Hauser bases

The magnesium amides of choice for the preparation of magnesium enolates via met-allation are the Hauser bases, such as 39 and 40, or (bis)amidomagnesium reagents, such as 46 and 47. The reaction has been successfully applied to the preparation of enolates derived from cyclic, acyclic and a-siloxyketones, benzyUc ketones, aldehydes, carboxylic esters and amides, even with the less hindered Hauser bases. 

Allan, J. F. Clegg, W. Henderson, K. W. Horsburgh, L. Kennedy, A. R. Solvent effects and molecular rearrangements during the reaction of Hauser bases with enolizable ketones structural characterization of [ tBuC (=C H2) OMgBr/HMPA 2] and [MgBr2/ (HMPA)2]./. Organomet. Chem. 1998, 559, 173-179. 

Magnesium amides, known as the Hauser bases [26], are able to magnesiate arenes and heteroarenes [27]. However, their low solubility in most organic solvents makes them difficult to handle. Furthermore, in noncoordinating solvents, these species behave as reducing agents for various functional groups such as keto, nitro, nitroso, and azoxy derivatives [28]. 

Garcia-Alvarez P, Graham DV, Hevia E, et al. Unmasking representative stmctures of TMP-active Hauser and turbo-Hauser bases. Angew Chem Int Ed. 2008 47(42) 8079-8081. 

Structure of the Reagent. Properties of 2,2,6,6-tetramethylpi-peridinatozinc chloride-lithium chloride such as high THF solubility and kinetic basicity are determined mainly by its assumed monomeric zincate-like structure (eq 1), which significantly differs from oligomeric Hauser bases. 

Unlike to their lithium analogues, Hauser bases (R2NMgBr) are much more stable in THF (up to reflux conditions). Eaton reported in 1989 the use of magnesium bis(2,2,6,6-tetramethylpiperamide), (TMP)2Mg, as a selective metalating reagent (Scheme 4.3) [21]. 

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