CHIRAL ORGANOMETALLIC REAGENTS .16. ENANTIOMERIZATION OF ALPHA-THIO-SUBSTITUTED, ALPHA-SELENO-SUBSTITUTED, AND ALPHA-TELLURO-SUBSTITUTED ALKYLLITHIUM COMPOUNDS - KINETIC AND MECHANISTIC STUDIES

The rate of enantiomerization of the racemic alpha-phenylselenoalkyllithium compound 6 has been determined by dynamic NMR spectroscopy in [D-8]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 alpha-duryl- and alpha-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 alpha-arylthio- and alpha-aryltelluroalkyllithium compounds 7j and 7f, but are absent with the alpha-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.