9-Fluorenyl cation carbocation reactivity

Both the 9-methyl and 9-trifluoromethyl-9-fluorenyl cations (7 and 8) have been generated by laser flash photolysis of corresponding alcohol precursors. The product ions were then studied by time-resolved spectroscopy. Consistent with previous studies related to carbocation-bearing electron-withdrawing groups, ion (8) exhibits a significant bathochromic shift in the UV absorption compared to (7). The ions were quenched with methanol and reaction rates showed cation (8) to be a more reactive species. 

The photochemical generation of arylmethyl cations from arylmethanols has been the topic of several papers this year. For mono-aryImethanols in which the aromatic group is phenyl, naphthyl, pyrenyl or phenanthryl, Wan has reported that ultra-violet light irradiation in aqueous alcoholic sulphuric acid solution yields the corresponding ether,presumably by way of an intermediate arylmethyl carbocation. Photolysis of (414) in aqueous alcohol to give (415) is found to proceed even in the absence of acid, while the non-cyclic analogue (416) reacts much less efficiently, even at low values of pH. This reactivity difference has been observed previously for fluorenol and benzhydrol and is attributed to extra stabilisation of the intermediate carbocation in the fluorenyl system. 

The solvolytic reactivity of the 9-CFs substituted fluorenyl tosylate 46 was strongly depressed compared to R = H, and a rate factor of 10 due to antiaromatic destabilization was estimated. Solvolytic studies with formation of 9-fluorenyl carbocations with COzR, - CONMez, and CR=NOCH3 substituents have also been reported. On the basis of the calculated geometry it was suggested by Creaiy et al. that such cations avoid antiaromatic structures with cyclopentadienyl cation moieties and resemble bis(dienyl) cations 28a. 

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