium ions 1 are characterized by 29Si NMR chemical shifts at δ29Si = 230–216.[18] For the silicon atoms in aromatic 3 and homoaromatic 4, an even stronger deshielding effect is noticed [i.e., δ29Si = 316 for the central tricoordinated silicon atom in the homoaromatic cation 4 and δ29Si = 208–288 in the trisilacyclopropenium ion 3 (Z = Si)].[19–21]

The synthetic efforts toward the isolation of silylium salts with an ideal trigonal planar coordinated positively charged silicon atom created a series of stabilized silyl cations in which either the interaction with the solvent, the counteranion, or intramolecular donor groups pacifies the high reactivity of the silyl cations. This electron donation leads to cationic species 7 in which the silicon atom adopts a distorted tetrahedral coordination environment (▶ Scheme 2). Siliconium ions 8, in which the silicon atom has expanded its coordination number to 5 by addition of two solvent molecules, have been structurally characterized. Intermolecular species 7 and 8 as well as intramolecular variants 9 and 10, which have both modes of stabilization, have been characterized.

Of particular interest are the solvent- or anion-stabilized tetracoordinated silyl cations 7. In the case of the solvent complexes 11–16, structural and/or NMR spectroscopic data clearly indicate a covalent interaction between the solvent and the silylium ion (▶ Scheme 3). For example, cations 12 should be described as silylated arenium ions, as indicated by their 29Si NMR chemical shifts of δ29Si = 84 and 98 for R1 = Me and Et, respectively.[8,23] Nevertheless, cations 12 as well as chloronium ions 11 and the bissilylated hydronium ions 13 are extremely valuable synthetic sources of silylium ions with an unmatched Lewis acidity. From a synthetic point of view, these species might be regarded as solventstabilized silylium ions. In particular, their reactivity is significantly greater than those of silylated oxonium ions 14, nitrilium ions 15, or pyridinium ions 16. Benzenium ions 12 are usually prepared using the weakly coordinating perfluorinated tetraphenylborate anion {[B(C6F5)4]–}. Switching to carborates as counteranions results in the formation of carborate-stabilized silylium ions; for example, when using the 7,8,9,10,11,12-hexachloro-1-carba-closo-dodecaborate(1–) {[HCB11H5Cl6]–} anion the silylium carborate 17 is formed (▶ Scheme 4). Extended structural studies of 17 and several related halogenated carborates and borates reveals in each case close contact between one halogen atom of the carborate anion and the silicon atom. This leads to a distorted tetrahedral coordination environment for the silicon atom.[8,24,25] The zwitterionic nature of the silylium carborate 17 is also reflected by the relatively small 29Si NMR chemical shift (δ29Si = 103 in benzene-d6) compared to that of free silylium ions. This 29Si NMR chemical shift is, however, significantly different from that measured for triisopropylsilylium tetrakis(pentafluorophenyl)borate {[iPr3Si][B(C6F5)4]} in the same solvent (δ29Si = 108).[8] This suggests that, depending on the weakly coordinating anion, different silyl cationic species are present in benzene solution. Although the interaction between the carborate anion and the silicon atom determines the structure and spectroscopic properties of these silylium carborates and distinguishes them from the free silylium ion, they represent possibly the nearest approach to simple trialkyl-substituted silylium ions in the condensed phase. Their high ability for silyl group transfer and their unprecedentedly high Lewis acidity, which outperforms by far those of the conventionally covalently bonded silyl Lewis acids 18–20, justifies, from a synthetic point of view, their being described as anion-stabilized silylium ions.