Flash photolysis study of a Friedel–Crafts alkylation. Reaction of the photogenerated 9-fluorenyl cation with aromatic compounds

Abstract

A combination of flash photolysis and product analysis is employed to investigate the reaction of aromatic compounds (ArH) with the 9-fluorenyl cation (Fl⁺) photogenerated from 9-fluorenol in 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP). The availability of the photochemical route to Fl⁺ means that the reaction of a benzylic-type cation with ArH can be directly followed by flash photolysis. An additional feature with electron-rich ArH is that the cyclohexadienyl cation is observed to grow as Fl⁺ decays. Thus both cationic intermediates of a Friedel–Crafts alkylation are observed in the same experiment. The formation of the cyclohexadienyl cation is demonstrated to be reversible, or at least quasi-reversible, with the kinetic analysis furnishing absolute rate constants for the formation of this cation, as well as for its loss of H⁺ and Fl⁺. Values of k_H:k_D for benzene: [²H₆]benzene and toluene: [²H₈]toluene are ∼ 1.5 and demonstrate that Fl⁺ addition is at least partly reversible with these compounds as well. The Hammett ρ⁺ value obtained for a series of the less electron-rich ArH is –8, indicative of a transition state with considerable cyclohexadienyl cation character. Anisole shows a negative deviation from the Hammett correlation line, explained by the addition of Fl⁺ to ArH becoming encounter-controlled. This behaviour is dramatically illustrated in a comparison of data for Fl⁺ and Br₂. For the less electron-rich ArH, rate constants for the two electrophiles are parallel. However, from m-xylene through pentamethylbenzene, the rate with Fl⁺ is unchanged, while the rate with Br₂ increases over 1000-fold. The concept of encounter control with Fl⁺ is strongly supported by the absolute rate constants, which for the the electron-rich ArH are all in the range 1–2 × 10⁹ dm³ mol^–1 s^–1, a magnitude typical of diffusion-controlled reactions. The electron-rich ArH also show no intermolecular selectivity since their reactions are encounter-controlled, but have a high intramolecular selectivity. It is suggested that a factor influencing the latter is the reversibility of formation of the cyclohexadienyl cation from the encounter complex.