Unexpectedly Small Ortho-Oxygen Substituent Effects on Stabilities of Benzylic Carbocations

Abstract

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 (kO/kCH2 = 1.2) or in the cyclohexyl ring of tetralol (kO/kCH2 = 0.6, kS/kCH2 = 0.3) on rates of carbocation formation. Values of pKR (KR = [ROH][H+]/[R+]) have been obtained by combining rate constants, kH, for the acid-catalyzed ionization of the alcohols with kH2O 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.

The protonation of carbenes: structural effects on α-proton acidity of carbocations

“…The stabilities of carbocations [1][2][3][4] and the protonation of carbenes [5][6][7][8] are interrelated topics which have attracted considerable attention over the past ten years. Normally, the stability of a carbocation in aqueous solution is expressed by equilibrium constants pK R or pK a , where pK R refers to the acid-promoted ionization of the alcohol (ROH + H + = R + + H 2 O) and pK a to loss of a proton from a carbon adjacent to the centre of positive charge.…”

Insights into unusual stability of 5‐membered‐ring endocyclic benzyl carbocations in aqueous solution

“…It is not a special case that the 5‐membered‐ring endocyclic o ‐oxygen stabilizes the benzyl carbocation 3 much better than the acyclic o ‐oxygen stabilizes the benzyl carbocation 1 in aqueous solution. The stability of the 5‐membered‐ring endocyclic benzyl carbocation 5 (p K R + = −11.6) is also around 2 orders of magnitude better than that of the acyclic benzyl carbocation 4 (p K R + = −13.5) in aqueous solution . Hence, the unusual stability of the 5‐membered‐ring endocyclic benzyl carbocations is not a special case but a general phenomenon.…”

“…It was reported that the 5‐membered‐ring endocyclic o ‐oxygen stabilizes the benzyl carbocation 3 (p K R + = −9.3) 2.4 orders of magnitude better than the acyclic o ‐oxygen stabilizes the benzyl carbocation 1 (p K R + = −11.7) in aqueous solution . In the acid‐catalyzed dehydration reactions of benzyl alcohols, the 5‐membered‐ring endocyclic o ‐oxygen accelerates formation of the benzyl carbocation 3 ( k H = 7.4 × 10 −3 M −1 second −1 ) 2.3‐fold more than the acyclic o ‐oxygen accelerates formation of the benzyl carbocation 1 ( k H = 3.2 × 10 −3 M −1 second −1 ) in aqueous solution .…”

“…It was reported that the 5‐membered‐ring endocyclic o ‐oxygen stabilizes the benzyl carbocation 3 (p K R + = −9.3) 2.4 orders of magnitude better than the acyclic o ‐oxygen stabilizes the benzyl carbocation 1 (p K R + = −11.7) in aqueous solution . In the acid‐catalyzed dehydration reactions of benzyl alcohols, the 5‐membered‐ring endocyclic o ‐oxygen accelerates formation of the benzyl carbocation 3 ( k H = 7.4 × 10 −3 M −1 second −1 ) 2.3‐fold more than the acyclic o ‐oxygen accelerates formation of the benzyl carbocation 1 ( k H = 3.2 × 10 −3 M −1 second −1 ) in aqueous solution . In the hydration reactions of benzyl carbocations, the 5‐membered‐ring endocyclic o ‐oxygen assists the hydration reaction of the benzyl carbocation 3 ( k H2O = 1.55 × 10 7 second −1 ) 103‐fold slower than the acyclic o ‐oxygen assists the hydration reaction of the benzyl carbocation 1 ( k H2O = 1.6 × 10 9 second −1 ) in aqueous solution .…”

“…In the acid‐catalyzed dehydration reactions of benzyl alcohols, the 5‐membered‐ring endocyclic o ‐oxygen accelerates formation of the benzyl carbocation 3 ( k H = 7.4 × 10 −3 M −1 second −1 ) 2.3‐fold more than the acyclic o ‐oxygen accelerates formation of the benzyl carbocation 1 ( k H = 3.2 × 10 −3 M −1 second −1 ) in aqueous solution . In the hydration reactions of benzyl carbocations, the 5‐membered‐ring endocyclic o ‐oxygen assists the hydration reaction of the benzyl carbocation 3 ( k H2O = 1.55 × 10 7 second −1 ) 103‐fold slower than the acyclic o ‐oxygen assists the hydration reaction of the benzyl carbocation 1 ( k H2O = 1.6 × 10 9 second −1 ) in aqueous solution . However, both the 5‐membered‐ring endocyclic o ‐oxygen and the acyclic o ‐oxygen are in‐plane with their respective benzyl carbocations 3 and 1 .…”