Substituent Effects on Cyclopropyl Anion Structure and Stability

Substituent Effects on Cyclopropyl Anion Structure and Stability 

In view of the enhanced acidity of protons attached to a cyclopropane ring, it might have been expected that conjugating groups Z would increase the acidity of the methine proton in 10 at least to the same extent as for the similar proton in the isopropyl model system (11). The discovery that nitrocyclopropanes (e.g. lOd) are generally weaker Br nsted acids 

Most of the preceding conclusions are based on kinetic acidity determinations and the assumption that relative rates reflect relative carbanion stabilities. This assumption has been appropriately challenged however, the limited equilibrium acidity data that are available are in complete accord with the above summary (see Table 2). 

Method A Br-Li exchange, n-BuLi/Et20 followed by CO2 quench. Method B NaNH2 promoted Haller-Bauer cleavage of phenyl ketone. Method C LDA/THF, — 72°C, quench with Mel or MeOD. Method D H-D exchange vs. racemization using NaOMe. Method E LDA/Et20 at 25°C. Method F DNMR. 

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C. Substituent Effects on Cyclopropyl Anion Structure and Stability... 

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