Bonds silyl cations

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... 

In the case of halocarbon matrices, the SOMO is of the a type and the cleavage is facilitated by the antibonding nature of the SOMO. In other ions, a or a orbitals of scissile bonds interact with a ti or 7i orbital, causing them to be weakened. Accordingly, radical anions of benzyl halides may generate benzylic radicals with loss of a halide ion. Conversely, benzylsilane radical cations may form benzylic radicals with loss of a silyl cation (Fig. 6.15). 

To study the possible stabilizing effect of [3-silyl cations, Olah and co-workers334 prepared the 2- [(1 -trimethylsilyl)vinyl]-2-adamantyl cation 132 [Eq. (3.43)] as well as the parent silicon-free carbocation. In contrast to the above observations, NMR data [the (Cl ), (C2), and (C2 ) carbons are more deshielded in 132 than in the parent ion] showed that cation 132 is destabilized compared with the silicon-free analog. Furthermore, at — 100°C the C(l) and C(3) carbons were found to be equivalent, whereas in the parent ion they were nonequivalent. This indicates a rapid rotation about the C(l)-C(3) bond in 132, which can be rationalized by assuming the intermediacy of the [3-silyl-stabilized cation 133. The difference between cation 132 and those having [3-silyl-stabilization discussed above may be the orthogonal arrangement of the [3-C-Si bond and the p-orbital of the carbocation center. 

In contrast, the Ln(silox)3 system appears to be thermally rather inert and the neodymium derivative sublimes at around 340 °C with slight decomposition [16]. The strength of the Si-O bond and the unfavorable separation of silyl cations (route C) might explain this behavior [34]. Thermal stability of the silox ligand was also detected in Ba2(silox)4(THF) [314]. 

Several major advances have been made in the field of R3M+ cations (M = Ge, Sn, Pb) in recent years, including the preparation of free cyclotrigermenium cations and possibly close to free trimesitylgermyl and trimesitylstannyl cations. However, many challenges remain, for example, a structural characterization of free R3M+ cations (M = Ge, Sn, Pb). In many respects these cations appear to be even more challenging than the silyl cations, due to their weaker and longer bonds to carbon. Presently inadequate tools which need to be improved are computational methods for the NMR chemical shifts of the Sn and Pb nuclei. 

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