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

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 krel., 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 krel., 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 krel., 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.