Rapid Nucleophilic Substitutions on Cyclopentadienyl Iodide and Bromide

Cyclopentadienyl iodide reacts with tetrabutylammonium bromide to afford cyclopentadienyl bromide ca. 10 times as rapidly as cyclopentyl iodide reacts under the same conditions. Cyclopentadienyl bromide reacts with tetrabutylammonium iodide ca. 10³ times as rapidly as does cyclopentyl bromide. to check for allylic substitution, 5-iodocyclopentadiene was metallated with potassium hexamethyldisilazide, and the resulting anion was quenched with D₂O/CF₃CO₂D to afford 5-iodo-5-deuteriocyclopentadiene. This could be trapped when kept very cold, but on warming to room temperature it underwent iodine migration that rapidly scrambled the deuterium position. the high reactivity of the cyclopentadienyl halides in substitution under these conditions is in sharp contrast to their low reactivity under solvolytic S_N1 conditions. the possible mechanisms and reasons involved are discussed.