Nucleophilic reactivity towards electrophilic fluorinating agents: reaction with N-fluorobenzenesulfonimide ((PhSO2)2NF)

Abstract

Second-order rate constants for the reaction of N-fluorobenzenesulfonimide (FBS) with nucleophilic reagents, k_Nu (M⁻¹ s⁻¹), have been measured in aqueous solution at 25 °C. Analysis of the reaction products shows that soft polarizable nucleophiles (I⁻, SCN⁻, Br⁻) react at fluorine, whereas hard nucleophiles (oxygen and nitrogen nucleophiles) react at sulfur. The ambident behaviour of this electrophile seems to be related to the relative contribution of electrostatic and orbital interactions in reaching the transition state. The preferential reaction of soft nucleophiles at fluorine and the correlation of k_Nu values with the one-electron oxidation potentials of the nucleophiles in water suggest that nucleophilic reactivity at fluorine is largely determined by the ease of one-electron transfer from the nucleophile to the electrophile. Nucleophilic addition to fluorine is far more sensitive to the nature of the attacking nucleophile than the corresponding reactions at both saturated (n scale) and unsaturated carbon (N₊ scale). Comparison of the rate constants for the reaction of nucleophiles at the sulfonyl group with those for reaction of the same nucleophiles with 2,4-dinitrophenyl acetate reveals a similar reactivity pattern for sulfonyl sulfur and carbonyl carbon as electrophilic centres.