A-Silylvinyl cation

The simplest a-silyl-substituted carbenium ion H3SiCH2+ (1) is not a minimum on the potential energy surface and it rearranges without a barrier to the more stable (by 49 kcalmol at MP4/6-31G(d)//3-21G(d)) silylium ion H2Si+Me (2). For a-silylethyl cation (3) and a-silylvinyl cation (4) calculations at the correlated MP2(fu)/6-31G(d,p) level predict hydrogen bridged structures just as for the parent ethyl and vinyl cations . 3 is separated by a barrier of only 5.2 kcalmol" from the ethylsilylium ion 5, which is by 25.9 kcalmol more stable [at MP2(fu)/6-31G(d,p)]. Similarly, the activation barrier for the 1,2-H shift from the a-silylvinyl cation (4) to the isomeric 1-silaallyl cation (6) is... 

Protonation of the A -aryl-Af-(3-triisopropylsilylpropargyl) carbamate 37 with trifluoromethanesulfonic acid generates a (3-silylvinyl cationic intermediate 38 that is attacked by the carbamate carbonyl group (but not the aromatic ring) to give good yields of the 2(3/7)-oxazolone 40 (Fig. 5.9). ... 

The classical open /l-silylethyl cation 9 and /f-silylvinyl cation 11 are no minima at higher level of theory3,4. They collapse to the bridged protonated silacyclopropane 21 and silacyclopropene 22, respectively. On the basis of their calculated structures (Figure 3) both cyclic molecules are best described as -complexes between a silylium ion and ethene or acetylene, respectively. 

FIGURE 12. 13C NMRof a -mesityl-/S-(triisopropyl)silylvinyl cation 375 at —135 °C in SO2CIF/SO2F2, internal reference CD2CI2. 

The a-aryl-p-triisopropylsilyl-substituted vinyl cations 1-3 are generated by protonation of the corresponding l-aryl-2-silyl-alkynes with superacids at low temperatures (Eq. 1). For die protonation of a-ferrocenyl-p-(triisopropyl)silylethyne to yield the l-ferrocenyl-2-(triisopropyl)silylvinyl cation 4 the weaker acid CF3COOH is sufficient (Eq. 2). 

When other electrophiles besides a,/ff-unsaturated ketones are used, the mechanism is analogous, however, care must be taken to ensure that direct desilylation of acyclic intermediate 31, to form acyclic alkyne 32 does not occur. It is interesting that the use of more sterically encumbered alkylsilanes, for example tert-butyldimethylsilyl rather than trimethylsilyl, increases the selectivity of the reaction for the cyclized product versus the acyclic alkyne 32 that is produced by desilylation of silylvinyl cation 31. ... 

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