Lithium aggregates

Figure 1.11 Two potential dimeric lithium aggregates of the 1 1 complex 68.

Wu and co-workers (Wu et al., 1999) have demonstrated a novel chiral lactone enolate-imine process to access 2-azetidinone diols such as 35 (Scheme 13.10). Treatment of 34 with LDA at — 25°C in THF followed by addition of imine 3, afforded only trace product. Addition of HMPA or the less toxic DMPU during the lithium enolate formation step improved the yield and the trans cis diastereoselectivity ( 90 10). Recrystallization improved the purity to >95 5 trans cis 2-azetidinone. Addition of an equivalent of lithium bromide accelerates the rate of ring closure, presumably by destabilizing the intermediate lithium aggregates. Side-chain manipulation of 35 was accomplished by sodium... 

In addition, one must consider the possibility of interaction between adjacent groups. This is of particular importance when dealing with the beryllium derivatives in which the metal nucleus is very small and may also be of significance in other systems such as the lithium aggregates. Unfortunately, little quantitative information has appeared with regard to this feature other than statements of distance observed in a few systems. 

The other structures all represent cases in which the Group IV element is interacting with 3 lithium atoms, and in each case a three-dimensional lithium aggregate is formed. The lithium-lithium and lithium-carbon distances are summarized in Table VIII for those structures that have been determined. In addition, lithium-carbon distances in several lithium-aromatic ion pair systems are included in Table VIII for comparison (18, 19), as well as the observed distances in the hexamer of trimethylsilyllithium. In the dimeric molecule, the Li—Li distance of... 

It, therefore, seems probable that the suggested Li—H interactions do not always have any appreciable effect on the stability of the lithium aggregate, even though they may have some influence on the conformation adopted by the cyclohexyl ring in cyclohexyllithium hexamer or in other selected cases. 

The influence of tetrahydrofuran on the propagation and association behavior of poly(isoprenyl)Iithiura in n-hexane has been examined47. As for the case of poly(styryl)lithium156), the rate of polymerization was found to first increase followed then by a decrease as the THF/active center ratio increased. This decrease ultimately reached the polymerization rate found in pure tetrahydrofuran at a THF active center ratio of ca. 2 x 103. This was for the case where the active center concentration was held constant and the tetrahydrofuran concentration varied. The maximum rate of polymerization was found to occur at a THF active center ratio of about 500 a value at which the viscometric measurements demonstrated 47 the virtual absence of poly(isoprenyl)lithium self-aggregation. As noted before in this review, the equilibrium constant for the process shown in Eq. (12) has the relatively small value of about 0.5 LM-1, which is in sharp contrast with the value of about 160 LM 1 found for the THF-poly(styryl)lithium system. The possibility of complexation of THF directly with the poly(isoprenyl)lithium aggregates, Eq. (13), was not considered by Morton and Fetters47. ... 

In the original paper n was taken to be four 189) although the burden of evidence suggests (Table 2) two is more likely. This scheme assumes that complexation need not result in disaggregation. Viscometric studies show that poly(butadienyl)lithium aggregates are broken on complexation 152). Calorimetric measurements89 on the interaction of TMEDA with poly(isoprenyl)lithium have yielded data that are not inconsistent with the formation of a complex between one molecule of the former and two of the latter. 

The presence of cross-associated species needs to be considered in the interpretation of copolymerization kinetics. It has been found 269) that the reaction of poly(butadie-nyl)lithium with p-divinylbenzene in benzene solution proceeds at a rate which increases markedly with time. Such a result implies that the poly(butadienyl)lithium aggregate is less reactive than the mixed aggregate formed between the butadienyl-and vinylbenzyllithium active centers. Interestingly, no accelerations with increasing reaction time were found with poly(butadienyl)lithium and m-divinylbenzene nor with poly(isoprenyl)lithium and either the m- or p-divinylbenzenes. This general behavior was subsequently verified 270) by a series of size exclusion chromatography measurements on polydiene stars (linked via divinylbenzene) as a function of conversion. 

Many chelated organolithium compounds can be obtained as 1 1 complexes, but only certain lithium aggregates appear to form insoluble... 

The fragmentation or complexation of alkyl lithium aggregates has a profound effect upon their ability to initiate anionic polymerization and it greatly affects their rate of initiation. Examples of such effects are discussed in the next section. 

The character of the initiation changes as the reaction proceeds because the growing polymers combine with alkyl lithium aggregates into mixed aggregates. The latter seem to be more dissociated than the former and this facilitates the initiation. Similar behaviour is observed in the presence of Lewis bases. 

This result indicated that the lithium aggregates equilibrated at 0°C to form an effective complex in the transfer of chirality. The experiments also revealed a nonlinear effect (NLE), suggesting that more than 1 mol of chiral auxiliary was involved in the chirality transfer (more detailed discussion about the origin of non-linearity in chirality transfer is given in Chap. 11). 

Equilibration of Lithium Aggregates and the Effect of Their Relative Stability on Enantioselectivity... 

In Table 4.2, some selected examples of lithium aggregates with structures determined by crystallographic studies are described. 

A basic three-step mechanism has been proposed to explain the DoM reaction (Scheme 11.2). This invokes the complex-induced proximity effect (CIPE) wherein the allq l lithium aggregate (RLi) coordinates to the DMG through an equilibrium process, and the complex thus formed places the base in close proximity to the ortho proton leading to a coordinated ortho-lithiated species, which then reacts with the electrophile to form the 1,2-disubstituted arene. ... 

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