7-Silyl carbocations formation

Attack of a nucleophile on the /1-silyl carbocation 127 or the cyclic siliconium ion 128 leads to desilylation and formation of the Sakurai product. When nucleophilic attack is disfavoured by steric hindrance at the silicon, competing intramolecular attack by the enolate becomes important. This 5-exo-tet cyclization gives the trimethylsilylcyclopentane product with high stereospecificity, the trimethylsilyl group having undergone a 1,2 shift. 

The ring contraction shown in equation 111 is also promoted by formation of a /J-silyl carbocation in the intermediate 146179. 

Clearly the course of these cyclization reactions is dependent upon the silicon group. In this regard, cyclization of (82) affords the steroid nucleus (83 Scheme 40). The formation of (83) was attributed, in part, to a transition state preference for the formation of the linear vinyl carbocation (84b) rather than the bent vinyl cation (84a), which would be produced in an endocyclic cyclization. The formation of (81) was controlled by the generation of the -silyl carbocation (85a), which may be a precursor to an a-silyl ketone, which undergoes protodesilylation. It is not known whether the formation of (81) as the major cyclization product occurs through a kinetic pathway or by Wagner-Meerwein rearrangement of the kinetically prefened linear carbocation (85b). 

Competition studies reported by Kuwajima, ° which also complement the results of Nakai, illustrate the limitations of the -effect as a tool for predicting the outcome of vinylsilane-terminated cyclizations (Scheme 4). Acylium ion initiated cyclizations of (7a) and (7b) gave the expected cyclopentenones (8a) and (8b). However, compound (7c), upon treatment with titanium tetrachloride, gave exclusively the cyclopentenone product (8c) arising from the chemoselective addition on the 1,1-disubstituted alkene followed by protodesilylation of the vinylsilane. The reversal observed in the mode of addition may be a reflection of the relative stabilities of the carbocation intermediates. The internal competition experiments of Kuwajima indicate that secondary B-silyl cations are generated in preference to secondary carbocations (compare Schemes 3 and 4), while tertiary caibocations appear to be more stable than secondary P-silyl carbocations, as judged by the formation of compound 9c). 

A second, less used, strategy encompasses the Lewis acid catalyzed intramolecular reaction of a silyl enol ether with a propargyl cation. The latter can be conveniently generated by a cobalt complexed propargyl ether. This complexation strongly helps the carbocation formation. By using cobalt complexation, intramolecular aldol type reactions (for R = OR ) have been accomplished. ... 

Allenylsilanes react with acetals to afford homopropargylic ethers (Table 9.37) [61]. These reactions are promoted by silyl and carbocation species. A variation of this conversion involves in situ formation of the acetal from an aldehyde and Me3SiOMe (Eq. 9.55). The success of this method indicates that conversion of the aldehyde to the acetal and its subsequent reaction with the silane must be faster than direct reaction of the aldehyde with the silane. 

The silyl fragmentation in superacids initiated by a controlled temperature increase is a method to generate persistent carbocations, such as the vinyl cation 378, which are not accessible by direct protonation of unsaturated hydrocarbons because of excessive formation of oligomeric and polymeric products. 

As described in Section II, Lewis acid catalyzed desilylative carbon-carbon bond formation with an electrophile has been shown to be very versatile in organic synthesis. Occasionally, depending on the nature of the substrates (e.g. the presence of appropriate functional groups), the carbon-silicon bond may remain intact. For example, treatment of 132 with a Lewis acid affords a mixture of cyclization products 133-135 (equation 113). The isolation of 133 indicates that the carbocation intermediate thus formed is trapped by the oxygen nucleophile before elimination of the silyl moiety occurs204. 

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