Tertiary alkyllithium preparation

The choice of solvent for reaction (a) is important. Alkyllithiums (except CH3Li) react with ethers. Primary alkyllithiums may be prepared in ether if storage of the reagent is not necessary, or in the even more reactive tetrahydrofuran (THF) at low T (ca. — 50°C) when the reagent is to be used promptly. For more reactive secondary and tertiary alkyllithiums, hydrocarbon solvents are used. The less reactive methyl-, aryl- and vinyllithiums are prepared in ethers. 

Tertiary alkyllithiums are difficult to prepare by reaction (a), but optimum conditions for the preparation of t-butyllithium and other tertiary alkyllithiums are available " (see below). 

Tertiary alkyllithiums are prepared in hydrocarbons from the appropriate alkyl chlorides with Li dispersions. Once the reaction is initiated, secondary reactions between the reagent and its precursor halide may cause low yields. Slow rates of addition of the alkyl chloride minimize these secondary reactions . Polycyclic and bridgehead reagents are less susceptable to such bimolecular reactions with their precursor chlorides. 

The cleavage of alkyl phenyl sulfides by naphthalenelithium or a lithium dispersion in THF to afford alkyllithium reagents has been studied initially by Screttas and Micha-Screttas [309,310) as part of their hydrolithiation of a-olefins process. They prepared primary, secondary and tertiary alkyllithium reagents in fair to good yields, as shown by carbonation. 

In alkyllithium initiated, solution polymerization of dienes, some polymerization conditions affect the configurations more than others. In general, the stereochemistry of polybutadiene and polyisoprene respond to the same variables Thus, solvent has a profound influence on the stereochemistry of polydienes when initiated with alkyllithium. Polymerization of isoprene in nonpolar solvents results largely in cis-unsaturation (70-90 percent) whereas in the case of butadiene, the polymer exhibits about equal amounts of cis- and trans-unsaturation. Aromatic solvents such as toluene tend to increase the 1,2 or 3,4 linkages. Polymers prepared in the presence of active polar compounds such as ethers, tertiary amines or sulfides show increased 1,2 (or 3,4 in the case of isoprene) and trans unsaturation.4. 1P U It appears that the solvent influences the ionic character of the propagating ion pair which in turn determines the stereochemistry. 

It was also discovered at Phillips. that the four rate constants discussed above can be altered by the addition of small amounts of an ether or a tertiary amine resulting in reduction or elimination of the block formation. Figures 13 and 14 illustrate the effect of diethyl ether on the rate of copolymerization and on the incorporation of styrene in the copolymer. Indeed, random copolymers of butadiene and styrene or isoprene and styrene can be prepared by using alkyllithium as initiator in the presence of small amounts of an ether or a tertiary amine. 

The first total synthesis of the Stemona alkaloid (-)-tuberostemonine was accomplished by P. Wipf and co-workers. " The installation of the butyrolactone moiety commenced with the preparation of a Weinreb s amide from a methyl ester. The tricyclic methyl ester substrate was exposed to A/,0-dimethylhydroxylamine hydrochloride and Me2AICI and the tertiary amide was isolated in excellent yield. Next, the bromo ortho ester was treated with LDBB in THF to generate the corresponding primary alkyllithium species, which cleanly and efficiently added to the Weinreb s amide to afford the desired ketone. 

Langer (13) has also disclosed the use of alkyllithium and dialkyl-magnesium tertiary diamine complexes as catalysts for copolymerization of ethylene and other monomers such as butadiene, styrene, and acrylonitrile to form block polymers. Examples are given in which polybuta-dienyllithium initiates a polyethylene block, as well as vice-versa. Random copolymers of these two were also prepared, and other investigators have used not only tertiary diamines but hexamethylphosphoramide (14) and tetramethylurea (15) as nitrogenous base cocatalysts in such polymerizations. Antkowiak and co-workers (11) showed the similarity of action of diglyme and TMEDA in copolymerizations of styrene and... 

Shapiro reaction. (2, 418-419 6, 598-600). Several laboratories have used EDA instead of an alkyllithium for decomposition of tosylhydrazones of ketones to olefins. Trisubstituted alkenes can be prepared by this modification in moderate yields from tosylhydrazones that contain only tertiary a-hydrogens. This modification also favors formation of the (Z)-disubstituted olefin. ... 

This difficulty does not arise with tertiary amines since the C—H bonds adjacent to nitrogen are less susceptible to nucleophilic attack than those next to oxygen in ethers. TMEDA (p. 38) complexes strongly with alkyllithiums. The n-butyllithium chelate is monomeric and very soluble in hydrocarbons. It is a very strong metallating agent. It reacts with toluene at room temperature and more slowly, with benzene. Butyllithium in the absence of TMEDA does not normally attack benzene or toluene and can even be prepared in these solvents. 

Polymers terminated with tertiary amino groups have been generated by reacting living polymers with a,cu-alkylene chloroamines. Living PB prepared with a tertiary amino-functionalized alkyllithium initiator was terminated in this way (Scheme 36). ... 

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