Tetrameric Lithium Amides

The chiral lithium amides can also be part of cubic tetrameric structures as shown by the mixed complex, Li-6/(n-BuLi)3, consisting of the chiral lithium amide Li-6 and three molecules of n-BuLi61. 

Another mixed aggregate complex consisting of Bu°Li and r-butoxide was reported in 1990 as the tetramer (45). Hiis complex was first isolated by Lochmann and has been shown to be tetrameric and dimeric in benzene and THF, respectively, by cryoscopic measurements, and it has also been studied by rapid injection NMR techniques. Hiis species has received much attention because it is related to the synthetically useful superbasic or LiKOR reagents prepared by mixing alkali metal alkoxides with lithium alkyls or lithium amides. ... 

Asymmetric butylation (91-98.5% ee) of prochiral aliphatic aldehydes is promoted much faster by mixtures of n-BuLi and chiral lithium amides in Et20-dimethoxy-methane than by tetrameric n-BuLi. " ... 

Lithium enolates of ketones exist as aggregates in solution.29-3l,34d,35 Mixed aggregates between the enolate anion and the amide base are also possible. In 1981, Seebach and co-workers confirmed by X-ray crystallography that the lithium enolates of pinacolone and cyclopentanone form a tetrameric aggregate in the solid state, and it was assumed that a similar species exited in solution. A THF solvated tetramer of lithium pinacolonate is shown (see 33), as it was reported by Seebach. Williard et al. reported the X-ray structure of... 

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