Transition metal-alkyl bonds

Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyze the copolymerization of olefins with carbon monoxide to form polyketones.594-596 Sulfur dioxide is an attractive monomer for catalytic copolymerizations with olefins since S02, like CO, is known to undergo facile insertion reactions into a variety of transition metal-alkyl bonds. Indeed, Drent has patented alternating copolymerization of ethylene with S02 using various palladium(II) complexes.597 In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF4 was an effective catalyst for the copolymerization of S02 with ethylene, propylene, and cyclopentene.598 There is a report of the insertion reactions of S02 into PdII-methyl bonds and the attempted spectroscopic detection of the copolymerization of ethylene and S02.599... 

E) Sigma-bond metathesis. Dihydrogen is observed to react with transition-metal-alkyl bonds even when the metal lacks lone pairs. In this case the reaction cannot be explained in terms of the oxidative-addition or reductive-elimination motif. Instead, we can view this reaction as a special type of insertion reaction whereby the ctmr bond pair takes the donor role of the metal lone pair and donates into the cthh antibond. When the M—R bonds are highly polarized as M+R, the process could also be described as a concerted electrophilic H2 activation in which R acts as the base accepting H+. 

At present, however, most experimental evidence supports the mechanism in which propagation takes place at the transition-metal-alkyl bond. Of the different interpretations, the one proposed by Cossee and Arlman303-306 is the most widely accepted.125 254 294... 

The strongest evidence in favor of propagation at the transition metal-alkyl bond is the existence of one-component, that is, metal-alkyl-free polymerization catalysts. Of these systems the Phillips catalyst was studied most thoroughly because of its commercial importance. Originally it was believed that Cr(VI) ions stabilized in the form of surface chromate and perhaps dichromate resulting from the interaction of Cr03 with surface hydroxyl groups above 400°C are the active species in polymerization 286,294... 

Few transition-metal-alkyl bond-dissociation energies are known reliably (16). Potential approaches to the estimation of such dissociation energies encompass the following ... 

Ng, F. T. T., Rempel, G. L., and Halpem, J., 1982, Ligand effects on transition-metal alkyl bond-dissociation energies, J. Am. Chem. Soc. 104 621n623. 

The two-component catalytic systems used for olefin polymerization (Ziegler-Natta catalysts) are combinations of a compound of a IV-VIII group transition metal (catalyst) and an organometallic compound of a I-III group non-transition element (cocatalyst) An active center (AC) of polymerization in these systems is a compound (at the surface in the case of solid catalysts) which contains a transition metal-alkyl bond into which monomer insertion occurs during the propagation reaction. In the case of two-component catalysts an AC is formed by alkylation of a transition metal compound with the cocatalyst, for example ... 

For both types of catalysts polymerization proceeds by insertion of the olefin into an active transition metal alkyl bond ... 

Similar to the CO2 insertion reactions mentioned in 11.3.3.4, 1,2-insertion of organic carbonyls into transition metal-alkyl bonds (excluding copper) is not frequently observed, probably as a result of steric constraints, as well as reduced nucleophilicity of transition metal alkyls. 

It is only during the past ten years that reliable and widely applicable methods for determining homolytic metal-alkyl bond dissociation energies of stable or-ganometallic compounds in solution have been developed and that information about such bond dissociation energies has become available. Today about one hundred transition metal-alkyl bond dissociation energies have been determined, the majority for cobalt-alkyl complexes. Most of these have been from kinetic measurements. The scope, limitations and results of such determinations are discussed. 

A knowledge of such BDE s is important for an understanding of the kinetic and thermodynamic stabilities of organometallic compounds and of the thermodynamics of the many organometallic reactions that involve the formation or dissociation of transition metal-alkyl bonds, for example, insertion or oxidative addition-reductive elimination (Equations 2 and 3, respectively)(1-3). 

In 1982 (23) we described a method of determining transition metal-alkyl bond dissociation energies, based on measuring the kinetics of reaction 1 and using the relation. 

Equilibrium Measurements. Measurements of the temperature dependence of equilibrium constants of reactions involving transition metal-alkyl bond disruption yield values of aH from which BDE s can be deduced through appropriate thermodynamic cycles. The first example of this application involved the determination of the Co-C BDE of Py(DH)2Co-CH(CH )Ph from measurements of the equilibrium constant of reaction 17, according to Equations 17-19 (28,29). 

Olefin Formation Accompanying Homolytic Transition Metal-Alkyl Bond Dissociation... 

Electrophilic metal ions, notably, Hg + can cleave transition metal alkyl bonds relatively easily. Two main pathways have been identified, one of which is attack at the a carbon of the alkyl, which can lead to inversion of configuration... 

When this mechanism was proposed, there was no clear example of insertion of an alkene into a transition metal alkyl bond, which is one of the key steps in the Cossee mechanism. Many complexes CpNiR(alkene) have been prepared, in which alkyl and alkene substituents coexist. When R = Ph, however, insertion does occur. 

Electrophilic metal ions, notably can cleave transition metal alkyl bonds relatively easily. Two main pathways have been identified, one of which is attack at the a carbon of the alkyl, which can lead to inversion of configuration at (bat carbon (Eq. 8.3S). In the other, attack occurs at the metal or at the M—C bond and retention of configuration is found (Eq. 8.36). The difference has been ascribed to the greater basicity of the metal in the iron case. ... 

Transition-metal-alkyl bonds can be formed by a variety of reactions that include metathetical replacement of a halide ion, oxidative addition, and insertion of an alkene into a metal-hydride bond. " A similar set of reactions is available for the synthesis of transition-metal-aryl bonds, although the analogous insertion of a benzyne intermediate into a metal-hydride bond is not particularly viable as a synthetic route. For alkyl complexes that have longer chains than methyl, thermal decomposition to give the metal-hydride complex by a j5-hydrogen transfer reaction is frequently observed at ambient temperature. 

Sulphur Dioxide.—Insertion reactions of sulphur dioxide into transition-metal-alkyl bonds and into metal-carbon bonds have been the subject of review articles. 

Several useful reviews have appeared on different aspects of the metal-carbon or bond. Halpern has collected data on bond dissociation energies of transition metal-alkyl bonds. An extensively referenced review on the breaking of organometallic bonds, R-M, in reactions in which R2, RH, and... 

Reviews on transition-metal-alkyl bond dissociation energies and the formation of C-H bonds by reductive elimination are also relevant to the mechanisms of catalytic processes. It has been proposed that [Ti(Me2PCH2CH2PMe2)(Et)Cl3] contains a direct-bonding interaction between the titanium atom and the j8-C-H system the evidence for such agostic C-H-M bonds in other systems has been reviewed. Obviously, such bonds have important implications in alkane activation and it is particularly exciting that kinetic studies of the exchange reaction shown in equation (1) indicate that a bimolecular pathway predominates. ... 

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