Hauser base

Hauser bases, also called magnesium amide bases, are magnesium compounds used in organic chemistry as bases for metalation reactions. These compounds were first described by Charles R. Hauser in 1947. Compared with organolithium reagents the magnesium compounds have more covalent and therefore less reactive metal-ligand bonds. This is why they display a higher functional group tolerance and a much greater chemoselectivity. Generally Hauser bases are used at room temperature while reactions with organolithium reagents are performed at low temperatures (e.g. -78°C).

Like all Grignard dimers TMP and HMDS Hauser bases are bridged by halides in the solid state. In contrast to Grignard reagents, dimeric amido bridged Hauser bases exist, too. All have one in common: They are bridged by less bulky amido ligands like Et₂N−, Ph₃P=N− and iPr₂N−. The displacement to halide bridges may be a result of the bulky groups on the amide ligand.

Although there is a great deal of information on the utility of these reagents, very little is known regarding the nature of Hauser bases in solution. One reason for that lack of information is that Hauser bases show a complex behaviour in solution. It was proposed that it could be similar to the Schlenk equilibrium of Grignard reagents in ether solution, where more than one magnesium containing species exists. In 2016, Neufeld et al. showed by Diffusion-Ordered Spectroscopy (DOSY) that the solution structure of iPr₂NMgCl is best represented by the common Schlenk equilibrium:

iPr₂NMgCl (A) ⇋ (iPr₂N)₂Mg (B) + MgCl₂

This equilibrium is highly temperature dependent with heteroleptic (A) to be the main species at high temperatures and homoleptic (B) at low temperatures. Dimeric species with bridging chlorides and amides are also present in the THF solution, although alkyl magnesium chlorides do not dimerize in THF. At low temperatures, where an excess of MgCl₂ is available MgCl₂ co-coordinated species are present in solution, too.

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