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 reacts in a slower process to give the substitution product 17. Kinetic analysis of a model involving an intermediate allyl cation/tosylate ion pair 18 gave a partitioning ratio in CD(3)CO(2)D at 99.6 degrees C for 18 of 7.7 for return with allylic rearrangement compared with solvent capture. Studies of 15 with specific (18)O 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 7. Normal salt effects in CF(3)CO(2)H predominate for 15, and the special salt effect involves no more than 14% capture of solvent-separated ion pairs by 0.551 M KO(2)CCF(3). 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 CF(3) group, respectively. The solvolysis of 15 is proposed to occur by formation of an ion pair with significant nucleophilic solvation 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-CF(3) group, a large extent of internal return, and with a normal salt effect.