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

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 degrees C. Analysis of the reaction products shows t hat soft polarizable nucleophiles (I-, SCN-, Br-) react at fluorine, wherea s hard nucleophiles (oxygen and nitrogen nucleophiles) react at sulfur. The ambident behaviour of this electrophile seems to be related to the relativ e contribution of electrostatic and orbital interactions in reaching the tr ansition state. The preferential reaction of soft nucleophiles at fluorine and the correlation of k(Nu) values with the one-electron oxidation potenti als of the nucleophiles in water suggest that nucleophilic reactivity at fl uorine is largely determined by the ease of one-electron transfer from the nucleophile to the electrophile. Nucleophilic addition to fluorine is far m ore sensitive to the nature of the attacking nucleophile than the correspon ding 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-d initrophenyl acetate reveals a similar reactivity pattern for sulfonyl sulf ur and carbonyl carbon as electrophilic centres.