Kinetics and mechanism of the reaction of 9-arylfuoren-9-yl cations with polymethylbenzenes in trifluoroacetic acid solution

Abstract

9-Arylfluoren-9-yl cations, ArFl⁺, generated from the alcohol in CF₃CO₂H solution, are reduced to ArFIH by the polymethylated aromatic hydrocarbons (Ar¹CH₃) hexamethylbenzene, pentamethylbenzene, 1,2,3,4-, 1,2,3,5-, and 1,2,4,5-tetramethylbenzene, 1,3,5-trimethylbenzene, and 9,10-dimethylanthracene. Simple hydride-ion transfer, ArFI⁺+ Ar¹CH₃→ ArFIH + Ar¹CH₂⁺, is thermodynamically unfavourable, but analysis indicates that Ar¹CH₂⁺ reacts further, to give stable products. Rates of disappearance of ArFI⁺(=k₂[ArFI⁺][Ar¹CH₃]) measured spectrophotometrically show wide variation depending on the nature of Ar and Ar¹, and kinetic isotope effects in the range 2.8–4.2 at 30°C resulting from experiments using perdeuteriohexamethylbenzene. The rate variation with changing Ar¹CH₃ correlates best with the ability of Ar¹CH₃to form charge-transfer complexes with fluoranil or to undergo one-electron oxidation. It is concluded that in the transition state for hydride transfer, electron transfer may have progressed further than nuclear motion of the migrating hydrogen. The possibility of intermediate formation of the pair ArFI˙–Ar¹CH₃⁺˙ is also considered. 4-Substituents in the phenyl group of PhFI⁺ have a remarkably constant effect on reaction rates with a range of Ar¹CH₃ of varying reactivity.