The effects of ortho-substituents on the stability of the phenyl anion. The base cleavage of aryltrimethylsilanes

Abstract

The rates of cleavage of some XC₆H₄SiMe₃ compounds by KOH in 9 : 1 v v Me₂SO–H₂0 at 40.0 °C have been measured : the values of k_rel., the rate relative to that for X = H, are (X), o-NO₂, 420 000; p-NO₂, 10 300; m-NO₂, 9 700; o-F, 4 000; o-Cl, 3 960; m-Cl, 375; p-Cl, 38; o-NMe₂, 14; o-OMe, 6.3; o-Me, 0.60. The values of tog k_rel., correlate satisfactorily with the calculated energy for the conversion XC₆H₅→ XC₆H₄-, except for X =o-NMe₂, for which steric effects may be significant. The previously reported log k_rel., values for hydrogen exchange in XC₆H₄²H in KNH₂–NH₃ also correlate satisfactorily with the deprotonation energies, confirming that rate-determining formation of the anions XC₆H₄- is involved in this reaction, as in the desilylation.