The Effect of Reaction Conditions on Activation Parameters for the Mild Introduction of Fluorine into Phenyl Substituted Alkenes with SelectfluorTM F-TEDA-BF4

Maja Papež-Iskraa Marko Zupan,a,b and Stojan Stavberb*
Department of Chemistry of the Faculty for Chemistry and Chemical Technology of the University of Ljubljana, Aškerčeva 5, 1000 Ljubljana, Slovenia, E-mail: stojan.stavber@ijs.sib “Jožef Stefan” Institute, Jamova 39, 1000 Ljubljana, Slovenia,

1-Chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis-(tetrafluoroborate) (SelectfluorTM F-TEDA-BF4) reacted with phenyl-substituted alkenes in acetonitrile and Ritter-type fluoroamidation was observed, while in the presence of a nucleophile (H2O, MeOH), the formation of vicinal fluorohydroxy or fluoromethoxy alkanes was the predominant process. Mild fluorine introduction obeys a simple rate equation: v= d[F–TEDA-BF4]/dt= k2 [F–TEDA-BF4] [alkene] and the following second order rate constants were determined in acetonitrile at 22 °C: k2=9.0´10–3M–1s–1 for trans-stilbene (1a), 2.0´10–3M–1s–1 for cis-stilbene (1b), 8.4´10–3M–1s–1 for 1,1-diphenylethene (1c) and 0.29´10–3M–1s–1 for styrene (1d) (calculated from k2=3.9´10–3M–1s–1 at 50 °C), while the presence of a nucleophile had no a significant effect on the second order rate constants. Activation enthalpies were between 72.7 kJmol–1 and 65.5 kJmol–1 in acetonitrile, while the presence of a nucleophile lowered the activation enthalpies for 1a, 1b, 1c, whereas higher values were observed for styrene. Activation entropies were between –37.9 e.u. and –68.7 e.u. in acetonitrile, the presence of nucleophile lowering the activation entropy for 1a, 1b, 1c, while the opposite effect was observed for styrene. Solvent polarity variation (Grunwald-Winstein Ybenzyl) had a very small effect on the rate of fluorination of styrene and 1,1-diphenylethene, indicating
a small change in the polarity of the rate determining transition state in comparison with the reactants. 

Key words: SelectfluorTM F-TEDA-BF4, alkenes, fluorination, kinetics, activation parameters