CURVE CROSSING ANALYSIS OF LFER DATA IN MICHAEL ADDITION-REACTIONS

Rate constants for the Michael addition of CN- in water, aqueous sulfolane, and DMSO, and MeO- in MeOH to eight mono- and disubstituted 1,1-diaryl-2,2-dinitroethylenes were measured. The p values are in the range 0.7 (CN-/H2O) - 1.6 (MeO-/MeOH). For each of the four systems a correlation was found between log k and the C-13 chemical shift of the carbon that undergoes the nucleophilic attack. However, the quality of the correlation as well as the slope decreases with decreasing p values. Analysis of the electronic configuration of the transition state using the curve crossing model shows that the traditional interpretation of the location of the transition state should be reversed. This analysis provides a rationale for the observed dependence of the chemical shift - rate correlation on the p value.