Dimeric Magnesium Amides

Dimers with three-coordinate magnesium. The simplest dimeric magnesium amides are unsolvated and contain a planar (MgN)2 core with trigonal planar coordinated magnesium atoms and bridging amides. The best known example is the crystalline silylamide [(Mg N-(SiMe3)2 )2], ° which has a planar (MgN)2 core with terminal Mg—N bond lengths = 1.97 A... 

More recent investigations have shown that the reaction of a Na(tmp)/Bu Mg(tmp)/ tmeda mixture with 0.5 equivalent of benzene leads to dideprotonation of the arene to afford [ (tmeda)Na(p-tmp)Mg(tmp)2 2C6H4]. 5 The reaction of the magnesium amide Mg(tmp)2 with LiN(SiMe5)2 resulted in a deprotonation/amine elimination process as shown in Scheme 3.7. The product, a dimeric lithium/magnesium mixed amide has a conformationaUy locked structure with two stereogenic nitrogen centres. ... 

Reaction mechanism Detailed mechanistic studies have been conducted on this reaction. While the stoichiometric protonolysis reaction of [ (ArNCMe)2CH Mg" Bu]2 with benzylamine, 2-methoxyethylamine, and pyrrolidine to yield the corresponding dimeric magnesium amides and methane proceeds rapidly and non-reversibly at room temperature [88], the reaction of [ (ArNCMe)2CH Ca N (SiMe3)2 (THF)] with benzylamine forms a quantifiable equilibrium between monomeric bis(trimethylsilyl)amide and dimeric benzylamide reaction products. A van t Hoff analysis of this equilbrium mixture allowed derivation of AG°(298 K) as —2.7 kcal mol consistent with facile precatalyst activation via protonolysis with a primary amine [76, 87]. 

The redox-neutral dimerization of aldehydes to form esters known as the Tischenko (or Tischtschenko) reaction may be catalyzed by group 2 hexamethyldisilazides [M N(SiMe3)2 2(THF)2] (M = Ca, Sr, and Ba), bio-inspired magnesium thiolate complexes or with bicyclic guanidinate-stabilized magnesium amide complexes (Scheme 23) [139-142]. 

As can be seen from the structure, the dimerization proceeds via nitrogen-magnesium interactions. To our knowledge, dimerization via Mg—R—Mg two-electron three-center bonds in alkylmagnesium amides has neither previously been found in the solid state nor proved by X-ray techniques, although the existence of such bonds has been claimed (10). 

A special case is 2.4 J151 which is dimeric with two bis(trimethvlsilyl)amide groups in the bridging position, yet has a incoordinate magnesium alom. 

A sigruficant use is as a catalyst in a multitude of reactions, such as the formation of acrjdic and methacrjdic acid amides from fatty acid amides (81) of cychc ketones such as 4-phen5icyclopentane-l,2-dione and 2,5-dihydroxy-/)-benzoquinone from benzaldehyde diethji acetal and 2,4-dioxo-5,5-dimethoxy-hexanoic acid methyl ester (82) of dimer aldehydes from propane (83) and for the polymerization of epoxy compounds (84). Mag nesium Kthylate. Magnesium ethoxide [2414-98-4]y Mg(OC2H 2 niol wt 114.4, is an almost white hygroscopic powder density,... 

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