Lithium alkylating agents

Polymers containing 90-98% of a c 5-1,4-structure can be produced using Ziegler-Natta catalyst systems based on titanium, cobalt or nickel compounds in conjuction with reducing agents such as aluminium alkyls or alkyl halides. Useful rubbers may also be obtained by using lithium alkyl catalysts but in which the cis content is as low as 44%. 

Sloop and Lerner [132] showed that SEI formation can be affected by treatment of the cross-linked polymer, poly-[oxymeth-ylene oligo(oxyethylene)] (PEM) with an alkylating agent. Cross-linked films of PEM do not form a stable interface with lithium however, upon treatment with methyl iodide, / Ej stabilizes at 2000 Hem"1. Such an SEI is characterized by low conductivity, from 10 to 10 Q-Icm2, which is linear over the temperature range of 25-85 °C. 

The important features of this transition structure are (1) the chelation of the methoxy group with the lithium ion, which establishes a rigid structure (2) the interaction of the lithium ion with the bromide leaving group, and (3) the steric effect of the benzyl group, which makes the underside the preferred direction of approach for the alkylating agent. 

Many organoarsenic compounds are prepared by reactions of AsC13 with alkyl group transfer agents such as Grignard reagents, lithium alkyls, or aluminum alkyls. Typical reactions include... 

Scheme 10.11. Reaction between R2CuLi LiX and an alkylating agent R Z. Solvent coordinated to lithium atoms is omitted.

Similar effects are also seen with enolates of simple ketones. For isopropyl phenyl ketone, the inclusion of one equivalent of 12-crown-4 in a DME solution of the lithium enolate changes the C/O-alkylation ratio from 1.2 1 to 1 3, with methyl sulfate as the alkylating agent.50 With methyl iodide as the alkylating agent, C-alkylation is strongly favored with or without 12-crown-4. 

The final step in the sequence can be accomplished with less reactive halides such as hexyl iodide with two modifications to the reaction conditions the lithium dialkylcuprate must b e generated in 1,2-dimethoxyethane instead of diethyl ether and HMPA must be added along with the alkylating agent as illustrated in the following example ... 

The alkylations are performed in the usual way. Deprotonation is achieved with a strong base, usually lithium diisopropylamide or sometimes butyllithium, added to the amide 1 in tetrahydrofuran at low temperature, usually —78 C7. Sometimes a mixture of solvents is used. To ensure complete enolate formation, warming to 20 °C is usually employed. An excess of alkylating agent is added at —78 or — 20 °C and the products 2 and 3 are isolated in the conventional way (see Tables 7 and 8). 

One hydrolytic method that is useful for the preparation of ketones is the hydrolysis of dithianes. 1.3-Dithiane can be alkylated by treatment with butyl lithium followed by an alkylating agent. The two sulfurs flanking the acetal carbon acidify the protons on that carbon such that butyl lithium can remove one giving a sulfur-stabilized anion. This anion reacts with alkyl halides or sulfonates to give alkylated products. This sequence can be repeated to give a bis-alkylated product. Hydrolysis then yields a ketone. Ditliioacetals are much more resistant to hydrolysis than acetals and thus Hg2+ is often used to promote efficient hydrolysis. 

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