Unexpectedly small ortho-oxygen substituent effects on Stabilities of benzylic carbocations

Equilibrium constants are reported for the ionization of benzylic alcohols to carbocations stabilized by cyclic or acyclic o-alkyl or o-oxygen substituents. The measurements were stimulated by the observation of small or inverse effects of replacement of an o-CH2 group by O or S in the cyclopentyl ring of indanol (k(O)/k(CH2) = 1.2) or in the cyclohexyl ring of tetralol (k(O)/k(CH2) = 0.6, k(S)/k(CH2) = 0.3) on rates of carbocation formation. Values of pK(R) (K-R = [ROH][H+]/[R+]) have been obtained by combining rate constants, k(H), for the acid-catalyzed ionization of the alcohols with k(H2O) for attack of water on the carbocation measured by the azide clock method. For carbocations derived from the following alcohols, values of pKR are as indicated: 1-indanol, -11.7; 2,3-dihydro-3-hydroxybenzofuran (benzofuran hydrate), -9.3; 1-tetralol, -12.2; 4-chromanol, -12.0; 4-thiochromanol, -12.3; o-methyl-1-phenylethanol, -13.8; o-methoxy-1-phenylethanol, -11.7. The measurements show that, in contrast to its small kinetic effect, the equilibrium effect of replacing the o-CH2 group by O in the cyclopentyl ring of indanol is 250-fold, whereas the effect of the same replacement in the cyclohexyl ring of tetralol is only 1.6. It is concluded (a) that the efficiency of conjugation of annular o-oxygen substituents to a benzylic carbocation center is sensitive to conformational restrictions arising from ring strain and (b) that, in the case of indanol, the kinetic effect of the same oxygen atom is subject to an imbalance of favorable resonance and unfavorable inductive effects at the transition state.