3-(trifluoromethyl)indenyl cation: Ion pair return in the formation of an antiaromatic and electron-deficient doubly destabilized carbocation

The solvolysis of 3-(trifluoromethyl)-3-indenyl tosylate (15) occurs with extensive isomerization to 1-(trifluoromethyl)-3-indenyl tosylate (16), which reads in a slower process to give the substitution product 17. Kinetic analysis of a model involving an intermediate allyl cation/tosylate ion pair PS gave a partitioning ratio in CD3CO2D at 99.6 degrees C for 18 of 7.7 for return with allylic rearrangement compared with solvent capture. Studies of 15 with specific O-18 labeling show no scrambling in. recovered 15 and partial scrambling in rearrangement to 16. The m value measuring the dependence of the reactivity of 15 on the solvent-ionizing parameter Y-OTs is 0.78, which is significantly less than that of 1.23 for the analogous 9-(trifluoromethyl)-9-fluorenyl tosylate a, Normal salt effects in CF3CO2H predominate far 15, and the special salt effect involves no more than 14% capture of solvent-separated ion pairs by 0.551 M KO2CCF3. The substrate 15 has a net diminution in reactivity of more than 10(9) relative to the secondary indanyl tosylate 22, with factors of 10(6) and 10(3) attributable to antiaromaticity and to the electron-withdrawing CF3 group, respectively. The solvolysis of 15 is proposed to occur by formation of an ion pair with significant nucleophilic salvation at the relatively unhindered allylic carbon, but internal return occurs in preference to solvent or salt capture. Solvolysis of the rearranged tosylate 16 occurs with a strong rate retardation by the gamma-CF3 group, a large extent of internal return, and with a nor;mal salt effect.