Alkali metal alkyls

The process of anionic polymerisation was first used some 60 or more years ago in the sodium-catalysed production of polybutadiene (Buna Rubbers). Typical catalysts include alkali metals, alkali metal alkyls and sodium naphthalene, and these may be used for opening either a double bond or a ring structure to bring about polymerisation. Although the process is not of major importance with the production of plastics materials, it is very important in the production of synthetic rubbers. In addition the method has certain special features that make it of particular interest. 

Ge, P, As, Sb, and Bi, can also be prepared in this manner." Except for alkali metal alkyls and Grignard reagents, the reaction between RM and M X is the most conunon method for the preparation of organometallic compounds." ... 

Reaction of the same neutral borabenzene-ligand adduct, C5H5B-PMe3, with a transition, rather than an alkali, metal alkyl or amide can furnish r 6-boratabenzene complexes in a single step (Scheme 8).17 This efficient transformation presumably proceeds through initial ir-coordination of CsHsB-PMes to the transition metal, followedby an intramolecular substitution reaction. In contrast to other approaches to the synthesis of T 6-boratabenzene complexes, this synthetic route does not have a parallel in if-cyclopentadienyl chemistry. 

If coordination were not important, the polymerization process would presumably resemble those initiated by alkali metal alkyls. The latter are very effective initiators for the polymerization of such monomers as styrene, butadiene, etc., and it is generally considered that propagation proceeds through intermediates of the type (43)... 

With very few exceptions, alkali metal complexes of phosphinomethanide ligands are synthesized by the deprotonation of a tertiary phosphine bearing an -CHR2 group with an alkali metal alkyl such as BunLi ... 

A number of metal alkyls add readily to double bonds. These include the titanium alkyls, chromium aryls and alkyls, the alkylmanganese carbonyls, acyl-cobalt carbonyls, alkali metal alkyls, the magnesium alkyls, and aluminum alkyls. 

Again several alkyls add—molybdenum, chromium, iron, cobalt, nickel, the alkali metal alkyls and aluminum alkyls react. A tin alkoxide has recently been studied by Russian workers and found to add to acetylenes. Mercury chloride, of course, adds and two cobalt—cobalt bonded compounds add to acetylene. The second is questionable because it dissociates in solution and the reaction may be a radical reaction, one cobalt adding to each end of the triple bond. 

Cobalt hydrocarbonyl, diborane, and aluminum hydrides add, I think, to all of these carbonyl compounds. Of course, there is the well known Grignard reagent and the alkyllithium additions to carbonyl compounds. Aluminum alkyls add, and we could have listed all the other alkali metal alkyls. Recent work has shown that the tin alkoxides add readily to all these derivatives, and similarly, a tin amide adds to most of these carbonyl compounds. 

For the anionic polymerization of methacrylonitrile (MAN), many initiators have been developed, which include alkali-metal alkyls such as butyllithium [42], triphenylmethylsodium [43], phenylisopropylpotassium [43], the disodium salt of living a-methylstyrene tetramer [44], alkali-metal amides [45], alkoxides [46], and hydroxide [47], alkali metal in liquid NH3 [48], quaternary ammonium hydroxide [49], and a silyl ketene acetal coupled with nucleophilic or Lewis acidic catalysts [50]. However, only a single example of the synthesis of PMAN with narrow molecular-weight distribution can be cited, and the reported number-average molecular weights were much higher than those calculated from the stoichiometry of the butyllithium initiator [42]. 

Lithium amides are the most important of the alkali metal amides. This is mainly due to the facility with which they can be prepared in solution by the simple reaction of the amine with commercially available LiBu". An analogous reaction with heavier metal alkyls is much more difficult due to the high reactivity of heavier alkali metal alkyls which attack many solvents. Another advantage of lithium amides is that they tend to be more soluble in hydrocarbons than their heavier element congeners. This is due to the small size (and hence greater polarizing power) of the lithium ion, which induces greater covalent character. 

Scheme 3.8 Metallation reactions of bis(benzene)chromium and bis(toluene)chromium with alkali metal alkyls and magnesium amides ...

Alkali metal alkyls, particularly n-butyl lithium, are the most frequently used reagents to form metallated intermediates.246 247 In certain cases (di- and triphenyl-methane, acetylene and 1-alkynes, cyclopentadiene) alkali metals can be directly applied. Grignard reagents are used to form magnesium acetylides and cyclopenta-dienyl complexes.248 Organolithium compounds with a bulky alkoxide, most notably M-BuLi-ferf-BuOK in THF/hexane mixture, known as the Lochmann-Schlosser reagent or LICKOR superbase, are more active and versatile reagents.249-252... 

