Transition alkyl compounds

Transesterification of methyl methacrylate with the appropriate alcohol is often the preferred method of preparing higher alkyl and functional methacrylates. The reaction is driven to completion by the use of excess methyl methacrylate and by removal of the methyl methacrylate—methanol a2eotrope. A variety of catalysts have been used, including acids and bases and transition-metal compounds such as dialkjitin oxides (57), titanium(IV) alkoxides (58), and zirconium acetoacetate (59). The use of the transition-metal catalysts allows reaction under nearly neutral conditions and is therefore more tolerant of sensitive functionality in the ester alcohol moiety. In addition, transition-metal catalysts often exhibit higher selectivities than acidic catalysts, particularly with respect to by-product ether formation. 

Organometallic chemistry (see p. 1199) is not particularly extensive even though gold alkyls were amongst the first organo-transition metal compounds to be prepared. Those of Au are the most stable in this group, while Cu and Ag (but not Au ) form complexes, of lower stability, with unsaturated hydrocarbons. 

Two-component systems are obtained by the interaction of transition metal compounds of groups IV-VIII of the periodic system with or-ganometallic compounds of groups I-III elements (Ziegler-Natta catalysts). An essential feature of the formation of the propagation centers in these catalysts is the alkylation of the transition metal ions by an organo-metallic cocatalyst. 

D. G. H. Ballard Polymerisation by homogeneous transition metal alkyl compounds, pp. 213-264 (25). 

II. Soluble Transition Metal Alkyl Compounds as Polymerization Catalysts. 266... 

III. Ligand Replacement in Transition Metal Alkyl Compounds and... 

Attempts to synthesize transition metal alkyl compounds have been continuous since 1952 when Herman and Nelson (1) reported the preparation of the compound C H6>Ti(OPri)3 in which the phenyl group was sigma bonded to the metal. This led to the synthesis by Piper and Wilkinson (2) of (jr-Cpd)2 Ti (CH3)2 in 1956 and a large number of compounds of titanium with a wide variety of ligands such as ir-Cpd, CO, pyridine, halogen, etc., all of which were inactive for polymerization. An important development was the synthesis of methyl titanium halides by Beerman and Bestian (3) and Ti(CH3)4 by Berthold and Groh (4). These compounds show weak activity for ethylene polymerization but are unstable at temperatures above — 70°C. At these temperatures polymerizations are difficult and irreproduceable and consequently the polymerization behavior of these compounds has been studied very little. In 1963 Wilke (5) described a new class of transition metal alkyl compounds—x-allyl complexes,... 

The results of polymerizing ethylene using varying sigma-bonded transition metal alkyl compounds are summarized in Table VII. It is evident that none of the catalysts are very active and are comparable with the simple allyl compounds listed in Table I. 

The above substitution effects appear to be independent of the nature of the ligand (16) since the benzyl compounds behave similarly, Table XI. It would appear from these observations that the introduction of anionic ligand would be sufficient to increase activity of transition metal alkyl compounds for polymerization. This, however, is probably an oversimplifica-... 

IV. Heterogeneous Polymerization Catalysts Derived from Transition Metal Alkyl Compounds... 

Transition metal alkyl compounds react with the -OH groups on the surface of silica in a manner similar to that described for the silanol [reaction (13)] and as with the latter more than one type of bonding is possible. Silica dried at 200°C reacts with Zr(allyl)4 to give two molecules of propene per metal atom and utilizing in the course of this process two -OH groups per metal atom. The chemistry of the process is accurately described by the equation... 

Polymerization of Ethylene with Transition Metal Alkyl Compounds in Toluene at 80°C... 

Polymerization of Propylene by Transition Metal Alkyl Compounds Toluene as Solvent, Temperature 65°C. Ethylene Pressure 10 atm (IS, 16)... 

Ballard et al. (15) have found that transition metal alkyl compounds of... 

It has been shown (p. 266) that transition metal alkyl compounds containing Cpd and C6H6 groups, ir-bonded to the metal inactivate the metal center for polymerization. It has also been shown by Nyholm and Aresta (45), in the platinum series, that five- or six-membered rings containing only sigma and ir-carbon-to-metal bonds are very stable compounds. These observations add chemical plausibility to reaction (29). 

Comparison of the homogeneous polymerizations of transition metal alkyl compounds with their heterogeneous equivalents shows that the higher activity of the latter is due to ... 

Olefin epoxidation by alkyl hydroperoxides catalyzed by transition metal compounds occupies an important place among modern catalytic oxidation reactions. This process occurs according to the following stoichiometric equation ... 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

The development of G. N. Lewis s octet rule for the s/p-block elements was strongly influenced by the stoichiometric ratios of atoms found in the common compounds and elemental forms (CH4, CCI4, CO2, CI2, etc.). Let us therefore begin analogously by examining the formulas of the common neutral binary chloride, oxide, and alkyl compounds of transition metals. (Here we substitute alkyl groups for hydrogen because only a small number of binary metal hydrides have been well characterized.)... 

Main-group elements X such as monovalent F, divalent O, and trivalent N are expected to form families of transition-metal compounds MX (M—F fluorides, M=0 oxides, M=N nitrides) that are analogous to the corresponding p-block compounds. In this section we wish to compare the geometries and NBO descriptors of transition-metal halides, oxides, and nitrides briefly with the isovalent hydrocarbon species (that is, we compare fluorides with hydrides or alkyls, oxides with alkylidenes, and nitrides with alkylidynes). However, these substitutions also bring in other important electronic variations whose effects will now be considered. 

Since the discovery by Ziegler and Natta that transition metal complexes, in the presence of aluminum alkyl compounds, can efficiently catalyze the polymerization of ethylene and propylene, significant efforts have been devoted to the development of new catalytic systems for polymerization of olefins. One of the... 

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