Solvolysis 5-silicon effect

Since then the kinetic a-silicon effect has been measured in solvolysis of various systems that are considered to involve ionization in the rate-determining step. The results are compiled in Table 2, which also includes for comparison data for the a-silicon effect... 

The effect of a-silyl substitution on the stability of a carbenium ion was qualitatively unclear for a long time. Early solvolytic studies by the groups of liaborn36 and Cartledge37 suggest a destabilizing effect of a-silyl substitution compared with alkyl. The measurement and interpretation of the kinetic a-silicon effect in solvolysis reactions is, however, often complicated by the fact that steric and ground state effects may play an important role and that, in addition, the rates of ionization often involve a contribution from nucleophilic solvent assistance. 

The small /J-silicon effect found by DeLucca and Paquette82 in the solvolysis of the cyclopropylmethyl trifluoroacetate 179 (equation 30) might be a result of the inherent stability of the cationic intermediate 180 and its delocalized structure. The 27-fold rate... 

The overall kinetic effect of the disilanyl group is, however, small due to the different solvolysis mechanisms. 224 solvolyses only 3.48 times faster than 226 in TFE at 100 °C. For the k process Shimizu and coworkers calculated the relative rates for 224 225 226 to be 1.60 1.0 10.7. They concluded that these small silicon effects reflect the concerted nature of the k solvolysis involving transition states with a highly delocalized charge distribution88. 

In the adamantyl framework the /-silicon effect was studied by Grob and coworkers96,97. The first-y-trimethylsilyl substituent (cf 245) enhances the solvolysis rate of 1-bromoadamantane 246 by a factor of only 8.6 and the second trimethylsilyl group in... 

Over the past few years, the debate over the origin of the p-silicon effect on carbocations has narrowed to one of the relative magnitudes of inductive and hyperconjugative factors. Theory and experiment are finally in agreement that hyperconjugation is by far the dominant factor—29 kcal/mol calculated to be from P-stabilization ( ) versus 9 kcal/mol from induction and polarization. The realization of these effects is dramatically revealed in the SnI solvolyses of the conformationally locked cyclohexyl trifluoroacetates (OTFA) (3-5), The relative solvolysis rates at 25 °C for compounds 3-5 are 1, 4 X 10, and 2.4 X 10, respectively. Compound 4 cannot attain the necessary anti-coplanar relationship of the Si-C and C-O bonds, which is present in 5 and required for full hyperconjugative interaction with the cation formed as the C-O bond suffers heterolysis. 

Solvolysis of 15 in 97% trifluoroethanol gave a secondary isotope effect of 1.17, which indicates a vertically stabilized transition state. Thus the highly unsymmetrical dihedral dependence of silicon participation can almost entirely be attributed to the hyperconjugation model with little non-vertical involvement of the silicon nucleophile. 

The /-effect of silicon has so far found little application in organic synthesis, apart from the solvolysis studies described previously. 

A quantitative scale for the structural effect of various silyl groups is established, as shown in entry 57 of Table 1, by the rates of solvolysis of 40 triorganosilyl chlorides in aqueous dioxane under neutral conditions69. The structural effect involves the steric effect and, in some examples, the electronic effect. Because little difference exists in the electronic effect among alkyl groups, their steric effect at silicon follows the order primary < secondary < tertiary substituents. 

TABLE 9. Kinetic 5-silicon, 5-germanium and 5-tin effects in solvolysis... 

A large kinetic silicon /,6-effect was found in the solvolysis of 4-trimethylsilylnorbor-nane triflate. Adcock and coworkers108 found a 1319-fold acceleration for 278 compared with 279. For the tin compound 280 the relative solvolysis rate was even 20486, while a 4-methyl group in 281 causes only a modest acceleration [fc(281)/fc(279) = 7.41]108. 

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