Effect of Lewis Bases

The terminations usually occur by chain transferring, either by an internal hydride transfer  

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Recently, Henderson has investigated the effect of Lewis base additives such as HMPA in enantioselective deprotonation of ketones mediated by chiral magnesium amide bases. In almost all reactions investigated, the additive HMPA could be replaced by DMPU without any undue effect on either selectivity or conversion (equation 69) ... 

Few detailed studies have been made on the effect of Lewis bases on the propagation rate, but the effect of tetrahydrofuran is known for... 

Lewis bases exert dramatic effects on the rate, stereochemistry, and reaction pathway in organolithium chemistry 4). A partial explanation for these observations can be deduced from the effects of Lewis bases on the degree of association of organolithium compounds as shown in Table 3. In general, the presence of basic molecules tends to decrease the average degree of association of organolithium compounds. Thus, simple alkyllithiums which are hexameric in hydrocarbon solution... 

The most dramatic effects of Lewis bases in organolithium chemistry are observed in polymerization reactions. Aside from colligative property measurements, there is little direct quantitative information on the nature of the organolithium-base interactions responsible for the observed effects. The calorimetric method has been used also to examine the fundamental nature of the interaction of bases with polymeric organolithium compounds 83,88,89). Information is now available on the ground-state interaction of bases with poly(styryl)lithium (PSLi), poly(isoprenyl)lithium (PILi) and poly(butadienyl)lithium (PBDLi). 

For ethylene polymerization, the iso- (syndio-) specific catalytic activity is not needed. For higher 1-alkene polymerizations it is, of course, indispensable crystalline polymers are in greater demand. For some time, third-generation catalysts could not be widely used because of their insufficient ability to induce tactic chain propagation. This drawback was overcome by the revamped discovery of the donor effect of Lewis bases [285]. 

NMR and temperature rising elution fractionatiOTi (TREF) polymer characterization data, on the other hand, pointed to a direct effect of Lewis bases on site enantioselectivity in fact, the highly isotactic polymer fraction yielded by catalysts modified with Lewis bases is not only much more abundant, but also more stereoregular compared with that of Lewis-base-free systems, while keeping the typical fingerprint of enantiomorphic-site control [42]. This suggests that Lewis base molecules are in nonbonded contact with the inherently chiral catalytic species, and shape their active pocket to make them better able to discriminate between the two enantiofaces of propene at the insertion step. 

Discuss the effect of Lewis bases on Ziegler-Natta-type coordinated anionic polymerizations. 

With the synergistic effect of Lewis base and Lewis acid, Denmark and coworkers have established several successful organic catalyst systems based on the strategy of Lewis acid activated Lewis base or Lewis base activation of Lewis acid, in which chiral phosphoramides expressed excellent diastereo- and stereoselectivity for the aldol reaction of silyl enolates and aldehydes (Scheme 65) (311-313). 

An interesting effect of Lewis bases on diene microstructure is the fact that in the presence of strongly coordinating bases such as TMEDA, 1,2 units are observed for polyisoprene. For example, the microstructure of polyisoprene formed in the presence of TMEDA ([TMEDA]/[Li] = 1) in cyclohexane corresponds to 21% 1,4, 12% 1,2, and 67% 3,4 (137). The formation of 1,2 units requires the formation of the less stable 1,4 chain ends versus 4,1 chain ends as shown in equation 52 ... 

Hydrogen was adjusted in order to maintain the 2.16 kg melt index near 1 g per 10 min. The effect of Lewis bases chosen in different chemical families is presented at different butene contents. Most of the ELB induces a MWD narrowing. Good results are obtained with ethers for instance. The best melt flow ratio indices are very low 22 to 24. 

An important feature of the effect of Lewis base additives on diene microstmaure is the fact that the amount of vinyl microstmcture generally decreases with increasing temperature as shown in Table 11 and illustrated in Figure a simple... 

Scheme 7 Oiemoselectivity effects of Lewis base additives ...

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