Silylium borates

Slow-spinning Si NMR CPMAS solid-state NMR spectroscopy for silylium borates 28[HCBnMe5Br6] and 452[Bi2Cli2] (see Fig. 3 for an example) revealed within the limits of accuracy an almost axial symmetric Si NMR chemical shift tensor (8n = 622 > 833 and skew k close to +1) in agreement with the local molecular C3-symmetry of silylium ions 28 and 45 (see Table 3) [20, 22]. A very large anisotropy of the Si NMR chemical shift tensor is indicated by the determined span ii = 811-833. Therefore, the anisotropy of the Si chemical shift tensor is significantly larger for triarylsilylium ions than for tetrahedral... 

Table 9 NMR and IR data of silylium borates and carborates having a Si-H-Si bond...

Stabilized silylium ions can be also used to transfer silyl groups to singlet carbene analogues. As outlined previously, silylation of N-heterocyclic silylenes gives access to aromatic silaimidazolium ions [27]. Likewise, the reaction of stannylene 104 with arene-stabilized silylium borates results in the formation of stable... 

Scheme 43 Examples for Mukaiyama aldol condensations (a) and Diels-Alder cyclizations (b, c) catalyzed by silylium borates [41, 114, 115]...

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>. 

Cations 1, with different substituents X, can be synthesized by hydride transfer reactions [3] from 2,5-disilaheptanes 2. The reaction of 2 with one equivalent trityl tetrakis(pentafluorophenyl) borate (TPFPB) in benzene at room temperature yields the trivalent silylium ions 3 and triphenylmethane (Scheme 1). The silylium ions 3 are only transient species and undergo intramolecular cyclization which yield the cations 1. 

It was already pointed out in the introductory remarks that non-nucleophilic reaction conditions are one prerequisite for the synthesis of silylium ions in the condensed phase. The use of weakly coordinating anions is particularly decisive. Halogenated closo-carborate monoanions and - more recently - closo-borate dianions have found the most widespread use besides fluorinated tetraarylborates. While with bulky substituents these anions form silylium ions salts with well-separated cations and borate anions [10, 20, 22, 42], silylium ions with smaller alkyl substituents tend to coordinate directly to the anion forming zwitterionic compounds [7, 31, 32, 47]. For example, upon reaction of trimethylsUane with trityl-cZoso-carborate 86 in excess trimethylsUane the trimethylsUylium carborate 87 is formed (Scheme 27) [44]. 

Stannylium borates 105[B(C6F5)4] (Scheme 32) [92]. These results indicate that in general the silylation of heavy carbene analogues using silylium ions or stabilized silylium ions is a promising route to the corresponding cationic species [93]. 

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