Substituent Effect of the Pentafluorophenyl Group: Is There a Correlation between X-ray Crystal Structures and Reactivity in Carbocation Formation?

Article abstract of DOI:10.1021/jo00228a004

The reactivity of C6F5CH(OTs)CH3 (7) in various solvents is found to be less than that of Ph(CH(OTs)CH3 by factors of 6E3 to 5E5, but the C6F5 substituent is still much more activating than hydrogen.The reactivity of 7 gives a poor correlation with the Y_OTs measure of solvent ionizing power, with an m value of 0.44, and while the reaction in TFA or HFIP evidently involves initial formation of an ion pair, there is strong nucleophilic assistance to ionization by less ionizing solvents.Crystal structures of C6H5C(CH3)2OPNB (10), C6F5C(CH3)2OPNB (11), and 3,5-(CF3)2C6H3CH(OTs)CH3 (12) have been determined and used to evaluate the proposal Kirby at al. (J.Am.Chem.Soc. 1986, 108, 7067-7073) that more reactive phenylethyl systems have geometries that more closely resemble solvolysis transition states.The geometries of 10 and 11 are similar, and the differences are predicted by the Kirby proposal, but the very short C-OTs bond length in 12 is contrary to expectations, although disorder in the CF3 groups complicates the interpretation.It is concluded that the above simple structure/reactivity correlation should only be applied with due caution.