The mechanism of base-promoted HF elimination from 4-fluoro-4-(4-nitrophenyl)butan-2-one is E1cB. Evidence from double isotopic fractionation experiments

Leaving-group fluorine and secondary deuterium multiple kinetic isotope effects (KIEs) have been determined for the base-promoted HF elimination from the 4-fluoro-4-(4'-nitrophenyl)-(1,1,1,3,3-H-2(5))butan-2-one. The fluorine KIE was determined by using the accelerator-produced short-lived radionuclide F-18 in combination with the naturally abundant F-19. The F-19 substrate was labeled with C-14 in a remote position to enable radioactivity measurements of both substrates. The size of the determined fluorine KIE is 1.0009 +/- 0.0010 when acetate is used as base. The secondary deuterium KIEs are 1.009 +/- 0.017, 1.000 +/- 0.018, and 1.010 +/- 0.023 for formate, acetate, and imidazole, respectively. The magnitudes of these KIEs are sigificantly smaller compared to the corresponding KIEs that we recently reported for the protic substrate. This new data clearly demonstrates that the elimination proceeds via an E1cB mechanism.