Olefins elimination from transition metal alkyls

The thermodynamic driving force of olefin insertion into a metal-hydrogen bond is, of course, equal and with opposite sign to the thermodynamic barrier that prevents /3-hydride elimination from the metal alkyl hydride compounds, that is, the reverse of reaction (91) (R = H see Scheme 1). This decomposition mechanism will thus be more favorable for middle and late transition complexes, where metal-hydrogen bonds are much stronger than metal-alkyl bonds. 

One of the most defining characteristics of the late metal a-diimine polymerization systems is the uniquely branched polyolefins that they afford. This arises from facile p-hydride elimination that late transition metal alkyl complexes undergo. The characteristics of the isomerization process have been the subject of much investigation, particularly with the more easily studied Pd(II) a-diimine system. The process is initiated by P-hydride elimination from the unsaturated alkyl agostic complex 1.17, followed by hydride reinsertion into olefin hydride intermediate 1.18 in a non-regioselective manner (Scheme 5). In doing so, the metal center may migrate... 

When a transition metal alkyl or a metal hydride reacts with olefin molecules to undergo successive insertions, chain growth of a polymer attached to the transition metal takes place. If -hydrogen elimination occurs from the polymer chain, a transition metal hydride coordinated with the olefin derived from the polymer chain will be produced. By displacement of the coordinated olefin from the transition metal by the other monomer olefin, the polymer with an unsaturated terminal bond is liberated with generation of a transition metal hydride coordinated with the olefin. New chain growth will follow from the hydride, with the net result of control of the molecular weight without termination of the polymerization process. The process is in fact a chain transfer process. 

In the preparative section 3.2 devoted to metal-carbene complexes, it is shown how the a-elimination reaction from high oxidation state early-transition-metal-alkyl complexes is one of the general methods of synthesis of Schrock s Ta and Nb alkylidene complexes. The other direction, formation of an alkylidene from an alkylidyne complex, can also be a valuable route to metal alkylidenes. For instance, Schrock s arylamino-tungsten-carbynes can be isomerized to imido-tungsten-carbene by using a catalytic amount of NEts as a base. These compounds are precursors of olefin metathesis catalysts by substitution of the two Cl ligands by bulky alkoxides (dimethoxyethane then decoordinates for steric reasons), and this route was extended to Mo complexes ... 

P-Carbon elimination from early transition metal alkyls has been extensively studied since it is related to chain termination of olefin polymerization. An early example is thermal decomposition of an isobutyllutetium complex, which formed a methyllutetium complex along with various byproducts (Scheme 1.37)... 

This mechanism is quite general for this substitution reaction in transition metal hydride-carbonyl complexes [52]. It is also known for intramolecular oxidative addition of a C-H bond [53], heterobimetallic elimination of methane [54], insertion of olefins [55], silylenes [56], and CO [57] into M-H bonds, extmsion of CO from metal-formyl complexes [11] and coenzyme B12- dependent rearrangements [58]. Likewise, the reduction of alkyl halides by metal hydrides often proceeds according to the ATC mechanism with both H-atom and halogen-atom transfer in the propagation steps [4, 53]. 

Understandably, metal complexes containing alkyl groups that have no hydrogen atoms in the fi position, such as methyl, benzyl, and neopentyl, are more stable than other alkyl derivatives. The decomposition of metal alkyls — the reverse of olefin insertion — is of importance in the transition metal catalyzed isomerization of olefins and as a chain-termination reaction in olefin polymerization. The a elimination reaction should also be mentioned here. It is mainly of importance in W and Mo complexes [Til]. Extraction of an a hydrogen atom from methyl compoimds leads to intermediate alkylidene species ... 

Simpler p-halide eliminations occur from late transition metal catalysts for olefin polymerization (Equations 10.25 and 10.26). Reactions of the cationic palladium-alkyl complexes occur in a similar fashion to the reactions of the cationic group 4 complexes, despite the softer nature of these species. In this case, propylene and the metal chloride are formed. Even a neutral nickel-hydrocarbyl complex (the salicaldimine complex in Equation 10.26) undergoes reactions with vinyl chloride that involve insertion followed by P-chloride elimination. 

Several alkyl transition metal compounds decompose by olefin elimination with transfer of hydrogen from a j8-carbon to the metal atom, as in... 

This process is simply the reverse of the olefin insertion reaction by which several alkyl metal compounds have been made (Section II,D). The elimination of ethylene from the ethylplatinum compound can be reversed under ethylene pressure (50). The elimination reaction appears to be involved in the formation of three extraordinarily reactive transition metal hydrides. 

Finally, note that it is possible to cleave a C-C bond of a ligand by p-alkyl transfer (extrusion of an olefin from the alkyl ligand), bnt only with early transition-metal and rare-earth complexes. This reaction, that parallels the more common P-H elimination, is the reverse of the insertion of an olefin into a metal-alkyl complex and is consequently discussed in Chap. 6 dealing with insertion and extrusion reactions. 

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