Enantioselective Synthesis with Lithium/(−)‐Sparteine Carbanion Pairs

Abstract

“Chiral carbanions”—that is, enentiomerically enriched lithium–carbanion pairs in which the carbanionic center carries the chiral information—were regarded until recently as “exotic species.” In the past ten years it has become clear that they can, in fact, play a meaningful role in enantioselective synthesis, since substitution for lithium occurs here stereospecifically, usually with retention of configuration. They are also more readily and commonly accessible than was originally assumed. The trick lies in the use of lithium cations with chiral ligands, whether in the form of alkyllithium species used as bases in kinetically controlled, enantiotopically discriminating deprotonation, or in thermodynamically controlled equilibration in configurationally labile epimeric ion pairs. The lupine alkaloid (−)‐sparteine has shown itself admirably suited as a chiral bidentate ligand, and its efficiency and breadth of application are so far unsurpassed. This contribution constitutes an overview of the preparation of chiral reagents, convering primarily “umpoled” synthons such as homoenolates. 1‐oxyalkanides with a broad pattern of substitution, and α‐aminobenzyl anions.