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Chiral organometallic reagents, XII. On the racemization of α-hetero-substituted benzyllithium compounds

✍ Scribed by Ahlbrecht, Hubertus ;Harbach, Jürgen ;Hoffmann, Reinhard W. ;Ruhland, Thomas

Publisher
John Wiley and Sons
Year
1995
Tongue
English
Weight
645 KB
Volume
1995
Category
Article
ISSN
0947-3440

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✦ Synopsis

Abstract

Benzyllithium compounds carrying a benzylthio substituent (6), a benzylseleno substituent (8), and an isopropyl(methyl)amino substituent (14) have been generated. Racemization barriers have been determined in THF by monitoring the coalescence of signals of diastereotopic groups in the ^1^H‐NMR spectra. The barrier for 6 amounts to a Δ__H__^≠^ of 7.5 kcal mol^−1^ and Δ__S__^≠^ of −12 cal mol^−1^ K^−1^ corresponding to a Δ__G__ of 9.9 kcal mol^−1^. The barrier for the other compounds investigated varies from 9 to 10 kcal mol^−1^. The rate‐determining step for the racemization is discussed to be the transformation of the contact ion pair to the solvent‐separated ion pair.

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## Abstract α‐Methylbenzyllithium (6) should possess either central or planar chirality. The rate of enantiomerization of 6 at — 78^°^C was found to be faster than its addition to the amide 7, the ketone 8, or to the aldehyde 10, according to a test based on kinetic resolution. The benzyllithium co