Disilyl cation

The novel bridged disilyl cations (33), X = H, F, were synthesized as their B(CeF5)4 salts and characterized by NMR and X-ray methods.74 In both cations the Si-X-Si linkage is symmetrical, corresponding to a single minimum potential. The novel cation (34), Ar = 2,6- PrCeH3, with a formal divalent silicon was obtained as a B(CeF5)4 salt.75... 

Detail mechanistic studies using IR and NMR have led to the following reaction mechanism being proposed (i) nucleophilic attack of an alkene on the 1,3-disilylpropyl cation (Figure 6.1) to form allylsilane and a secondary carboca-tion intermediate stabihzed by the silyl group at the P-position (ii) nucleophihc attack of an allylsilane to the intermediate to form a new C-C bond and (iii) formation of the allylsilylated product and disilyl cation by nucleophilic addition of another allylsilane. The formation of the 1,3-disilylpropyl cation has also been discussed. 

Panisch R, Bolte M, Miiller T (2006) Hydrogen and fluorine-bridged disilyl cations and their use in catalytic C-F activation. J Am Chem Soc 128 9676... 

These reactions presumably proceed by catalytic cycles in which the carbonyl component is silylated. The silyl ether can then act as a nucleophile, and an oxonium ion is generated by elimination of a disilyl ether. The reduction of the oxonium ion regenerates the silyl cation, which can continue the catalytic cycle. 

The rhodium-catalyzed cyclization/hydrosilylation of internal diyne proceeds efficiently with high stereoselectivity (Scheme 106). However, terminal diynes show low reactivity to rhodium cationic complexes. Tolerance of functionalities seems to be equivalent between the rhodium and platinum catalysts. The bulkiness of the hydrosilane used is very important for the regioselectivity of the rhodium-catalyzed cyclization/hydrosilylation. For example, less-hindered dimethylethylsilane gives disilylated diene without cyclization (resulting in the double hydrosilylation of the two alkynes), and /-butyldimethylsilane leads to the formation of cyclotrimerization compound. 

The results indicate that the formation of long-lived trimethyl substituted silyl cations, in the presence of aromatic solvents, as claimed by Lambert et al.95 is not feasible under these conditions. Persistent silicenium ions require sterically more shielding substituents at silicon or hypercoordinative stabilization.96 98 13C and 29Si NMR chemical shifts were calculated for a series of disilylated arenium ions 85 using density functional theory (DFT). The calculations predict consistently the unsaturated carbon atoms to be too deshielded by 8-15 ppm. Applying an empirical correction, the deviation between experiment and theory was reduced to -0.4 to 9 ppm, and the 13C NMR chemical shift of the highly diagnostic cipso is reproduced by the calculations (Ad = -3.8 to 2.7 ppm).99... 

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. 

Silyl substituents have also distinct effects on the stability of aryl cations. For example, the disilyl (23) and trisilyl-substituted (24) phenyl cations are calculated to be more stable than the parent phenyl cation 25 by 25 kcalmol-1 (HF3-21G)11 and by 22.4 kcalmol-1 (HF6-31G(d))12, respectively. 

Anodic oxidation of hydroquinone disilyl ethers also takes place easily at around 1 V vs SCE, as shown in Table 1044. It was proposed that an initial one-electron oxidation generates a cation radical which decomposes by a Si—O bond cleavage to form quinones (equation 40)44. 

Muller et al.332 have characterized a number of a-aryl-, (3,(3 -disilyl-substituted vinyl cations (130). Both 13C NMR data and calculations show the effect of both Tt-conjugation with the aromatic moiety and O-delocalization of the (3-Si—C bonds. Jt-Delocalization is manifested by the marked low-field shift of the ortho and para carbon atoms compared to those of the precursor alkynes, whereas o-delocalization is shown by the deshielding of the 29Si NMR resonance relative to the precursor silylalkyne (A829Si 29.5-41.4). Computed structures [B3LYP/6-31G(d) level] are all very similar, that is, calculation is not able to show the subtle interplay between O- and 7t-delocalization, which is evident from the NMR spectroscopic data. 

Cyclohexylidenecyclopropanone acetal 4a-c was treated with TiCU at -78 °C in dichloromethane in the presence of furan to give adducts of furan, 23 and 24 (Table IV). All products formed result from C2-C3 bond cleavage in 4a-c, and are rationalized by a reaction mechanism including alkylideneallyl cation intermediate 5. The reactions of 4a and 4c preferentially gave a furanyl product 24 (entries 1, 2, and 6) in contrast to preferential formation of [4 + 3] cycloadduct 23 in the reaction of the disilyl acetal substrate 4b (entries 3 and 4). The reaction also took place by using SnCl4 (entry 5). 

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