Ladder-trimer

Collum and co-workers investigated the aggregation structure of LDA with LiX. At high LiCl concentrations the mixed-dimer structure MD-1 is the major LDA species at lower concentrations two distinctive structures are possible, the mixed-cyclic trimer MCT-1 and the ladder-trimer LT-1 (Fig. 4) [47]. Rigorous establishment of the presence of LTMP-LiX aggregates has not been forthcoming, because the various LTMP... 

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 tertiary structure of retrocyclin-2 in a membrane-mimicking environment was determined recently and consists of a well-defined (3-hairpin braced by the three laddered disulphide bonds.193 Analytical ultracentrifugation and NMR diffusion experiments indicated that retrocyclin-2 self-associates to form a trimer. Self-association is an important feature of some antimicrobial peptides but the role of this self-association in the mechanism of action of the retrocyclins is yet to be determined. 

That these compounds are polymeric there can be little doubt, and spectroscopic evidence favors a cross-linked, reticular lattice rather than a ladder structure (151, and references therein). However, if bulky hydrocarbon groups are present on tin then soluble oligomers are formed, which to date have all been trimers [Eq. (42)] (152-154). The (SnO)3 ring (51) is planar... 

Later, the same amide was crystallized as a cyclic trimer, (Li-1)3, from the noncoordinating solvent hexane28 showing that the nitrogen substituents are too bulky to allow lateral association into ladders. 

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. 

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. " ... 

Collum and co-worker studied the solution structure of lithium diethylamide (16) in THF and in ether. Not only dimers and trimers, but also four-, five-, and six-rung ladder structures such as 22 were detected. 

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