Monomeric Lithium Amides

Table XIII (189-199) gives details of solid-state lithium amide monomeric complexes (69)—(87). These include just three [(79), (80), and (87)] solvent-separated ion pairs. The remainder are contact-ion pairs, each with an (amido)N—Li bond. Association to dimers or higher oligomers is prevented sterically. The size of the R and/or R group in the RR N- anions can lead to monomers even when Li+ is complexed only by a single bidentate (e.g., TMEDA) or by two monodentate (e.g., THF or Et20) ligands. In such cases [(69), (71), (72), (75)-(78), and (81)—(83) ], the lithium centers are only three coordinate. Electronic factors in the anion [notably, B N multiple bonding in (75)—(78) ] also may reduce the charge density at N, and lower the ability to bridge two...

Monomeric Lithium Amide Complexes Key Structural Parameters (X-Ray Diffraction Data)... 

Computational chemistry has been employed to calculate energy differences between diastereomeric activated complexes in the stereoselective deprotonations of cyclohexene oxide by monomeric, homo- and heterodimeric lithium amides (see Section II.A.2). Computational chemistry has also been used as a tool for design of highly stereoselective amides. Such a design approach has resulted in the homochiral base 20 and its enantiomer. These are readily available from both enantiomers of norephedrine, by inexpensive routes... 

By using a combination of gas-phase synthesis and millimeter/submillimeter-wave spectroscopy, LiNH2 was found to be a monomeric unsolvated planar molecule. The lithium amide [H2NCH2CH2N(H)Li]co has a polymeric ladder structure with two types of (NLi)2 ring which alternate throughout its infinite length. ... 

In the development of chiral lithium amides which result in higher ee, the effect of a diverse set of substituents R and R in 38 was examined. It was shown that ee increases as the size of substituent R becomes bulkier, and also as the amount of fluorine in R increases. In THF, 38a occurs as a monomeric structure M-38a in either the presence or absence of HMPA. Fluorinated base 38b has also been shown to be monomeric in THF, consistent with structure M-38b where the fluorine atoms do not act as internal chelating ligands. In the presence of LiCl, the solution structure of labeled 38b was examined by Li and NMR in THF-ds- The Li- N coupling patterns showed that mixed dimer MD-6 was formed, as also illustrated with 38a. The absolute configuration of the products renders the OD-1 structure of transition state TS-1 most likely (Fig. 7) [58]. 

From the experimentally obtained product composition in THF 90% of (5)-3 and 10% of (R)-3, a free energy difference (AG) between the two rate-limiting diastereoisomeric activated complexes of 1.33 kcal moP is calculated. In Table 1, energy differences and product compositions obtained by computational chemistry at various levels of theory are presented for activated complexes built from one molecule of epoxide 2, monomeric or dimeric lithium amide 4 and no or one specifically solvating THF or DEE molecule. 

Grotjahn DB, Sheridan PM, Al Jihad I, Ziurys LM (2001) First synthesis and structural determination of a monomeric, unsolvated lithium amide, LiNH2. J Am Chem Soc 123 5489-5494... 

The regioselectivity, reactivity, and structure of the aluminum adducts of type 65 and of the aluminum ate base 64 have been carefully studied [22, 23]. Alternatively, LiCl-monomerized TMP bases such as 68 and 69 allow the smooth deprotonation of various aromatic and heterocyclic compounds. These bases are soluble in THF (ca. 0.3 M) and decompose in THF at 25°C within 12 h. They are prepared in almost quantitative yield from corresponding lithium amides (Scheme 4) [24]. 

Hitherto no monometalated molecular pnictide exists without solvation of the main group metal atom. Therefore, the monomeric species L (Fig. 2) can only be stabilized if the Li ion has its coordination sphere enlarged through donor solvation. More importantly, the lithium phosphanides of the type K undergo oligomerization processes to form dimer, tetramer, hexamer, or polymeric assemblies M—Q (Fig. 2), which dissociate in solution more easily than related amides (2, 11, 12). 

A variable-temperature Li, C, NMR spectroscopy study was carried out on the ligand structure and association behavior of [ Li, N]-enriched lithium bis(trimethylsilyl)-amide in hydrocarbon solntion. The system behaves as shown, for example, in equation 38, with the solnte adopting a dimeric form (209), which tnms to the monomeric 210 on... 

Other monomeric silylamido complexes have also been obtained by the addition of tmeda (A, A, A A"-tetramethylethylenediamine) or pmdeta (N,N,N ,N" - pentamethyl-diethylenetriamine) to afford [(tmeda)LiN(SiMe3)2] which has three coordinate lithium Li—N(amide)= 1.893(3) A and [(pmdeta)LiN(SiMe3)2 with 4-coordinate Li and Li—N (amide = 1.988(6) A. The use of very large substituents at the amido ligand can also afford monomeric structures. Examples include [Li(thf)3N(SiMe2Ph)2], [Li(thf)2N(SiPh3)2l/ [Li(th )2N(SiBu 2F)2] [Li(thf)2N (C6F5)(SiBu 2F) ] and [Li(thf)3 NMes (SiPr 2X) ]... 

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