Lithium alkyls and aryls

Many other organometaUic compounds also react with carbonyl groups. Lithium alkyls and aryls add to the ester carbonyl group to give either an alcohol or an olefin. Lithium dimethyl cuprate has been used to prepare ketones from esters (41). Tebbe s reagent, Cp2TiCH2AlCl(CH2)2, where Cp = clyclopentadienyl, and other metal carbene complexes can convert the C=0 of esters to C=CR2 (42,43). 

Pyridine reacts with lithium alkyls and aryls under rather vigorous conditions (e.g. xylene at 100°C) to afford 2-alkyl- and 2-aryl-pyridines. The reaction proceeds by way of the corresponding dihydropyridines (e.g. 275 or a tautomer), and these may be isolated at lower temperatures. The less reactive Grignard reagents give poorer yields of the same products. 

This scheme requires the assumption of extremely strong association of all lithium-oiganics down to at least 10-4 molar concentration if the observed reaction orders are to be obeyed. It assumes in agreement with earlier workers that only unassociated lithium alkyls and aryls are reactive. The six-fold association of butyllithium required is in agreement with physical measurements although admittedly these were carried out at much higher concentrations. Morton and co-workers (69) have shown that the polymer molecules are indeed associated into dimers in this system from a quantitative study of the decrease in solution viscosity on removal of the charged species at the ends of the polymer molecules. 

Caution. The lithium alkyl and aryl compounds and the resulting organolanthanides smolder in contact with air and react violently with water. A fire extinguisher should be ready to use nearby. 

The simple kinetics of anionic polymerization described earlier exists because the initiator is converted completely from the inactive form, CA, to the active form, C "A" (or C -bA ) before any propagation reactions take place. However, some initiators (e.g, lithium alkyls and aryls) maintain an equilibrium between the active form and the inactive form. Moreover, this equilibrium may extend to the growing anionic chains also. In such a situation we must write the initiation steps as... 

Lithium alkyls and aryls are associated in solutions, but the nature of the species depends on the nature of the solvent, the steric nature of the organic radical and temperature.27 Cryoscopy and nmr study with 13C, 7Li and resonances28 29 show that in hydrocarbon solvents MeLi, EtLi, -PrLi and some others are hexamers but terf-butylithium, which presumably is too bulky, is only tetrameric. On addition of ethers or amines, or in these as solvent, solvated tetramers are formed. The formation of dimers, or aggregates less than tetramers, seems not to occur. 

With lithium alkyls and aryls, complexes such as Li[ZnPh3]and Li2[CdMe4] may be formed,42 while hydrido complexes such as Li[(Et2Zn)2H] appear to have Zn—H—Zn bridges.43... 

Compounds with As—N and As—As bonds are cleaved by lithium alkyls and aryls (91a, 91b, 91c). 

Arsenic hydrides are useful sources of this t)rpe of compound, reacting with sodium in liquid ammonia 215) or tetrahydrofuran 288), with potassium in dioxane or benzene 218), and with lithium, lithium alkyls and aryls, lithium alkoxides, lithium amides in various solvents 218,280a, 287a). Derivatives such as (C6Hs)2AsK and C HsAsHK can be isolated from dioxane as solvated crystals 163,218). 

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