Radical-Anionic Nature of the Transition State in the Michael Addition Reaction

The kinetics of the addition of CN⁻ to a series of para mono-and disubstituted 1,1-diaryl-2-nitroethylene was studied in water and DMSO. log k correlates with σ⁰ better than with σ with a ρ value of 0.24 in water and 1.16 in DMSO. The points for the mono-and dimethoxy derivatives (1 and 3) in water deviate positively from the Hammett type line. When a Yukawa-Tsuno type equation is used, a linear plot that includes the p-MeO derivatives is obtained with R = -0.5. Contrary to expectations, the ρ value in DMSO is ca. 5-fold larger than that in water and the data points for the p-MeO substituents do not deviate from the Hammett type correlation. In the reaction of the p-dinitro derivative 9 in DMSO, 10% of a product rising from nucleophilic attack at the other terminus of the double bond (α to the nitro group) was observed. These unusual results are rationalized by using current theories, which assign the transition state a significant contribution of charge-transfer configurations (configurations B and C). The relative and absolute contribution of each of the configurations to the transition state is responsible for the observed phenomena.