Effect of Reaction Conditions on the Kinetic and Activation Parameters for the Mild Introduction of Fluorine into Phenyl-Substituted Alkenes with Accufluor™ NFTh

Evidence for a bimolecular process in mild fluorine transfer from the F–L type of reagent (electrophilic fluorinating reagent) is presented. The corresponding second-order rate constants for the fluorination of phenyl-substituted alkenes with 1-fluoro-4-hydroxy-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (NFTh) in acetonitrile at 24°C with methanol as nucleophile, giving vicinal fluoro-methoxy adducts with a Markovnikov type of regioselectivity, are: k2=9.1×10−3 M−1 s−1 for 1,1-diphenylethene (1), 2.7×10−2 M−1 s−1 for triphenylethene (2), 2.0×10−2 M−1 s−1 for tetraphenylethene (3), 5.5×10−3 M−1 s−1 for acenaphthylene (7) and 4.7×10−3 M−1 s− 1 for 9-benzylidenefluorene (10a). The substitution of methanol as nucleophile for water had a negligible effect on the rate of the process, as well as the change of solvent polarity, indicating little change in the polarity of the rate-determining transition state in comparison with the reactants. Activation enthalpies (between 62 and 74 kJ mol−1) and activation enthropies (between −75 and −37 J mol−1 K−1) were determined for fluorination of 1 and 7 with NFTh in acetonitrile in the presence of methanol and water as nucleophiles. Hammett correlation analysis of the reaction of substituted 9-benzylidenefluorenes with NFTh in CH3CN/CH3OH gave a reaction constant ρ+ of −0.95 which supports our belief in the mainly non-polar character of the rate-determining transition state.