Lithium, alkyls vinyls

The refined grade s fastest growing use is as a commercial extraction solvent and reaction medium. Other uses are as a solvent for radical-free copolymerization of maleic anhydride and an alkyl vinyl ether, and as a solvent for the polymerization of butadiene and isoprene usiag lithium alkyls as catalyst. Other laboratory appHcations include use as a solvent for Grignard reagents, and also for phase-transfer catalysts. 

The difficulties encountered in the early studies of anionic polymerization of methyl methacrylate arose from the unfortunate choice of experimental conditions the use of hydrocarbon solvents and of lithium alkyl initiators. The latter are strong bases. Even at —60 °C they not only initiate the conventional vinyl poly-addition, but attack also the ester group of the monomer yielding a vinyl ketone1, a very reactive monomer, and alkoxide 23). Such a process is described by the scheme. 

Lithium dialkylcopper reagents can be oxidized to symmetrical dimers by O2 at -78°C in THF. The reaction is successful for R = primary and secondary alkyl, vinylic, or aryl. Other oxidizing agents (e.g., nitrobenzene) can be used instead of O2. Vinylic copper reagents dimerize on treatment with oxygen, or simply on standing at 0°C for several days or at 25°C for several hours, to yield LS-dienes." ... 

The activity of transition metal allyl compounds for the polymerization of vinyl monomers has been studied by Ballard, Janes, and Medinger (10) and their results are summarized in Table II. Monomers that polymerize readily with anionic initiators, such as sodium or lithium alkyls, polymerize vigorously with allyl compounds typical of these are acrylonitrile, methyl methacrylate, and the diene isoprene. Vinyl acetate, vinyl chloride, ethyl acrylate, and allylic monomers do not respond to these initiators under the conditions given in Table II. 

Alkyl vinyl tellurides can also be prepared by a sequential reaction of divinylic tel-lurides with lithium and alkyl halides in liquid ammonia. ... 

In view of the fact that alkyl vinyl ethers are that easily metalated, the generation of lithiated vinyl ethers by halogen-lithium or tin-lithium exchange is seldom applied. Nevertheless, 1-lithio-l-methoxyethene 56 can be generated in this way... 

Whereas carbenoid character is definitely present in metalated alkyl vinyl ethers, lithiated alkyl and aryl vinyl sulfides and thioesters, which are easily available by hydrogen-lithium exchange, do not display carbenoid-typical reactions . They rather behave like nucleophilic reagents, so that their discussion is beyond the scope of this overview despite their utility in synthesis The same appiies to various derivatives of enamines, deprotonated in the vinyiic a-nitrogen position - . 

The absence of a-alkylated allylic sulfoxides or the corresponding 2,3-sigmatropically rearranged allylic alcohols as well as of y-alkylated vinylic sulfoxides supports the intermediacy of a vinylic rather than an allylic lithium species. 

One requirement is diat the pKa of die new organometallic compound is lower than die pKa of die starting organometallic. This in essence means that the equilibrium is driven to products by die formation of a more stable anion. This metiiod is commonly used to make vinyl lithiums from vinyl halides and alkyl lithiums and aryl lithiums from aryl halides and alkyl litiiiums because the electron pair in an sp2 orbital of a vinyl or aryl litiiium compound is more stable than the electron pah in an sp3 orbital of an alkyl lithium. 

Negishi and coworkers have shown that trialkylsilylalkynes are able to trap intramolec-ularly alkyl-, vinyl-, allenyl- and aryllithiums126. For instance, allene 366 cyclizes to cyclopentane derivative 367 under treatment with f-BuLi and TMEDA. This is a remarkable example of a carbolithiation reaction initiated by a deprotonation that affords allenyl-lithium 368 which cyclizes onto the alkyne (Scheme 96). 

Although this was one of the first examples of a living anionic polymerization, there are a number of drawbacks. A high level of polar solvent must be present, so that a diene block formed by this process will have a fairly high vinyl (low 1,4) content and the polymerization must generally be executed at a low temperature. It is also difficult to determine quantitatively the initiator concentration, so control of the molecular weight is difficult. A second approach involves addition of 2 mol of a lithium alkyl to a nonpolymerizable diolefin... 

Addition of polar solvents like ethers and amines causes an increase in side chain vinyl content resulting from 1,2 or 3,4 polymerization. This elTect is particularly marked in polymerizations with lithium alkyls, which are the only alkali metal alkyls that are soluble in bulk monomer or hydrocarbon solutions. Polar media also tend to increase the proportion of 1,4 units with trans configurations. 

The quasi-complex compounds, as well as the complexes discussed above, led us into a domain which was nearly terra incognita at that time, namely, the substituted (j3-chloro, j8-alkyl, )3-keto) vinyl organometallics. Thus, the simplest of metal vinyls became the objective of our work, and the vinyl, isopropenyl, propenyl, and styryl derivatives of elements such as Hg, B, Tl, Ge, Sn, Si, P, As, Sb, and Bi were studied in collaboration with Freidlina and Borisov. Apart from the procedures mentioned in Section V, vinyl-lithium and vinyl Grignard compounds were used for the synthesis. Note that the highest valence alkenyl derivatives of the type RjSb were reported by us (181-205). 

With lithium alkyl or aryl initiation in tetrahydrofuran, information is available only on the addition reaction to 1,1-diphenylethylene. Preliminary assessment of the rates showed a wide variation in initiator efficiency [100]. Direct comparison of initiation rates is difficult, however, since the orders in initiator vary between one quarter and unity [101] (Fig. 13) although the order in diphenylethylene is, as expected, unity. The reaction order in lithium compound is 0.27 0.03 (methyl-Li) 0.34 0.1 (vinyl-Li) 0.66 0.04 (phenyl-Li) 1.1 0.2 (benzyl-Li). 

Fig. 13. The reaction of 1,1-diphenylethylene with various lithium compounds in tetrahydrofuran. Variation of rate with formal concentration of lithium alkyl or aryl. (A) n-Butyllithium ( ) benzyllithium (- ) allyllithium (o) methyllithium ( ) vinyl-lithium (0) phenyllithium. Solvent tetrahydrofuran [101].

Unsubstituted vinyl-lithium and vinyl Grignard 14 reagents can be made directly from the halide by oxidative insertion of Li(0) and Mg(0). Vinyl-lithium is available as a 2M solution in THF from Alfa and vinyl magnesium bromide, which must be prepared in THF, is available in THF from Aldrich. These are quite stable o-complexes because alkenyl anions are more stable than saturated alkyl anions. They add as nucleophiles to carbonyl groups, e.g. cyclobutanone to give 15 and prefer direct to conjugate addition with enones to give e.g. 13. We have already used them in enone synthesis (chapter 5). 

The dissociation of the tetramers into a monomer and a trimer seems unlikely because fragments involving an odd number of Li atoms are hardly seen in the mass-spectra of the aggregated alkyl lithiums. Nevertheless, it is probable that monomeric lithium alkyls are present in solutions of the aggregates and these could be the direct initiators of polymerization of vinyl and diene monomers. Their low concentration might be offset by their very high reactivity. ... 

Lithium alkyls initiate polymerization of polar monomers like methyl-methacrylate, vinyl pyridine, acrylo-nitrile, etc. However, these reactions are more complex. The desired addition to the C=C bond is accompanied by other processes, e.g., attack on the -COOEt group with the formation of ketones and lithium methoxide, or in vinyl pyridine polymerization by the metalation of the pyridine moiety. 

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