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-1,4diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (NFTh) in acetonitrile at 24ЊC with methanol as nucleophile, giving vicinal fluoromethoxy adducts with a Markovnikov type of regioselectivity, are:
for triphenylethene (2), 2.0×10 Ϫ2 M Ϫ1 s Ϫ1 for tetraphenylethene (3), 5.5×10 Ϫ3 M Ϫ1 s Ϫ1 for acenaphthylene (7) and 4.7×10 Ϫ3 M Ϫ1 s Ϫ 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 Ϫ1 ) and activation enthropies (between Ϫ75 and Ϫ37 J mol Ϫ1 K Ϫ1 ) 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 CH 3 CN/CH 3 OH gave a reaction constant r ϩ of Ϫ0.95 which supports our belief in the mainly non-polar character of the rate-determining transition state.