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

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 f_p) 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.