The effect of an electron-donating β-substituent on the configurational stability and reactivity of vinyl carbanions

The effect of an electron donating β-substituent A (A = N(CH ₂)₂O(CH₂)₂, NCH₂CH ₂CH₂CH₂, OMe, SMe, or OPh) on the relative configurational stabilities and nucleophilic reactivities of the dimethyl esters of α-lithiated fumaric acid (17)-Li and maleic acid (18)-Li was studied in THF, by comparing their reaction products with electrophiles (MeOH, Ph ₂CO, aldehydes). It was confirmed that when located at a trans position to the vinyl carbanion, the substituent A had two inter-related effects: decreasing the configurational stability of the vinyl carbanion and increasing its nucleophilic reactivity. The rapidly-established (17)-Li ⇌ (18)-Li equilibrium was completely on the (18)-Li side with A = OR, SR, NR ₂. The products formed were those derived from (18)-Li only (with MeOH) or from both (17)-Li and (18)-Li depending on both the relative nucleophilic reactivities of the intermediates and on the electrophilicity of the electrophile. The products derived from (18)-Li predominated with the more reactive electrophiles.