Chiral Organometallic Reagents, XVI. Enantiomerization of α‐Thio‐, α‐Seleno‐, and α‐Telluro‐Substituted Alkyllithium Compounds; Kinetic and Mechanistic Studies

The rate of enantiomerization of the racemic α‐phenylselenoalkyllithium compound 6 has been determined by dynamic NMR spectroscopy in [D8]THF. The enantiomerization rate was found to be first order with respect to monomeric 6 and to show no conspicuous solvent dependence (diethyl ether; toluene + 1 eq. of THF) or change upon addition of LiClO4. The marked steric effects on the enantiomerization rate found with the α‐duryl‐ and α‐mesityl‐selenoalkyllithium compounds 7c and 7d suggest that rotation about the carbanion‐selenium bond may be the rate‐determining step in those sterically hindered systems. Similar steric effects were detected for the enantiomerization of the corresponding α‐arylthio‐ and α‐aryltelluroalkyllithium compounds 7j and 7f, but are absent with the α‐arylsilyl‐substituted alkyllithium compound 7o. This finding, along with the fact that the phenyltelluro‐ (7e), phenylseleno‐ (6), and phenylthio‐alkyllithium compounds (7g) have essentially the same enantiomerization barrier, lead us to propose that in these cases a reorganization within the contact ion pair is the rate limiting step for the enantiomerization.