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. Consequently, they display a higher degree of 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, commonly at −78 °C.
Structures
Hauser bases have the empirical formula R2NMgX. The crystallize as dimers with halide bridges. Attached to Mg is amido ligands derived from secondary amines 2,2,6,6-tetramethylpiperidine.Amido-bridged Hauser bases exist when the amido ligand is less bulky, such as Et2N− and Ph3P=N−.
The structures of Hauser bases in solution have been investigated by diffusion-ordered NMR spectroscopy. These studies indicate that iPr2NMgCl is subject to the Schlenk equilibrium:
This equilibrium is temperature-dependent: heteroleptic are the main species at high temperatures and homoleptic dominate at lower temperatures. Dimeric species with bridging chlorides and amides are also present in the THF solution. At low temperatures, adducts of MgCl2 are present in solution.
Preparation and reactions
The Hauser bases are prepared by treating a secondary amine with a Grignard reagent:Like many organolithium reagents, Hauser bases are generally used for metalation reagents. iPr2NMgBr selectively magnesiate carboxamides.
iPr2NMgX effect the deprotonation thiophenes. and phenylsulphonyl-substituted indoles.