Trimeric Lithium Amides

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... 

Despite their extensive synthetic use, it is only quite recently that the NLi species have been isolated and examined. The first singlecrystal structure of a lithium amide was the [(Me3Si)2NLi]3 trimer. This structure was fully described in 1978 (17) [although a powder diffraction study in 1969 also indicated a trimer to be present (18)]. Some synthetic advantages of isolating crystalline lithium amides... 

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. 

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.

Section III(B,2 described how results from ab initio and MNDO calculations can explain many of the structural features found in the solid state for uncomplexed lithium amides (RR NLi) . In particular, they explain (1) why rings formed experimentally, with n = 2,3, and 4, have R,R groups perpendicular to the (NLi) ring plane (2) why a trimer (n = 3) is favored over a dimer (n = 2) and (3) why the only known... 

In summary, chelating chiral lithium amides exist in either of four major structural motifs or mixtures of them (Scheme 3). Non-coordinating solvents generally favor cyclic trimers, A. Ladder tetramers are favored for pyrrolidide amides in the absence of coordinating solvents. 

NMR studies of the chiral lithium amide Li-10 showed that in the absence of coordinating solvents, e.g. in hexane or toluene, mixed trimers (Li-10)2/n-BuLi dominate, both... 

The rate constant for the exchange of lithiums within the trimer (Li-10)2/w-BuLi is reported to be 0.8 s-1 at — 33 °C, corresponding to an exchange barrier AG 24o of 14.7 kcalmol-1. The rate of lithium-lithium exchange is suggested to be faster within mixed complexes of the chiral lithium amides with n-BuLi than within the homoaggregates49. 

The mixed (heterogeneous) complexes of a lithium amide (LDA or LiTMP) and a ketone lithium enolate (acetone, cyclohexanone or diisopropyl ketone) have been examined by semiempirical methods (MNDO) by Romesberg and Collum48. If the stabilization associated with these mixed complexes was not determined, the solvation (by THF and HMPA) of the mixed cyclic dimers and trimers was calculated to be generally exothermic (but decreasingly with the steric demand of the enolate) and led to disolvated entities. A set of solvated dimers, trimers and tetramers, cyclic or not, has thus been identified... 

Pratt and Streitwieser performed ab initio (HF/6-31G and HF/6-311-FG ) calculations to examine the formation of mixed dimer and trimer aggregates between the lithium enolate of acetaldehyde (lithium vinyloxide, LiOVi) and lithium chloride, lithium bromide and lithium amides. Gas-phase calculations showed that in the absence of solvation effects, the mixed trimer (LiOVi)2 LiX (20) was the most favored species. 

Simple examples of cyclic oligomers formed through ionic interactions are the trimer of lithium bis(trimethylsilyl)amide, [LiN(SiMe3)2]3 (Scheme 13), " and hexameric [LilSCeHs (CH2NMe2)2-2,6 ]6 More intricate cage, ladder, and other polycyclic structures are formed in various other compounds. " ... 

The rates of formation of the dimeric and trimeric products are inversely proportional to the ratio of the metal amide to metallocyclopropene concentrations and, under similar conditions, the relative rates of dimerization and trimerization for potassium, sodium and lithium are 1 9 960 and 1 7 12, respectively. With each amide an equilibrium is gradually established, but the position of this is highly dependent on the metal. By choosing the conditions carefully each of the products can be obtained in reasonable yield (Table 4).63.64... 

Li-C couplings have been collected for a series of lithium reagents by Reich and co-workers, who studied their chelation and aggregation with potential 5-, 6-, and 7-ring chelating ether and amine ortho substituents. The couplings have been also applied by Hilmersson and Malmros in their studies on mixed dimer and mixed trimer complexes of -BuLi and a chiral hthium amide. 

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