Nucleophilic displacement reactions in ionic liquids: substrate and solvent effect in the reaction of NaN(3) and KCN with alkyl halides and tosylates.

Abstract

Room-temperature ionic liquids have been used as environmentally benign solvents for the preparation of primary and secondary alkyl azides and nitriles under solid-RTIL phase-transfer conditions. The reaction of primary, secondary, and tertiary halides or tosylates with KCN and NaN(3) has been investigated in three ionic liquids ([bmim][PF(6)], [bmim][N(Tf)(2)], and [hpyr] [N(Tf)(2)]). The observed nucleofugacity scales for the reaction of NaN(3) are similar to those reported for the same process in cyclohexane, indicating that in these solvents it is possible to evidence the intrinsic ability to depart of leaving groups. Changes in the nature of the IL cation or anion determine significant modifications in reactivity of the investigated substrates. Reactivity has been interpreted considering a gradual shift of the mechanism from concerted S(N)2 (primary substrates) to stepwise S(N)1 (tertiary substrate, 3), through the nucleophilically assisted formation of an ion pair intermediate, in the case of 2d.