Solvation chiral lithium amides

Studies of lithium ion solvation of organolithium compounds are important for a thorough understanding of the behavior of these complex reagents. The chiral lithium amide... 

FIGURE 4. PM3-optimized THF-solvated diastereoisomeric TSs for deprotonation of 1 with chiral lithium amide 20. Most hydrogen atoms are omitted for clarity... 

The two chiral lithium amides were found to form symmetrically solvated dimers in diethyl ether (DEE). The addition of tetrahydrofuran (THF) and of l,3-dimethyl-3,4,5,6-tetrahydro-2-(l//)-pyrimidinone (DMPU) did not affect the Li NMR chemical shift due to a very strong internal coordination. The Li and Li MAS-NMR signals of fast ionic conducting Li2 2xMgi+xCl4 have... 

Similar to the LDA dimers 52 in solution and in the crystal, the chiral amide 72a forms a bis-solvated dimer 82 as shown by the crystal structure [92] and NMR studies in THF [93]. The dimeric structure 83 was found in the case of Koga s base 75 (X = CH2) wherein hthium adopts a threefold coordination by chelation and not by coordination to THF [94] (Scheme 2.23). Similar dimeric structures were confirmed more recently by a variety of NMR techniques for chiral lithium amides derived from valinol [95]. 

The lithium amide analogue with only one chiral center derived from (1-phenylethyl) benzyl amine (2) has been found to crystallize as a disolvated dimer from a THF solution, (Li-2)2 2THF. With PMDTA added, the lithium amide crystallized as a monomer solvated by one triamine molecule, Li-2 PMDTA, showing the coordinating strength of PMDTA29. 

The lithium derivative of the chiral chelating diamine (3 )-2-(l-pyrrolidinylmethyl)-pyrrolidine (6) has been used extensively in stereoselective synthesis, i.e. in the deprotonation of ketones and rearrangement of epoxides to homoallylic alcohols. The lithium amide has been crystallized from toluene solution, and X-ray analysis revealed that it forms a ladder-type tetramer with the two pyrrolidine nitrogens solvating the two lithiums at the end of the ladder38, (Li-6)4. 

In the presence of the corresponding pyrrolidine diamine, the chiral lithium pyrrolidide amide yields dimeric chelates composed of a lithium pyrrolidide amide dimer solvated by a pyrrolidine diamine, (Li-6)2 6, as shown by NMR spectroscopy39. The lithium amide gives two 6Li NMR signals in a 1 1 ratio. The addition of TMEDA to Li-6 results in a similar complex where TMEDA coordinates to the lithium pyrrolidide amide dimer, (Li-6)2 TMEDA. 

---