Silyl-substituted carbocations Vinyl cations

Silyl-substituted carbocations corresponding a-mesityl-substituted vinyl cation 372. 

The generation of a-ferrocenyl-P-silyl substituted vinyl cations of type 28 does not require superacidic conditions, they can be generated by protonation of l-ferrocenyl-2-trialkylsilyl alkynes with trifluoroacetic acid at room temperature. The SiR3-groups with larger alkyl substituents increase the lifetime of this type of carbocations. 

The 13 C NMR chemical shifts of the para carbon for various phenyl-substituted sp2-and sp-hybridized carbocations (Figure 15) indicate that the demand for rr-aryl delocalization of the positive charge for the -silyl-substituted vinyl cation 393 is lower compared with that in a-phenylethyl (78), a-methyl-a-phenylpropyl (397) and cumyl (292) cations150 153. The stabilizing effect of the /)-silyl group in 393 is comparable to that of the cyclopropyl substituent in 1-cyclopropylbenzyl cation (398). The para carbon shift in the /)-cr-silyl... 

Summary 1-Ary 1-2-trialkyIsilyl-substituted vinyl cations are characterized in solution by NMR spectroscopy. The NMR chemical shift data reveal the stabilization of the positive charge by a (5-silyl substituent. The order of hyperconjugative stabilization of a positive charge by P-substituents is H < alkyl < silyl. The P-silyl effect is dependent on the electron demand of the carbocation and decreases with better electron donating a-substituents. NMR spectroscopy is a suitable tool to investigate the competition between 7i-resonance and CT-hyperconjugation in these type of carbocations. 

The effect of P-silyl groups was investigated in a-aryl-substituted vinyl cations. In these disubstituted carbocations the silyl substituent is attached at the doubly bonded P-carbon. The Cp-Si-bond is thus fixed in plane with the vacant orbital at the C -carbon allowing maximum overlap of the p-a-C-Si bond and the vacant 2p(C -orbital. 

Comparing p-silyl-substituted vinyl cations however with different a-aryl substituents shows that the stabilizing effect of a P-silyl groiq> is not constant but is dependent on the electron demand of the carbocation. Fig. 4 shows the pora-carbon C NMR chemical shift difference A8 between the p-silyl and P-unsubstituted vinyl cations (For the a-ferrocenyl substituted cation A8 C3,4 is used to probe the silyl effect). 

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). 

The first step of the mechanism involves the initial complexation of titanium tetrachloride to the carbonyl group of the electron-deficient alkene (enone) to give an alkoxy-substituted allylic carbocation. The allylic carbocation attacks the (trimethylsilyl)allene regiospecifically at C3 to generate vinyl cation I, which is stabilized by the interaction of the adjacent C-Si bond. The allylic Ji-bond is only coplanar with the C-Si bond in (trimethylsilyl)allenes, so only a C3 substitution can lead to the formation of a stabilized cation. A[1,2]-shift of the silyl group follows to afford an isomeric vinyl cation (II), which is intercepted by the titanium enolate to produce the highly substituted five-membered ring. Side products (III - V) may be formed from vinyl cation I. 

The addition of silylium ions to CC unsaturated compounds results in the formation of carbocations that are stabilized due their p-silyl substitution. As already mentioned in Sect. 3.3.5 p-silyl-substituted vinyl cations 93 were prepared by intramolecular addition of an incipient silylium ion to a C=C triple bond. A prominent example is ferf.-butyl-substituted vinyl cation 108 that could be isolated in the form of its [B(CgF5)4] salt and was investigated by XRD (Scheme 34). The molecular structures of 108 and related vinyl cations showed the linear coordination at the positively charged carbon atom and indicated the structural consequences of p-sUyl hyperconjugation [83, 84]. While... 

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