Halodehydroxylation of alcohols to yield benzylic and alkyl halides in ionic liquids

Background: Alcohols are widely used, and sometimes renewable, reagents but the hydroxyl moiety is a relatively poor leaving group under mild conditions. Direct nucleophilic substitution of alcohols is a desirable reaction for synthetic and process chemists. Results: Synthesis of twelve alkyl and benzyl halides was achieved in [Bmim]PF6 (Bmim = 1-butyl-3 methylimidazolium) from their parent alcohols using ammonium halides as the halogenating agents. Trends in reactivity based on the alcohol and halide were discovered. Mechanistic evidence suggests that the reaction proceeds via SN2 substitution of the hydroxyl group, which is activated via hydrogen-bonding with the acidic proton of the imidazolium cation. Also, for benzyl substrates, equilibria involving formation of dibenzyl ether complicate the reactions and reduce optimum yields. Conclusions: Ammonium halides are useful, solid and relatively safe reagents for the conversion of some primary alcohols to organohalides in ionic liquids (yields up to 81 %). Indanol under the same conditions yields biindenylidene (GC yield 63 %)