Protodesilylation

Most of the kinetic studies have been carried out by Eaborn and by Benkeser and their coworkers, and the first study was that of Eaborn656, who measured, spectrophotometrically, the rates of cleavage of 4-methoxyphenyltrimethylsilane by hydrochloric and perchloric acids in methanol, and by hydrochloric acid in aqueous dioxan, both at 47.9 °C (Table 226). The reaction was first-order in silane 

RATE COEFFICIENTS656 FOR REACTION OF 4-MeOC6H4SiMe3 WITH ACID AT 49.7 °C 

Further mechanistic evidence was provided by Benkeser and Krysiak658, who determined the effects of added salts and water on the rates of cleavage of xylyltrimethylsilanes by p-toluenesulphonic acid in acetic acid at 25 °C, the progress of the reaction being followed by dilatometry the first-order rate coefficients are given in Table 227. Clearly the addition of water retards the reaction, as 

The above results do not rule out the possibility that nucleophilic attack on silicon assists the cleavage, a transition state such as (XL), below being involved, 

A further examination of the solvent isotope effect in 65-100 mole % H20-CF3C00H or D20-CF3C00D at 25 °C showed a maximum in the rate with both media at ca. 85 mole % acid (Table 228), as was also observed for 

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Combined yield over 2 steps addition + protodesilylation. 

Initial protonation of iron in protodesilylation of trimethylsilylferrocene was not, however, favoured as a mechanism by Marr and Webster689, who measured rates by the spectroscopic method using hydrochloric acid in 20 vol. % aqueous methanol (Table 235) and found that the rate of desilylation of the ferrocene compound was little more than that for the 4-methoxyphenyl and 2,4-dimethyl compounds. The similarity of the spread of rates in the different media and the similar activation energies and entropies were considered as evidence that the transition states for reaction of all three compounds were similar. The lower activation energy obtained for the 4-methoxyphenyl relative to the ferrocene compound may arise from the different media involved the difference in entropy seems, however, to be rather larger than one might have expected even allowing for the solvent differences. 

The log kTel values for protodegermylation were shown to give an excellent correlation against the log kiel values for protodesilylation, which may be regarded as confirmation that the overlap kinetic technique is satisfactory. 

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