Silyl-substituted phenyl cation

Hi. a-Tolyl- and a-phenyl-fi-silyl-substituted vinyl cations. Protonation of l-(p-tolyl)-2-triisopropylsilylethyne 388a leads to formation of the a-(p-tolyl)-/J-triisopropylsilylvinyl cation 389a (equation 65)114,140. The 13C NMR chemical shifts and /ch coupling constants are summarized in Table 16. 

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... 

The tricarbonylcyclohexadienyliumiron salts (32) are synthetically equivalent to substituted phenyl cations/ and react with silyl enol ethers to give, after decomplexation, the a-(cyclohexa-l,3-dienyl) ketones, which are readily oxidized to a-aryl ketones. 2-Substituted cyclopent-2-enones can be prepared from the salts and l,2-bis(trimethyIsilyloxy)cyclopentene (Scheme 52).A... 

Silyl substituents have also distinct effects on the stability of aryl cations. For example, the disilyl (23) and trisilyl-substituted (24) phenyl cations are calculated to be more stable than the parent phenyl cation 25 by 25 kcalmol-1 (HF3-21G)11 and by 22.4 kcalmol-1 (HF6-31G(d))12, respectively. 

The Si NMR chemical shifts in the a-phenyl-, a-tolyl-, a-/7-anisyl-, and a-ferrocenyl-substituted P-silyl ethyl cations, shown in Fig. 7, are 66 34 ppm, 56.92 ppm, 38 88 ppm, and 23.48 ppm, respectively. Si NMR shifts are thus suitable for monitoring the electron demand in P-silyl substituted carbocations. 

In contrast to the FVP synthesis of indenocorannulene (98) from phenyl-corannulene, the fluoro-substituted analogue 29 (X=F) turns out to be very efficient [126] (Scheme 28). Catalytic cyclization is initiated by reaction of a silyl cation with an aryl fluoride to generate a phenyl cation and, subsequently, Friedel-Crafts reaction to an intramolecular aryl coupling, flowed by deprotonation, to give 98, The enabling feature of this reaction is the exchange of carbon-fluorine for sihcon-fluoiine bond enthalpies. 

Si NMR spectroscopy is a suitable tool to monitor the electron demand in -silyl-substituted carbocations. The Si NMR chemical shifts in the a-phenyl-, a-p-tolyl,a-p-anisyl- and a-ferrocenyl-substituted /J-silylethyl cations 331, 333, 323 and 339 are 3 = 66.3, 56.9, 38.9 and 23.5 ppm, respectively, decreasing regularly as expected on increasing the electron-donating capability of the a-substituent. 

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. 

Styrenes with less bulky alkyl groups at silicon, such as 1-phenyl-2-trimethylsilylethene 335, l-p-tolyl-2-trimethylsilylethene 337 and even the relatively more bulky substituted l-p-tolyl-2-dimethylisopropylsilyl ethene 338, on protonation using either FSC>3H/SbF5 or FSO3H, do not lead to solutions of persistent /i-trimethylsilyl-substituted carbocations such as 336 from 335 but undergo fast /i-silyl fragmentation to silyl-free cations such as 78 (equation 51). 

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