Shima, Smid and Szwarc (56) studied the effect of the methyl substitution in the polymerization of butadiene, isoprene and dimethyl-butadiene. They showed that the electron-donating methyl group decreased the rate of polymerization catalysed by polystyrylsodium. This same electron releasing effect of the methyl is seen, since the 3.4-structure, not 1.2-structure, is produced predominantly from isoprene. This results from the anionic propagation mechanism of the alkali metal alkyl catalysed polymerization of dienes which produced 1.2 and 3.4-structures. 

Unlike alcohols, ethers are not acidic and usually do not react with bases. However, exceptionally strong basic reagents, particularly certain alkali-metal alkyls, will react destructively with many ethers ... 

Alkyl/alkoxide metathesis employing OfBu as alkoxide component revealed to be even more complicated and unpredictable [222-224]. The reactivity of well-characterized (CpR)3 Ln(OtBu)/ species towards alkali metal alkyls seems to be quite sensitive to specific reaction conditions, including choice of solvent, number and type of cyclopentadienyl co-ligands or reaction temperature. 

Further support of the contention that a di(alkali metal) alkyl initiated polymerization was found by substituting lithium for sodium and changing the monomer to 2,3-dimethyIbutadiene or piperylene. Schlenk (55) had earlier shown that lithium adds more rapidly than sodium does to olefins, but the rate of addition of the lithium adduct under these conditions is slower than that of the corresponding sodium... 

Note Added in Proof Coordinated anionic polymerization of dienes by alkali metal alkyl catalysts and the effect of cation solvation on mechnism have been discussed in a similar manner by Medvedev and Gantmakher (231a). 

Consistent with the discussion on alkali metal alkyls, the least stereospecific catalysts for vinyl polymerizations should be those which are derived from the least electronegative metals having the weakest p or d bonding orbitals. On this basis, one expects increasing stereospecificity for making isotactic or cis-1,4 products in the order Ba < Sr < < Ca Mg Be, with some variations due to monomer structure. 

Preparation of gem-Dinitroalkanes. A methanolic solution of alkali metal hydroxide (Li, Na, or K) was treated with 10% excess nitroalkane and stirred for 30 minutes. The solution was evaporated to dryness in vacuo, and the alkali metal alkyl nitronate was aned over phosphorus pentoxide at reduced pressure (0.1 mm.) for 24 hours. (Caution nitronate salts may be shock sensitive and have been known to explode after prolonged storage.) A slurry of 3.3 grams (0.02 mole) of lithium 1-nitro-cyclohexane in 50 ml. of acetonitrile was cooled to —40°C., and 2.7 grams (0.02 mole) of nitronium tetrafluoroborate were slowly added. The reaction was not exothermic, and the reaction mixture turned brilliant blue upon adding nitronium tetrafluoroborate. The reaction mixture was stirred for 2 hours at —30° to —40°C., then filtered to give a quantitative yield of lithium tetrafluoroborate. The filtrate was quenched into 100... 

Treatment of alkali metal alkyl or aryltellurolates (Sections) with alkyl halides, aryl halides, alkenes, or alkynes have been nsed to prepare unsymmetrical tellurides, as exemplified by the reaction of sodium phenyl teUurolate or paramethylphenyl tellnrolate with PhC CHCOPh. ... 

Initiation. The three most common kinds of initiators for anionic polymerization are alkali-metal alkyls, metal amides, and elementary alkali metals. 

In initiation by an alkali-metal alkyl, the alkyl links up with the monomer to form a carbanion, leaving the metal as a cation to compensate the negative charge. An example is the initiation of styrene polymerization by butyl lithium [70-72] ... 

The two principal anionic initiation processes are nucleophilic attack on the monomer and electron transfer. Nucleophilic attack is essentially addition of a negatively charged entity to the monomer and involves mainly alkali metal alkyls, living polymers, metal alkoxides, metal amides, and Grignard reagents. The general initiation process is... 

A major interest in narrow distribution polymers is for research and molecular weight calibrations in gel permeation chromatography. Narrow-molecular-weight polystyrenes are made by initiation with alkali metal alkyls that are particularly effective in this application but that only polymerize conjugated monomers like styrene or butadiene. 

Addition of polar solvents like ethers and amines causes an increase in side chain vinyl content resulting from 1,2 or 3,4 polymerization. This elTect is particularly marked in polymerizations with lithium alkyls, which are the only alkali metal alkyls that are soluble in bulk monomer or hydrocarbon solutions. Polar media also tend to increase the proportion of 1,4 units with trans configurations. 

This reaction is valuable in the preparation of certain monoalkyl aromatic hydrocarbons and aliphatic hydrocarbons having quaternary carbon atoms. The organometallic reagents most frequently used are Grignard reagents, zinc alkyls, and alkali-metal alkyls. 

Hydrolysis of alkali-metal alkyl- or arylgermanides yields the corresponding Ge hydrides in reactions most useful for Ge—D bond formation ... 

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