Doubly Destabilized Carbocations. Strong Aryl Delocalization and the Attenuation of Rate Decelerating Effects of CF3 and CN Groups

Article abstract of DOI:10.1021/ja00272a050

Solvolysis rates of ArC(CF3)2OTs in CF3CO2H give an excellent correlation log k = -10.7?⁺-8.69, and a point for the p-Me2N compound derived from the rate of the p-nitrobenzoate also fits this correlation, giving a span of 1E19 in reactivity.Limiting values of ρ⁺ of -10 to -12 for generation of benzylic cations in the gas phase and solution have been reached.Rate ratios k(ArCH(OTs)CF3)/k(ArC(CF3)2OTs) increase from 2.4 to 54 with decreasing aryl donor power; these k(H)/k(CF3) ratios are much less than typical values of 1E6 in more stabilized systems.Displacement of the p-MeO group occurs in solvolysis of p-MeOC6H4C(CF3)2OTs (9) in EtOH and CD3OH.Trifluoroacetolysis of ArC(OTs)(CF3)CN (Ar = p-Tol (13) and Ph) gives ρ⁺ = -12.1, and for 13 in different solvents the reactivity is greater than p-TolCH(OTs)CF3 by factors of 8.6 to 175.This is the first observation of k(CN)/k(H) ratios greater than 1.0.All these results, plus the rather modest dependence of the rates on solvent polarity (m = 0.76 for 9 and 0.66 for 13), are interpreted in terms of carbocationic transition states and intermediates in which the charge is largely delocalized onto the aryl group.