Aromatic reactivity. Part LVII. Detritiation of o- and p-Me3Si[CH2]nC6H4·3H in trifluoroacetic acid

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Abstract
The following rate factors f(the specific rates relative to that for [3H]benzene) have been obtained for detritiation in trifluoroacetic acid at 70° of o- and p-Me3Si[CH2]n·C6H4·3H with n= 1–4 : (n= 1) 1, o- 9300, p- 82,000; 2, o- 450, p- 810; 3, o- 270, p- 580; 4, o- 270, p- 690. Some cleavage of the C–Si bonds also occurs, but it is concluded that this does not significantly influence the detritiation rates. The values of the (log f0) : (log fp) ratio all fall within the range 0·865 ± 0·05 previously observed to apply to a wide range of monosubstituted benzenes in electrophilic aromatic substitutions. The rate factors correlate quantitatively with the corresponding factors for cleavage of the compounds o- and p-Me3Si[CH2]n·C6H4·SiMe3 by aqueous methanolic perchloric acid except for the o-isomers with n= 1, and it is suggested that an unusual steric effect reduces the rate of the protodesilylation in this case. The significance of the rate factor for p-Me3SiCH2·C6H4·3H for understanding the ease of and cleavage of ArSiMe3 compounds is discussed, with reference to the importance of hyperconjugative electron-release from Me3Si–CH2bonds. It is argued that, contrary to a recent conclusion, such hyperconjugation can be significant in ground states as well as carbonium ions and transition states.

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