Alkynylsilanes structure

An interesting X-ray structure of the highly strained, sp hybridized vinyl cation 8 was recently reported by Muller et al. The synthesis of 8 was accomplished by the reaction of alkynylsilane 7 and triphenylmethyl (trityl) cation. 

As with vinylsilanes and alkynylsilanes, substitution is favoured over addition for allylsilanes. However, this can be affected by the steric and electronic effects of the silicon substituents. Mayr and Hagen have studied the reactivities of allylsilanes towards the p-methoxy substituted diphenylcarbocation (Scheme 7)136. 37 Relative rate data and observed products are summarized in Table 11 for allylsilanes of the structures 119-122, with various silicon substituents. 

The treatment of alkynylsilanes in acetonitrile with an iodine(III) reagent prepared from iodosylbenzene and two molar equivalents of triflic acid, believed to exhibit structure... 

Alkynylsilanes can function as carbon nucleophiles in addition reactions to electrophilic ir-systems. In principle electrophilic addition reactions to alkynylsilanes can occur to produce a- or P-silyl-substituted vinyl cations, as illustrated in Scheme 37. The a-silyl carbocation is not the most stabilized cation, the reason being that the caibon-silicon bond can achieve coplanarity with the vacant orbital on the -carbo-nium ion, making possible 3-stabilization through hyperconjugation. Depending on the configuration of the carbocation, the developing vacant orbital can exist as a p-orbital, as in structure (75a), or an rp -hy-brid, as in structure (75b). 

---