Reactions lithium alkyls

The formation of the above anions ("enolate type) depend on equilibria between the carbon compounds, the base, and the solvent. To ensure a substantial concentration of the anionic synthons in solution the pA" of both the conjugated acid of the base and of the solvent must be higher than the pAT -value of the carbon compound. Alkali hydroxides in water (p/T, 16), alkoxides in the corresponding alcohols (pAT, 20), sodium amide in liquid ammonia (pATj 35), dimsyl sodium in dimethyl sulfoxide (pAT, = 35), sodium hydride, lithium amides, or lithium alkyls in ether or hydrocarbon solvents (pAT, > 40) are common combinations used in synthesis. Sometimes the bases (e.g. methoxides, amides, lithium alkyls) react as nucleophiles, in other words they do not abstract a proton, but their anion undergoes addition and substitution reactions with the carbon compound. If such is the case, sterically hindered bases are employed. A few examples are given below (H.O. House, 1972 I. Kuwajima, 1976). 

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 alkyl lithium method gives high yields of -olefins from 17-ketones via the tosylhydrazones. A -Olefins are formed from 6- and 7-ketones. (Compare with the Bamford-Stevens reaction which gives A - and A -olefins, respectively.) In the presence of an excess of alkyl lithium, alkylation may occur. 

Many other routes are now available but the most used involve the reaction of Grignard reagents or lithium alkyls on orthoborates or boron trihalides ... 

Heterocyclic structures analogous to the intermediate complex result from azinium derivatives and amines, hydroxide or alkoxides, or Grignard reagents from quinazoline and orgahometallics, cyanide, bisulfite, etc. from various heterocycles with amide ion, metal hydrides,or lithium alkyls from A-acylazinium compounds and cyanide ion (Reissert compounds) many other examples are known. Factors favorable to nucleophilic addition rather than substitution reactions have been discussed by Albert, who has studied examples of easy covalent hydration of heterocycles. 

For example, the reaction enthalpy for the reduction of PC proceeding at lithium amalgam to form propylene gas and lithium carbonate is estimated to be -I41kcal (molPC)-1 [149]. PC is reduced at noble-metal electrodes at potentials below 1.5 V vs. Li, and yields lithium alkyl carbonates when lithium salts are the supporting electrolytes. Reduction occurs at 0.7-0.8 V vs. Li with Bu4NC104as supporting electrolyte [150],... 

The regiochcmistry for stoichiometric alkylation with butyl(cyano)copper magnesium bromide is the same as that for the copper cyanide catalyzed reaction. The regiochemistry with dibutyl-copper magnesium bromide is also very similar to that of the copper(I) bromide catalyzed reaction. Lithium cuprates do not exhibit y regioselectivity in this biased system. 

E. Reacticms Involving Organometallic Compounds 1. Reaction of lithium alkyls with organic halides... 

Tertiary amines also depolymerize lithium alkyl tetramers and hexamers and can be used to trigger reactions with alkyl halides (Lepley, 1968). However, when triethylamine is used to initiate the butyl... 

Studies of alkyl halide-lithium alkyl reactions have been almost wholly concerned with proton polarization. The one exception to date is an investigation of polarization in the reaction of n-butyl lithium with p-fluorobenzyl chloride giving p,p -difluorobibenzyl (A/E multi-plet) and l-fluoro-4-pentylbenzene (E/A) (Rakshys, 1970). Surprisingly H-polarization is not observed. 

On the basis of reaction-product structures, it might be expected that the reactions of organic halides with sodium naphthalene (Scheme 9) might resemble mechanistically the reactions of organic halides with lithium alkyls. CIDNP studies have shown that they are in fact quite different, in particular in the mechanism by which polarization occurs. The observations are as follows (Garst et al., 1970). 

A methyl group can be placed on cobalt in dicyano[cobyrinic acid heptamethyl ester] when it is treated with excess methyl magnesium iodide (the ester side chains being converted into tertiary alcohol groups). The alkylation could also be achieved using lithium alkyls (176). Presumably this reaction would be successful with any corrinoid which is soluble in solvents compatible with Grignard reagents. 

Further evidence has been adduced for the configurational stability of phosphoranyl radicals. Thus photolysis of iodobenzene in the presence of (11) gave a 95% yield of (12). Reaction of the phosphonium salt (13) with lithium alkyls produces the phosphoranyl radical (14). ... 

The most studied catalyst family of this type are lithium alkyls. With relatively non-bulky substituents, for example nBuLi, the polymerization of MMA is complicated by side reactions.4 0 These may be suppressed if bulkier initiators such as 1,1-diphenylhexyllithium are used,431 especially at low temperature (typically —78 °C), allowing the synthesis of block copolymers.432,433 The addition of bulky lithium alkoxides to alkyllithium initiators also retards the rate of intramolecular cyclization, thus allowing the polymerization temperature to be raised.427 LiCl has been used to similar effect, allowing monodisperse PMMA (Mw/Mn = 1.2) to be prepared at —20 °C.434 Sterically hindered lithium aluminum alkyls have been used at ambient (or higher) temperature to polymerize MMA in a controlled way.435 This process has been termed screened anionic polymerization since the bulky alkyl substituents screen the propagating terminus from side reactions. 

Anionic complexes [R2Au] are present in salts with metal or large quaternary cations, but only a few compounds of this type have been isolated.1,2 Solutions of the lithium salts generated in situ are substrates for addition reactions with alkyl halides leading to alkylgold(m) compounds.26... 

Organometallic compounds vary widely in their properties and reactivity, just as do the elements from which they are produced. These compounds may be lithium alkyls, Grignard reagents, or organotin compounds. Accordingly, there is no universal method for preparing the compounds, but we will present here some of the types of reactions that have been widely employed. 

There is an enormous organometallic chemistry associated with the group IA metals, particularly lithium and sodium. Lithium alkyls can be prepared by the reaction of the metal and an alkyl halide,... 

Lithium alkyls are used in processes such as polymerization and in transfer of alkyl groups in many types of reactions. Some examples are the following ... 

Because beryllium is a Lewis acid, it remains attached to ether that is the solvent in the reaction. Beryllium alkyls can also be produced by the reaction of the chloride with a lithium alkyl,... 

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 9 Reactions of 5a and 6a with lithium alkyls (Ar = 2,6-i-Pr2C6H3)...

The reactions of alkyl lithium compounds, although closely related to those of the Grignard reagents, are somewhat easier to study because of the greater simplicity of the organometallic reagent. They are very fast reactions but some rates have been successfully measured by resort to the flow method. The reaction is second order and a transition state like LXXII has been suggested.891... 

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