Effect of reaction conditions on the kinetic and activation parameters for the mild introduction of fluorine into α-substituted styrenes with AccufluorTM NFTh

Abstract

Kinetic studies on fluorination of α-substituted styrenes with 1-fluoro-4-hydroxy-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (Accufluor^TM NFTh) in acetonitrile or in acetonitrile in the presence of methanol or water as nucleophiles were carried out. These reactions exhibited overall second-order kinetics and formed Markovnikov type products. The corresponding second-order rate constants for fluorination of the studied substrates in the presence of methanol at 38.8 °C are: k₂ = 1.1 × 10⁻³ M⁻¹ s⁻¹ for styrene (1a), 3.3 × 10⁻² M⁻¹ s⁻¹ for α-methylstyrene (1b) and 3.4 × 10⁻² M⁻¹ s⁻¹ for 1,1-diphenylethene (1c). The substitution of methanol as nucleophile by water had very little effect on the rate of the process. Solvent polarity variation (Grunwald–Winstein Y_benzyl) showed a similar small effect, too, indicating negligible change in the polarity of the rate-determining transition state in comparison with the reactants. Activation enthalpies (between 62 and 80 kJ mol⁻¹) and activation entropies (between −74 and −32 J mol⁻¹ K⁻¹) were determined for fluorination of α-substituted styrenes with NFTh in acetonitrile in the presence of methanol and water as nucleophiles. Hammett correlation analysis of the reaction of substituted styrenes with NFTh in MeCN, MeCN–H₂O and MeCN–MeOH gave reaction constants ρ⁺ of −1.48, −1.52 and −1.80, respectively, which support our belief in the mainly non-polar character of the rate-determining transition state of the studied reactions.