Lithium, amido

Scheme2.1 Lithium amido complexes formed upon addition of 12-crown-4 a/ h/ c/ d, e, > f ...

Scheme 2.2, D) and tetrameric eight-membered (LiN)4 ring and cubane (Scheme 2.2, E and F) structures are also known. Comprehensive reviews of dimeric and other lithium amido stnictures were published in the 1990s. As a result only selected examples from the preceding literature are covered. ... 

Magnesium amides can form a wide range of mixed metal amido complexes with alkali metal ions (M2[Mg(NH2)4]) (M = K, Rb or Cs).These contain tetrahedral Mg-centered [Mg(NH2)4] ions connected in three-dimensional networks by coordination of the amido groups to the group 1 metal ions. The most common hetero metal is lithium and lithium amido magnesiates are readily accessible by the addition of a lithium amide to a magnesium amide. 

Several asymmetric 1,2-additions of various organolithium reagents (methyllithium, n-butyllithium, phenyllithium, lithioacetonitrile, lithium n-propylacetylide, and lithium (g) phenylacetylide) to aldehydes result in decent to excellent ee% (65-98%) when performed in the presence of a chiral lithium amido sulfide [e.g. (14)], 75 The chiral lithium amido sulfides invariably have exhibited higher levels of enantioselectivity compared to the structurally similar chiral lithium amido ethers and the chiral lithium amide without a chelating group. 

FIGLfRE 2. Calculated Gibbs free-energy values of unsolvated and solvated mixed complexes between a chiral lithium amido ether and methyllithium... 

Although these mixed complexes with chiral lithium amido sulfides appear promising structures for asymmetric addition reactions in general, it should be noted that they are only stable at low temperatures (<50 °C). At higher temperatures they readily decompose, most likely due to deprotonation of the acidic a-protons next to the sulfur. 

Bulky substituents lead to the formation of small-ring compounds. For example, LiF elimination from l-lithium-amido-3-fluoro-l,3-disiloxanes leads to four-membered (SiNSiO)-ring systems (Eq. 13). 

Potassium and sodium borohydride show greater selectivity in action than lithium aluminium hydride thus ketones or aldehydes may be reduced to alcohols whilst the cyano, nitro, amido and carbalkoxy groups remain unaffected. Furthermore, the reagent may be used in aqueous or aqueous-alcoholic solution. One simple application of its use will be described, viz., the reduction of m-nitrobenzaldehyde to m-nitrobenzyl alcohol ... 

Four equivalents of lithium phenylacetylide reacted with bis bis(trimethylsilyl)amido zinc (Scheme 51) to form the ion-paired dilithiotetra(phenylacetylido)zincate 65, whose structure is shown in Figure 33.121 The zinc-carbon bonds (2.05 A) are somewhat longer than those observed in the tri(phenylacetylido)zincate 62a, due to the increase in both the coordination number and the negative charge on the ion. Each lithium ion is associated with the zincate ion through coordination to two of the triple bonds. 

Substitutions A substituent exchange has been observed on the treatment of (AlCp )4 3 with lithium bis(trimethylsilyl)amid leading to (AlCp )3[Al-N(SiMe3)2] 50 [73]. The Al-Al distances in the tetrahedron became different, with the shorter ones being to the aluminum atom that is attached to the amido group. This observation is in accordance with the bonding situation in these clusters and reflects... 

A large variety of cuprates are known nowadays. They include heteroleptic derivatives R(Y)CuM (Y = alkynyl, halide, amido, alkoxide, thiolato, phosphide M = Li or Mg), and have found widespread application in organic chemistry. Their syntheses and applications are discussed in the other chapters of this book. In addition, compounds in which the copper to lithium (or magnesium) ratio differs from 1 1 are also known examples are R3CuLi2 and the so-called higher order cyanocuprates introduced by Lipshutz et al. [99]. 

The broader subject of the interaction of stable carbenes with main-group compounds has recently been reviewed. Accordingly, the following discussion focuses on metallic elements of the s and p blocks. Dimeric NHC-alkali adducts have been characterized for lithium, sodium, and potassium. For imidazolin-2-ylidenes, alkoxy-bridged lithium dimer 20 and a lithium-cyclopentadienyl derivative 21 have been reported. For tetrahydropyrimid-2-ylidenes, amido-bridged dimers 22 have been characterized for lithium, sodium, and potassium. Since one of the synthetic approaches to stable NHCs involves the deprotonation of imidazolium cations with alkali metal bases, the interactions of alkali metal cations with NHCs are considered to be important for understanding the solution behavior of NHCs. 

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