Dimeric Lithium Amides

In contrast, it has been suggested that for HCLA bases of type B, the stereodifferentiation proceeds via a complex composed of a lithium amide dimer and one molecule of oxirane. Indeed, these bases are known to aggregate in solution to C2 symmetric homodimers of type 62, as shown by multinuclear ( C, Li, N) NMR for HCLA 57 (Figure 4) . ... 

Isolated uncomplexed lithium amide dimers are not known in the solid state. However, electron diffraction shows (56) to be dimeric in the gas phase (80). The major change in structural parameters with ring... 

A kinetic investigation using 20 in the deprotonation of cyclohexene oxide revealed that the composition of the activated complexes was different from that assumed in the theoretical model. The reaction orders showed that an activated complex is built from one molecule of chiral lithium amide dimer and one molecule of epoxide 1. Such activated complexes have been computationally modeled by the use of PM3 and optimized structures are displayed in Figure A44. 

Flgnre 1 Li (74 MHz) and Li (194 MHz) NMR signals of a lithium amide dimer in diethylether at — 80°C external reference 0.3 M > LiQ in CD3OD. (Reprinted with permission from ref. [34]. Copyright (1995) American Chemical Society)... 

In 1994, lithium amide 23 was used in the conjugate addition of 2-cyclohexenone to afford optically active adduct with up to 97% ee (Scheme 13).28-29 A dimeric structure was proposed as the intermediate, where the phenyl group in 23 blocked the bottom face and the cyclohexenone substrate approached from the upper face. 

Considering the importance of alkali metal phosphanides it is not surprising that numerous review articles have dealt with this subject [34-36]. The solid state and solution structures vary from dimers with central M2 P2 cycles to larger rings and from chain to ladder structures as described for the lithium amides (see Sections 3.6.1 and 3.6.2). Cage compounds in the field of lithium phosphanides are unusual... 

In contrast to the lithium amides and phosphanides, dimeric alkaline-earth metal bis(phosphanides) of the heavier group 2 metals show bicyclic structures of the... 

Irradiation of matrix-isolated imidazole-2-carboxylic acid gave the 2,3-dihydro-imidazol-2-ylidene-C02 complex (31) characterized by IR spectroscopy and calculated to lie 15.9 kcal mol above the starting material. A series of non-aromatic nucleophilic carbenes (32) were prepared by desulfurization of the corresponding thiones by molten potassium in boiling THF. The most hindered of the series (32 R = Bu) is stable indefinitely under exclusion of air and water and can be distilled without decomposition. The less hindered carbenes slowly dimerize to the corresponding alkenes. Stable aminoxy- and aminothiocarbenes (33 X = O, S) were prepared by deprotonation of iminium salts with lithium amide bases. The carbene carbon resonance appears at 260-297 ppm in the NMR spectrum and an X-ray structure determination of an aminooxycarbene indicated that electron donation from the nitrogen is more important than that from oxygen. These carbenes do not dimerize. 

A recent study has indicated that the skeletal rearrangement step in the B12-catalysed isomerization of methylmalonyl-CoA to succinyl-CoA occurs not by a radical pathway but by an anionic or organocobalt pathway. A computational study of the isomerization of allyl alcohol into homoallyl alcohol by lithium amide has pointed to a process proceeding via a transition state in which the proton is half transferred between carbon and nitrogen in a hetero-dimer. l,l-Dilithio-2,2-diphenylethene... 

The reaction depicted in equation 43 between a nitrile and a lithium amide takes place as a 1,2-addition to the cyano group. The product crystallizes as a dimer (236) in which the lithium atoms are solvated by nitrile molecules and differently bonded to the amidine moieties, as shown by XRD analysis. Low-temperature H NMR spectrum in solution points to uniform chemical environments for both the aryl groups and for the Me—Si groups, and to rapid rearrangement of the Li—N coordination structures. Acidolysis of the dimer in solution yields the corresponding amidine (237) . The crystal structure of the THF-solvated analog of 236 shows dissimilar N—Li bond lengths for the two Li atoms... 

Dimeric and higher aggregate lithium amides can generally be classified into the coordination motifs illustrated in Scheme 2.2. The four-membered (LiN)2 ring is ubiquitous in lithium amide chemistry and is observed both in discrete dimeric structures in either planar (Scheme 2.2, A) or non-planar (Scheme 2.2, B) geometries as well as in oligomeric and polymeric (ladder) frameworks (Scheme 2.2, C). Trimeric six-membered (LiN), ring... 

The dimeric (LiN)2 structural motif (discussed in Section 2.2.3) is of central importance in a mode of lithium amide association known as laddering (Structure C). The phenomenon was first described by Snaith and co-workers with the structure... 

Fig. 26. Structural types for uncomplexed lithium amides (RR NLi) .- (a) dimeric ring (n = 2), showing the projection of R,R groups above and below the ring plane (b) rings with n = 2,3, and 4 (c) further association of rings into ladders.

Very few studies on uncompleted, lithium amides have been carried out in solution, e.g., in hydrocarbon solvents. Only five oligomeric, ligand-free compounds of this type have been characterized structurally f (54)—(58) Table VI Section III,B,1]. Studies carried out on solutions in polar solvents would, in effect, involve complexes. For example, the unsolvated crystalline trimers (55) and (56) both form dimeric etherate complexes (19, 20, 76, 81). Solution studies of com-plexed lithium amides will be described in Section III,C,3. 

The major structural types found for lithium amide complexes in the solid state are illustrated in Fig. 34. These comprise ladders of limited extent when the L Li ratio is less than 1 1 (Fig. 34a), dimeric (NLi)2 rings, when this ratio is 1 1 and, usually, when the complexants are monodentate (Fig. 34b), and monomers, both contact-ion pairs (CIPs) and solvent-separated ion pairs (SSIPs) (Fig. 34c). Monomers occur always when there are two or more monodentate complexants per Li. This also is usual with bidentate ligands, and is always found when the ligands have higher denticity. 

Fig. 34. Structural types for complexed lithium amides (RR NLixLi, L = a Lewis base (a) ladders with n = 2 and 3 (b) dimeric rings (c) monomers (CIPs and SSIPs). ...

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

Fig. 38. Coordination arcs available for attachment of Lewis base molecules (a) to a trimeric lithium amide ring and (b) to a dimeric lithium amide ring.

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...

These calculational results concur extremely well with the experimental findings. Lithium amide complexes are dimeric rings (or monomers) if the complexant Li ratio is 1 1 or greater, and if every Li bears (at least) one complexant. However, there is a switch of structural preference from ring to ladder if a base is not coordinated to each Li. In such cases, only the end-Li centers of the ladder are complexed, even though a 1 1 complexant Li ratio may be present under the experimental conditions. 

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