Allylic metal compounds

Dienes and allylarcncs can be prepared by the Pd-catalyzcd coupling of allylic compounds with hard carbon nucleophiles derived from alkenyl and aryl compounds of main group metals. Allylic compounds with various leaving groups can be used. Some of them are unreactive with soft nucleophiles, but... 

Compounds of the type Zr(7r-Cpd)2, Ti(Tr-Cpd)2, and Cr(CaH6)2, were found to be completely inactive with all monomers whereas a significant number of transition metal allyl compounds were found to have weak activity for ethylene polymerization. The latter results are summarized in Table I. Despite the fact that many transition metal allyl compounds are unstable above 0°C, in the presence of monomer, the metal allyl structure... 

The activity of transition metal allyl compounds for the polymerization of vinyl monomers has been studied by Ballard, Janes, and Medinger (10) and their results are summarized in Table II. Monomers that polymerize readily with anionic initiators, such as sodium or lithium alkyls, polymerize vigorously with allyl compounds typical of these are acrylonitrile, methyl methacrylate, and the diene isoprene. Vinyl acetate, vinyl chloride, ethyl acrylate, and allylic monomers do not respond to these initiators under the conditions given in Table II. 

Polymerization of Substituted Transition Metal Allyl Compounds Temperature 50°C, Ethylene Pressure Jfi aims, Ethylene. Catalyst Concentration 24 X 10 M... 

Initial studies of the polymerization of propylene with transition metal allyl compounds suggested that this monomer could not be polymerized by any of the soluble catalysts available. Subsequent work (16) has revealed, however, that the propylene polymerization is much more susceptible to impurities, in particular traces of ether which compete with the monomer for the coordination sites. When this and other impurities are removed, weak activity is detected. These results are summarized in Table XIII. 

A detailed study of the mechanism of the insertion reaction of monomer between the metal-carbon bond requires quantitative information on the kinetics of the process. For this information to be meaningful, studies should be carried out on a homogeneous system. Whereas olefins and compounds such as Zr(benzyl)4 and Cr(2-Me-allyl)3, etc. are very soluble in hydrocarbon solvents, the polymers formed are crystalline and therefore insoluble below the melting temperature of the polyolefine formed. It is therefore not possible to use olefins for kinetic studies. Two completely homogeneous systems have been identified that can be used to study the polymerization quantitatively. These are the polymerization of styrene by Zr(benzyl)4 in toluene (16, 25) and the polymerization of methyl methacrylate by Cr(allyl)3 and Cr(2-Me-allyl)3 (12)- The latter system is unusual since esters normally react with transition metal allyl compounds (10) but a-methyl esters such as methyl methacrylate do not (p. 270) and the only product of reaction is polymethylmethacrylate. Also it has been shown with both systems that polymerization occurs without a change in the oxidation state of the metal. 

It is more difficult to study equilibria between transition metal allyl compounds and bases, olefins, etc. In the case of Zr (allyl) 4 and pyridine, a valency change occurs as shown by Eq. (8), and the process is irreversible. The polymerization is considered to be preceded by displacement of one allyl group by the monomer (12) as shown in Eq. (1). In the methyl methacrylate/Cr(allyl)3 system it was not possible to detect any interaction between the olefin and catalyst with infrared radiation, even with equimolar concentrations because of the strong absorption by the allyl groups not involved in the displacement processes. Due to the latter, evidence for equilibrium between monomer and catalyst is less likely to be found with these compounds than with the transition metal benzyl compounds. 

The structure and chemical properties of metal-allyl compounds (ir-allylic, dynamic and a-allylic) which can be considered as models of a living polymer chain in butadiene polymerization have been studied. The polymerization of dienes proceeds only in dynamic allylic systems through the metal-ligand ir-bond in a-isomers. 

Various possible configurations of such a system may be represented by corresponding model metal allylic compounds. 

According to data reported in the literature at least three types of metal allylic compounds are usually distinguished ... 

It seemed reasonable to investigate some physical and chemical properties as well as the catalytic activity of all types of metal allylic compounds, and hence we chose compounds for our investigations. 

Various types of metal allylic compounds differ significantly in their physical and chemical properties. Dynamic allylic compounds exhibit the highest chemical activity. 

Butadiene Polymerization Initiated by Model Metal-Allylic Compounds... 

Metal allylic compounds, whose structure and reactions have been described in the preceding section, were used as catalysts for diene polymerization. 

Many complexes react with allene to afford metal allyl compounds [equations (6.217)-(6.219)]. 

Some carbonyl complexes may react with allyl halides to give metal allyl compounds. The metal as well as carbon monoxide undergo oxidation ... 

Transition metal complexes containing heteroallyl ligands, such as those in structures (7.79), have been less studied than metal allyl compounds. " Complexes con-... 

Some allyl complxes may be reduced to alkene and the metal. The complex dodeca-2, 6, 10-triene-l,12-diylnickel is reduced by hydrogen with the formation of nickel and dodecane. Many other metal allyl compounds react in a similar manner. 

The alkali metal allyl compounds show splittings that are one order of magnitude smaller. No temperature-dependence was reported or is evident from comparison of different investigations. It was concluded that the allyl magnesium bromide has a o-covalent unsymmetrical structure with a monohapto ligand-metal interaction. A fast, reversible equilibrium (33) interconverts structure [26a] and its tautomeric structure [26b]. In contrast the alkali metal allyl compounds can be described as more or less symmetric 7t-complexes [26c] with trihapto interaction between metal and ligand. 

The values for the small isotope shifts and splittings in the deuteriated alkali metal allyl compounds vary slightly in different investigations, most probably due to different experimental conditions i.e. concentration and temperature. In a 90 MHz nmr reinvestigation of alkali metal allyl compounds, Schlosser and Strahle (1981) report small upheld shifts for both... 

The high electronegativity of the nitrogen atom activates the monomer toward attack by carbanions. The polymerization can be initiated by carbanions [571] by electron-transfer initiators [573] electrochemically [574] by sodium in liquid ammonia [575] by a Grignard reagent [576] by magnesium, beryllium, and lithium compounds [577,578] or by transition metal allyl compounds [579]. 

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