Surprisingly small effect of an .alpha.-trifluromethyl-for-.alpha.-methyl substitution on 1-(4-methoxyphenyl)ethyl cation reactivity

Dynamics for reactions of ion pairs in aqueous solution: reactivity of tosylate anion ion paired with the highly destabilized 1‐(4‐methylphenyl)‐2,2,2‐trifluoroethyl carbocation

“…[14–16] This shows that the increase in the reactivity of 4 + compared to 1 + toward nucleophile addition caused by the large effect of the α-CF 3 substituent on the thermodynamic reaction driving force is essentially completely offset by a decrease in reactivity of 4 + due to an increase in the Marcus intrinsic barrier for carbocation-nucleophile addition. [23–25]…”

“…[14–16] We were interested in examining the effect of this strongly electron-withdrawing α-CF 3 substituent on the partitioning of ion pair intermediates of solvolysis of 1-(4-methylphenyl)ethyl derivatives.…”

“…[5,6] By comparison, carbocation-anion pairs to the much more unstable 1-(4-methylphenyl)-2,2,2-trifluorethyl carbocation can only be generated using a more weakly basic leaving group anion, such as tosylate anion. [12,14,15] There is little or no data for addition of tosylate anion to carbocations in aqueous solution, so that it is not clear whether addition of this weakly basic anion competes effectively with addition of the relatively nucleophilic solvent water. We were therefore interested in determining whether the ion pair intermediate of the reaction of 1-(4-methylphenyl)-2,2,2-trifluorethyl tosylate would undergo internal return of the weakly nucleophilic sulfonate anion in the nucleophilic solvent water.…”

Substituent effects on the formation and nucleophile selectivity of ring‐substituted phenonium ions in aqueous solution

“…Surprisingly, the lifetimes of a-substituted 4-methoxybenzyl carbocations are not strongly affected by a strongly electronwithdrawing substituent at the a-carbon, such as a-CF 3 and a-COOEt. [16][17][18][19][20] This is because the increase in k s due to the strong inductive substituent effect is offset by a decrease in k s due to an increasing intrinsic reaction barrier that arises from the tendency of these strongly electron-withdrawing groups to enhance resonance delocalization of charge onto the 4-methoxyphenyl ring substituent. [21,22] Bridging phenonium ions were at one time the subject of intense interest in studies to characterize internal [anchimeric] nucleophilic assistance to solvolysis, [23] and the chemical community continues to study the generation and reaction of these electrophiles.…”

Formation and mechanism for reactions of ring‐substituted phenonium ions in aqueous solution

“…A sharp decrease in k az / k s , with increasing electrophile reactivity (Fig. ) is observed for the partitioning of ring‐substituted 1‐phenylethyl, 1‐phenyl‐2,2,2‐trifluoroethyl, and cumyl carbocations . In each case, the fast addition of azide anion serves as a “clock” for the slower reaction of solvent: the rate constant k s for solvent addition may be estimated from the value of k az / k s using k az = k d ≈ 5 × 10 9 M −1 s −1 (Fig.…”