Journal of the American Chemical Society p. 5626 - 5634 (1992)
Update date:2022-08-04
Topics:
Richard, John P.
Amyes, Tina L.
Vontor, Tomas
The effect of changing carbocation reactivity on nucleophile selectivity has been determined for the reactions of ring-substituted 1-phenyl-2,2,2-trifluoroethyl carbocations, XArCH(CF3)+, with amines, alcohols, and carboxylate ions. Rate constants, Ks, for the capture of XArCH(CF3)+ by 50/50 (v/v) trifluoroethanol/water range from 1 × 1010 s-1 for 4-MeArCH(CF3)+ to ≤200 s-1 for 4-Me2NArCH(CF3)+. βnuc = 0.29 was determined for the reaction of alkylamines with 4-Me2NArCH(CF3)+. βnuc for reaction of RCO2- decreases from 0.35 for 4-Me2NArCH(CF3)+ to 0.05 for 4-MeOArCH(CF3)+. This decrease is due, at least in part, to a Hammond effect on the location of the reaction transition state along the reaction coordinate. βnuc for reaction of alcohols decreases from 0.48 for 4-Me2NArCH(CF3)+ to 0.09 for 4-MeArCH(CF3)+. The plot of log (KEtOH/KTFE for capture of XArCH(CF3)+ by ethanol and trifluoroethanol against log ks has a shallow negative slope for the more stable XArCH(CF3)+, which steepens with destabilization of the carbocation. This change in slope is due, in part, to a change in the magnitude of the Hammond effect, which corresponds to a third-derivative structure-reactivity effect, p*yyy = ?pyy-?σ > 0. There is considerable overlap between the reactivities of the most unstable triarylmethyl carbocations and the most stable XArCH(CF3)+, and there are also marked similarities in the reactivity-selectivity behavior of these species in the region of this overlap. Models are considered to explain the spectrum of reactivity-selectivity behavior that is observed on moving from very unreactive to very reactive carbocations.
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