Lithium alkoxide aggregation

The structure of the major aggregate was identified by labeling studies. Since the major set has two equal intensity 6Li signals, these signals could be assigned as a 1 1 complex 68 of lithium acetylide and lithium alkoxide or a dimer (such as 69) of the 1 1 complex 68 shown in Figure 1.9. Both structures have two different Li species. In order to discriminate between 68 and 69, a terminal acetylene carbon of 37 was labeled with 13C. In the case of 68, both lithium signals will be a doublet... 

In the lithium and cesium enolates of o-methoxyacetophenone, the methoxy oxygen coordinates with the smaller lithium cation but not with the cesium cation . Other examples of lithium enolate chemistry include a thermochemical analysis of the aldol reaction of lithiopinacolonate with pivalaldehyde and a comparison of the proton affinities and aggregation states of lithium alkoxides, phenolates, enolates, -dicarbonyl enolates, carboxylates and amidates. Although the lithium enolate of cyclopropanone itself remains unknown, derivatives (accompanied by their aUenoxide isomer) have been implicated in the reaction of a-(trimethylsilyl) vinyl lithium with CO. That both species are seemingly formed is surprising because cyclopropanone enolate is expected to be much less stable than its acyclic isomer cyclopropene is less stable than allene by almost 90 kJmol-. ... 

Crystallization in the presence of tmeda affords a tetrameric aggregate of composition [(tmeda)NaCH2C6H2Me2(OLi)]4, 205 [178]. A distorted U4O4 cube forms the central part of the molecule. Both the lithium and sodium atoms act as bridges between the methylene units and the oxygen atoms. The compound can be regarded as a model intermolecular superbase and overcomes the problem of differing solubilities of lithium alkoxides and the heavier alkali metal hydrocarbyls. 

It is important to consider the possible reasons for the association effects which lead to bimodal molecular weight distributions for polymers formed using 90 as initiator in the absence of added Lewis base or lithium alkoxide. Leitz and Hocker [199] proposed that double diphenylethylene-based dilithium initiators form dimeric dianion aggregates (93) and that is why they are soluble in hydrocarbon solutions compared to other dilithium species. This type of dimeric structure is consistent also with the dimeric association... 

The existence of mixed n-butyllithium/potassium alkoxide aggregates is established without any doubt. The simple metatheti-cal exchange between lithium and potassium can be ruled out by comparative experiments. Butylpotassium is known to attack concomitantly the allylic and vinylic protons in alkenes, whereas the n-butyllithium/potassium ferf-butoxide mixture offers greater selectivity and higher yields. ... 

Crotonaldehyde dimethyl acetal (7 Scheme 1) can undergo metallo-dehydrogenation or nucleophilic addition 9 for the example of n-butyllithium, the products of different experimental conditions are shown. The alternative pathways have been modelled computationally by examining the reactions of (7) with methyllithiurn and methylpotassium. The role of the potassium alkoxide in diverting the reaction towards diene is twofold it de-aggregates (RLi) , and promotes a partial cleavage of the carbon-lithium bond. 

Another mixed aggregate complex consisting of Bu°Li and r-butoxide was reported in 1990 as the tetramer (45). Hiis complex was first isolated by Lochmann and has been shown to be tetrameric and dimeric in benzene and THF, respectively, by cryoscopic measurements, and it has also been studied by rapid injection NMR techniques. Hiis species has received much attention because it is related to the synthetically useful superbasic or LiKOR reagents prepared by mixing alkali metal alkoxides with lithium alkyls or lithium amides. ... 

Lithium Alkyls. Organolithium compounds have been widely used as initiators, being readily available and experimentally very convenient. Their aggregated form in hydrocarbon solvents is readily broken down by addition of donor molecules, and initiation becomes fast and efficient. The presence of common impurities such as alkoxides can have a strong influence, almost certainly through cross-association, and may increase the rate of initiation in some solvents while depressing it in others. ... 

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