The rate of enantiomerization of the racemic a-phenylsele-in those sterically hindered systems. Similar steric effects noalkyllithium compound 6 has been determined by dy-were detected for the enantiomerization of the corresponding namic NMR spectroscopy in [D,]THF. The enantiomerization a-arylthio-and a-aryltelluroalkyllithium compounds ?j and rate was found to be first order with respect to monomeric 6 7f, but are absent with the a-arylsilyl-substituted alkyland to show no conspicuous solvent dependence (diethyl lithium compound 7 0 . This finding, along with the fact that ether; toluene + 1 eq. of THF) or change upon addition of the phenyltelluro-(?e), phenylseleno-(6), and phenylthio-alLiC104. The marked steric effects on the enantiomerization kyllithium compounds (irg) have essentially the same enanrate found with the a-duryl-and a-mesityl-selenoalkyl-tiomerization barrier, lead us to propose that in these cases a lithium compounds 7 c and 7d suggest that rotation about the reorganization within the contact ion pair is the rate limiting carbanion-selenium bond may be the rate-determining step step for the enantiomerization.a-Heterosubstituted alkyllithium compounds 1 are d' synthonsL21 in organic synthesis, and since the entity 1 is chiral, these compounds are of interest as chiral building blocks for stereoselective synthesis.Accordingly, attention was focused on their configurational stability: Various studies showed that o~y g e n -[~.~] and amino-sub~tituted[~I derivatives of 1 are configurationally stable on a macroscopic time scale, i.e. for ca. 10 min at -78 "C. On the other hand, ~u l f u r -[~~~~~] and seleno-L9] substituted derivatives of 1 are configurationally labile on this time scale. However, fast (e.g. intramolecular) trapping reactions are sometimes faster than enantiomerization in such cases[6,'01. Such species may be called configurationally stable on a microscopic time scale. Our goal was therefore to increase the configurational stability of such organolithium compounds by some modification, which obviously would require knowledge of the enantiomerization mechanism. Calculations" ' 1 and experiments [12] were carried out on the epimerizationtenantiomerization of alkyllithium compounds, suggesting an associative process between two or more alkyllithium entitites. However, the enantiomerization of a-heterosubstituted alkyllithium compounds 1 may also be described by other mechanistic schemes. For this reason, the groups of H. J. Reich and ourselves initiated studies on the enantiomerization mechanism of alkyllithium compounds 1 with X = SR, SeR, and SiR3. In this paper we detail our results. Results and DiscussionA limitation of all mechanistic studies is, that they rely on proposed reaction pathways, which can be disproven by experiments but not proven. A non-associative mechanism for the enantiomerization of 1 may involve the following steps:The first step may be described as a decoordination of the carbanion from the lithium to form a contact ion pair. This would allow a relative motion...

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Ab initio calculations showed that the two elementary steps in the racemization of the a-methylthioethyl anion ( 7 ) , inversion of the pyramidal anionic center, and rotation around the C--S bond, occur in sequence and are not concerted. The former process passes over a barrier of 1.1 kcal/mol, the latter over one of 10.9 kcal/mol. MNDO calculations on the aphenylthioethyl anion (8) revealed a preferred conformation of the S-aryl bond, allowing maximum delocalization of the sulfur lone pairs into the ?I* orbital of the aryl group. This orientation is maintained during rotation around the C--S bond and is the origin of steric hindrance in the racemization of certain a-arylthio-, a-arylseleno-, and a-aryltelluroalkyllithium compounds 1 to 6.

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New possibilities for increasing the stability of the configuration of α‐heteroatomsubstituted alkyllithium compounds follow from the unexpected finding that ortho methyl groups influence the rate of racemization of α‐arylselenoalkyllithium compounds. According to this observation, rotation about the CX bond (X  Se) [Eq. (a)] can be the rate‐determining step.