Electron Transfer in Reactions of Ketones with Organolithium Reagents. A Carbon-14 Kinetic Isotope Effect Probe

Article abstract of DOI:10.1021/ja00248a027

Kinetic isotope effects have been determined for reactions of ketones labeled with carbon-14 at the carbonyl carbon with MeLi and Me2CuLi in diethyl ether at 0 deg C.Observed isotope effects were as follows: (C6H5)2C=O + MeLi, ¹²k/¹⁴k = 1.000 +/- 0.002; (C6H5)2C=O + Me2CuLi, 1.029 +/- 0.005; 2,4,6-Me3C6H2COC6H5 + MeLi, 1.023 +/- 0.004.The relative reactivities of ortho-, meta-, and para-substituted benzophenones with these reagents were also determined by the competition experiments.These results are consistent with an electron transfer step which is followed by a carbon-carbon bond-formimg step that is or is not rate determining depending on the structure of ketones and reagents.The reaction of benzophenone with MeLi proceeds via rate-determining electron transfer; the change in nucleophile from MeLi to Me2CuLi shifts the rate-determining step from electron-transfer to recombination; the change in ketone from benzophenone to 2,4,6-trimethylbenzophenone also shifts the rate-determining step from electron transfer to recombination because the latter step becomes slower for the more hindered ketone.The extent of the geometrical change of the substrate at the electron-transfer transition state of the reaction of benzophenone with MeLi was estimated to be small on the basis of the magnitude of the KIE and teh ρ value of the Hammett correlation.