Enolates halogen-magnesium exchange

The halogen-magnesium exchange of a-halo carbonyl compounds has been reported to afford magnesium enolates which react with aldehydes to yield aldol products [107, 108]. The application of this reaction to the synthesis of penicillin derivatives has been reported (Scheme 3.94) [109-111]. 

Several years ago, Castro, Villieras and coworkers described the preparation of the magnesium enolate derived from an alkyl a,a-dihaloacetate by halogen-metal exchange between isopropyhnagnesium chloride and alkyl trihaloacetate. THF is required as solvent (equation 7) °. 

Scheme 2.46 Formation of lithium, magnesium, and zinc enolates by halogen-metal exchange.

If the metal-halogen interconversions were originally used to prepare magnesium enolates, other heteroatoms than halogen can undergo the exchange. Sulfur atom is often employed. 

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