Elimination Reactions Promoted by Fluoride Ion in Acetonitrile. Elimination Reaction from 2-Arylethyl Derivatives

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Abstract
Rates of bimolecular elimination have been determined for the reaction of 2-arylethyl bromides, chlorides, and tosylates with anhydrous tetraethylammonium fluoride in acetonitrile. The rates of elimination reactions in this new base system were much faster than those in conventional base systems (alkoxide-alcohol). The Hammett ρ values were found to be ρ=2.033 (X=Cl) at 25.0°C and ρ=1.879 (X=Br) at 5.0°C for p-Y-C6H4CH2CH2X. Kinetic deuterium isotope effects were determined using 2-phenylethyl-2,2-d2 bromide and chloride to be kH/kD=3.99 (X=Cl) and 5.03 (X=Br). These data and the low kOTs/kBr ratio suggest that fluoride-ionpromoted elimination reaction proceeds through a very tight transition state where β-proton is less than half transferred to the base.