Group exchange, organolithium compounds

Although the subject of exchange reactions of organolithium compounds has recently been dealt with in detail (14, 15), a brief account is included here because of the many reactions between these compounds and the alkyl derivatives of Groups II and III. 

Organolithium compounds occur in solution as dimeric, tetrameric, or hexameric aggregates held together by electron-deficient bridge bonds (14). The actual degree of association depends on the alkyl group involved and the solvent. The nature of the association in these derivatives permits two types of exchange ... 

The ester was cleaved by reduction with DIBAL (Ml AlH) and an achiral version of the normal protecting group put in place. It would obviously be silly to create unnecessary diastereo meric mixtures in these reactions. Then the tin could be exchanged first with lithium and then with an elec-tuophile, even an alkyl halide, with retention of configuration and without loss of enantiomeric purity. The intermediate organolithium compound must have had a stable configuration. 

The tetraalkyltins show little tendency to increase their coordination with an external ligand. The exception is the organic nucleophile from organolithium compounds which gives, reversibly, the 5-coordinate tin anion which can be observed by NMR spectroscopy, and which leads to the exchange of alkyl groups (equation 5-6). These reactions are discussed in Sections 5.3.5 and 22.1. An NMR study of the product from treating tribu-tyl(hydroxymethyl)tin with butyllithium implied that it had the 6-coordinate structure 5-2.25... 

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