Cyclopropyl carbanions derived from esters of 2-phenylcyclopropylcarboxylic acids: Configurational stability and reaction with electrophiles

The base-catalyzed cis-trans isomerization of cis and trans-alkyl 2-phenylcyclopropylcarboxylates was studied in several aprotic solvents. It was found that contact ion-pain of the derived carbanion lithium salts were configurationally more stable than the solvent-separated ones. Reaction of either the cis or the trans esters with MeI, and the reaction of the cis ester with Ph₂C = O, resulted in the corresponding I-substituted derivatives having cis geometry only. This was explained as due to steric hindrance to elctrophilic attack exerted by the 2-phenyl group in the case of the trans substrate. The trans esters did not react with Ph₂C=O. Inhibition of the trans-to-cis isomerization of the carbanion salt of the trans substrate in presence of Ph₂C=O could lead to this result.