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) (AccufluorTM 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 Ybenzyl) 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.