Substituent Effect on the Enantiomer-Differentiating Reaction of Lithiomethyl p-Tolyl Sulfoxide with Meta- or Para-Substituted (R)-(−)-Menthyl Benzoates

Treatment of (R)-(−)-menthyl benzoates, which have a variety of meta- or para-substituents, with 2 equivalents of racemic lithiomethyl p-tolyl sulfoxide displays the feature of an enantiomer-differentiating reaction, affording the corresponding optically active β-keto sulfoxides. The degree and the direction of enantioselectivity were affected by the nature of the substituent on benzene ring. The electron-releasing substituents trend to increase the %e.e. value. The reversal in the configuration with the variation in the substituent which has a high electron-withdrawing p-CN group was also observed. The R⁄S values thus obtained gave a good correlation with Hammett’s a values (r=0.975), affording a negative straight line. Based on these observations, a plausible stereochemical course of this reaction has been discussed.