Kinetic investigations of the reactions of hindered­phenols with N-fluoro-1,4-diazoniabicyclo[2.2.2]octane­salt analogues

The kinetics of reactions of hindered phenols (2,4,6‐trimethylphenol, 2,6‐di‐tert‐butyl‐4‐methylphenol and 2,4,6‐tri‐tert‐butylphenol) with 1‐fluoro‐4‐chloromethyl‐1,4‐diazoniabicyclo [2.2.2]octane bis(tetrafluoroborate) (Selectfluor™, F‐TEDA‐BF4) in acetonitrile in the absence or presence of various sources of external nucleophile (alcohols, water, trifluoroacetic acid) were studied. The reactions exhibited overall second‐order kinetics, whereas products formation was strongly dependent on reaction conditions. Fluorination took place in neat MeCN; in the presence of water or alcohols, para quinols or para quinol ethers were formed exclusively; under acidic conditions (TFA), Ritter‐type amidation was found to be the predominant process. Values for second‐order rate constants k2 were found to be dependent on the structure of the target phenol and decreased with bulkiness of the substrate. Methanol slightly increased the rate, whereas water decreased it considerably. Activation enthalpies (between 72 and 78 kJ mol−1) and activation entropies (between −5 and −42 J mol−1 K−1) were obtained. Activation entropies were found to be higher for less‐hindered substrates, indicating that the rate‐determining steps were mainly regulated by steric factors. Grunwald–Winstein correlation of log k2 with solvent ionizing power Y revealed a relatively small correlation factor m. An electron‐transfer‐type process following the rate‐determining formation of substrate–reagent complex was postulated as the key step of the reaction pathway. Copyright © 2001 John Wiley & Sons, Ltd.