Vinyl lithium determination

It appears that the lithiation site is determined by the geometry of the C=N bond. When a second (third ) lithiation is induced, it occurs on the same side as the first - and this is now the more crowded side 46. The regiochemistry is determined by the C=N bond but the stereochemistry remains as before. When the Shapiro reaction is allowed to go to completion, by warming to 0 °C, the unsymmetrical Z-vinyl lithium Z-47 is produced. This is the isomer that cannot be made by the Shapiro reaction from the methyl ketone 48. 

Three-co-ordinate sulphur. The most inorganic example has been the study of the kinetics and mechanism of hydrolysis of dithionites. Kinetics of attack of iodide and of thiourea at the 55 -dimethylsulphinium cation (M62S-NH2+) in water and in aqueous dimethyl sulphoxide indicate pre-equilibrium protonation followed by a rate-determining associative process. The overall activation entropy is negative. Nucleophilic substitution, by vinyl-lithium, at the triphenylsulphonium cation (PhaS ) is also associative. Indeed the species (30) is an intermediate rather than a transition state. Comparison of the stereochemical courses of reactions at tetrahedral phosphorus and at three-co-ordinate sulphur, which is... 

As shown by C, Li coupling constants and multiplicities in the NMR spectra, vinyl-lithium exists in a dimer-tetramer equilibrium in THF. The tetramer undergoes an intra-aggregate C-Li bond exchange process. A comprehensive analysis of the NMR spin-spin coupling constants was made. The Jc c values in vinyl-lithium were determined to be very small,viz 35.0 Hz for the dimer and 36.3 Hz for the tetramer. 

The vinyl lithium concentration of the solution is determined quantitatively by comparing the amount of ethylene formed on hydrolysis with known ethylene standards (Figure 1). Impurities such as acetylene and 1-3 butadiene are separated from ethylene on the second gas chromatograph. 

Hiller and Funke obtained easily dissolvable linear macromolecules of PVS by anionic polymerization of 1,4-DVB up to conversions of 80-90% [230,231]. In these experiments very low concentrations of n-butyl lithium (n-BuLi) were used and tetrahydrofuran (THF) as solvent. The reactions were carried out at -78 °C and for 7 min. The contents of pendant vinyl groups in the polymer were determined by infrared spectroscopy, mercury-II-acetate addition and catalytic... 

In order to get some quantitative information about the activating power of organoelement groups compared with the phenyl residue we did several competition experiments in which 1,3-diphenyl-azaallyl-lithium (i) was reacted in each case with two activated vinyl compounds in the mole ratio 1 1 1 as it is shown in an example in Scheme 7. By those experiments we determined the relative rate constants stated beneath the reaction scheme. 

It will be seen that there is an almost equal distribution of the charge between a and Y positions in THF for the heavier alkali metal counter-ions. If we suppose that increased charge produces an increased reactivity at a given position, then more vinyl unsaturation will be produced in THF than in hydrocarbon solvents and the highest vinyl content with heavier alkali metal counterions. The order in THF is however reversed, i.e. the highest, vinyl structures are produced by lithium catalysis (17) although microstructure determinations in this solvent normally apply to reactions with an appreciable free anion contribution and hence cannot be simply interpreted. In dioxane (18) and diethylether... 

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... 

Medium-c/5 lithium-polybutadiene was first developed by Firestone Tire and Rubber Company in 1955 [86]. Solution polymerization using anionic catalysts is usually based on butyllithium. Alkyllithium initiation does not have the high stereospecificity of the coordination catalysts based on titanium, cobalt, nickel, or neodymium compounds. Polymerization in aliphatic hydrocarbon solvents such as hexane or cyclohexane yields a polymer of about 40 % cis, 50 % trans structure with 10 % 1,2-addition. However, there is no need for higher cis content because a completely amorphous structure is desired for mbber applications the glass transition temperature is determined by the vinyl content. The vinyl content of the polybutadiene can be increased up to 90 % by addition of small amounts of polar substances such as ethers. 

The synthesis of 2-trimethylsilyoxy-l,3-butadiene by treatment of methyl vinyl ketone with chlorotrimethylsilane, lithium bromide and triethylamine in tetrahydrofuran was discussed in section 12.5.6. It was discussed how the stoichiometry of the reaction was determined by canonical analysis of the response surface model, and how this analysis made it possible to establish experimental conditions which afforded a quantitative conversion. However, before the response surface model could be established it was necessary to find a reaction system worth optimizing. 

Rappoport has presented a detailed outline of the mechanisms of the reactions of vinyl halides with nucleophilic reagents. Modena et /. " have provided further evidence in support of a spectrum of transition states for elimination from activated vinyl halides induced by alkoxide bases. Cristol and Whittemore have shown that the stereoselectivity of elimination from vinyl halides is largely determined by the choice of basic reagent alkoxide bases encourage am/-elimination, whereas syn-elimination and alpha-elimination become dominant with lithium alkyls. 

Since the first preparation of stereoregular poly(methyl methacrylate) by Fox et al. and Miller et al. in 1958, a large number of papers have been published on the steieospecific polymerization of methyl methacrylate, while the NMR technique for the determination of microstructure developed by Bovey and Tiers and Nishioka et al. enabled us to accumulate the extensive information on this polymerization. Mostly anionic initiators have been used for the pdymerization. A review on the polymerization by lithium compounds was presented by Bywater In a recent review by Pino and Suter were discussed some of the factors which can influence the stereoregulation in the polymerization of vinyl monomers including a-substituted acrylate. A variety of magnesium and aluminum compounds can be utilized as stereospecific initiators. Besides methyl methacrylate, not only methacrylates with various ester groups, but also a-substituted acrylates, such as a-ethyl- or o-phenyl-acrylate, were also subjected to the stereospecific polymerization by anionic initiator. The stereospecificity in the copolymerization between the monomers described above is also a matter of interest. 

The mechanism for this reduction, shown in the preceding box, involves successive electron transfers from lithium (or sodium) atoms and proton transfers from amines (or ammonia). In the first step, a lithium atom transfers an electron to the aikyne to produce an intermediate that bears a negative charge and has an unpaired electron, called a radical anion. In the second step, an amine transfers a proton to produce a vinylic radical. Then, transfer of another electron gives a vinylic anion. It is this step that determines the stereochemistry of the reaction. The trans-vinyWc anion is formed preferentially because it is more stable the bulky alkyl groups are farther apart. Protonation of the trans-v my ic anion leads to the nam-alkene. 

Addition methods were used for the determination of m-saturated compounds and olefins. The methods are based on the additions of bromine to unsaturated bonds, and the waves for the brominated compounds corresponding to the reduction of o, j8-dibromides (involving elimination) are measured. Their heights are proportional to the concentration of the unsaturated compound. Thus vinylchloride and 1,2-dichloroethylene were transformed into l-chloro-l,2-dibromoethane and l,2-dichloro-l,2-dibromoethane, by the action of a 3 M solution of bromine in methanol saturated with sodium bromide. The excess of bromine was removed with ammonia and the polarographic analysis was performed with sodium sulphite or lithium chloride as a supporting electrolyte. On the other hand, acetylene, vinyl-chloride, 1,2-dichloroethylene and 1,1,2-trichloroethylene were determined ) after a 24 hr reaction with bromine in glacial acetic acid (1 1). The excess bromine was removed with a stream of nitrogen or carbon dioxide. An aliquot portion is diluted (1 10) with a 3 M solution of sodium acetate in 80 per cent acetic acid and after deaeration the curve is recorded. 

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