Silicon carbocations containing

Amine 17 was prepared from4 by reaction with aniline at 80 °C. Treatment of 17 with mercury(II) acetate followed by borohydride reduction gave the azasilacycle (18) in modest yield (eq 9). This result is in accord with the 8-carbocation stabilizing effect of a silicon atom. Synthon 4 is thus a useful building block for the preparation of silicon-containing heterocycles. 

Other, removable cation-stabilizing auxiliaries have been investigated for polyene cyclizations. For example, a sdyl-assisted carbocation cyclization has been used in an efficient total synthesis of lanosterol. The key step, treatment of (257) with methyl aluminum chloride in methylene chloride at —78° C, followed by acylation and chromatographic separation, affords (258) in 55% yield (two steps). When this cyclization was attempted on similar compounds that did not contain the C7P-silicon substituent, no tetracycHc products were observed. Steroid (258) is converted to lanosterol (77) in three additional chemical steps (225). 

The methoxymethyl cation can be obtained as a stable solid, MeOCH SbF Carbocations containing either a, p, or y silicon atom are also stabilized, relative to similar ions without the silicon atom. 

However, the directing influence of silicon can be overcome if the vinylsilane contains another substituent that can stabilize a carbocation more strongly than silicon. For example, when the silyl group is attached to C-2 of a terminal alkene, reaction occurs to give the more substituted carbocation 82 (equation 44)107. Similarly, if the silicon is bound to the same carbon atom as a phenyl group, reaction occurs via the benzyl cation to give the product shown in equation 45108. 

Nucleophilic attack in a 1,6 fashion by the allylsilane double bond at the doubly activated Michael acceptor produces A containing the silicon-stabilized carbocation. Loss of the trimethylsilyl group generates the exocyclic methylene moiety. 

The methoxymethyl cation can be obtained as a stable sohd, MeOCHj SbF, Carbocations containing either a, p, or y silicon atom are also stabilized, relative to similar ions without the silicon atom. In super acid solution, ions such as CX (X = Cl, Br, I) have been prepared. Vinyl-stabilized halonium ions are also known. ... 

Hypercarbon chemistry is not restricted only for carbocations and their borane analogs but a few of examples of silicon- and germanium-containing bridged and homoaromatic compounds are also known. These will be discussed briefly in this section. 

Carbocations Containing Silicon and Other Group 14 Elements. 291... 

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