Metal alkyl mechanism

Two mechanisms have been proposed for acetylene and substituted acetylene polymerization by transition metal catalysts one is the metal-alkyl mechanism and the other is the metal-carbene mechanism. In general, it has been proposed that the polymerization of acetylenes by Ziegler-Natta catalysts proceeds by the metal-alkyl mechanism, while the metal-carbene mechanism has been accepted for the polymerization of substituted acetylenes by metathesis catalysts whose main components are halides or complexes of group 5 and 6 transition metals. The latter will be discussed in Section III. 

Scheme 2-S Isomerization ofa-olefins by the metal alkyl mechanism...

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

The insertion of a carbonyl group into a metal-alkyl or metal-aryl bond, and the reverse reaction involving decarbonylation of an acyl complex, have been studied from both the synthetic and mechanistic points of view. The mechanism proposed for this type of reaction seems well established and is... 

Leconte and Basset [161-166] proposed two other possible mechanisms (Scheme 39) the first one implies a 1,2 carbon-carbon activation which invokes the de-insertion of a methylidene fragment from a surface metal-alkyl species, and the second implies a 1,3 carbon-carbon bond activation in which the key steps are the formation of a dimetallacyle by y-H activation from a metal-alkyl followed by carbon-carbon bond cleavage via a concerted electron transfer. 

The reactions of oxygen with metal-alkyl derivatives typically involve radical intermediates (73, 74), and a likely mechanism for the reactions of [TpBut]MgR with 02 is illustrated in Scheme 6. Specific... 

The way in which aluminum alkyls and titanium halides combine together to form propagating centers have been discussed in depth for the last 15 years without any one mechanism taking precedence over another (6). The simplest explanation is that aluminum alkyl alkylates the transition metal in the crystal lattice to give a transition metal alkyl center. Polymerization takes place by... 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

Depending on the nature of surface chain growth species, on the other hand, one is confronted mainly with the alkyl mechanism,6 based on the insertion of a methylene species C CHj) into the metal-alkyl bond, or with the alkenyl mechanism,2 wherein a surface vinyl species ( CH=CH2) reacts with a surface methylene ( CII2) to form an allyl species ( CH2CH=CH2). 

However, the practical, direct synthesis of functionalized linear polyolefins via coordination copolymerization olefins with polar monomers (CH2 = CHX) remains a challenging and industrially important goal. In the mid-1990s Brookhart et al. [25, 27] reported that cationic (a-diimine)palladium complexes with weakly coordinating anions catalyze the copolymerization of ethylene with alkylacrylates to afford hyperbranched copolymers with the acrylate functions located almost exclusively at the chain ends, via a chain-walking mechanism that has been meticulously studied and elucidated by Brookhart and his collaborators at DuPont [25, 27], Indeed, this seminal work demonstrated for the first time that the insertion of acrylate monomers into certain late transition metal alkyl species is a surprisingly facile process. It spawned almost a decade of intense research by several groups to understand and advance this new science and to attempt to exploit it commercially [30-33, 61]. 

As indicated in Chapter 8, the production of alkanes, as by-products, frequently accompanies the two-phase metal carbonyl promoted carbonylation of haloalkanes. In the case of the cobalt carbonyl mediated reactions, it has been assumed that both the reductive dehalogenation reactions and the carbonylation reactions proceed via a common initial nucleophilic substitution reaction and that a base-catalysed anionic (or radical) cleavage of the metal-alkyl bond is in competition with the carbonylation step [l]. Although such a mechanism is not entirely satisfactory, there is no evidence for any other intermediate metal carbonyl species. 

A reaction which is rather new and not mentioned in older textbooks is the so-called o-bond metathesis. It is a concerted 2+2 reaction immediately followed by its retrograde reaction giving metathesis. Both late and early transition metal alkyls are prone to this reaction, but for d° early transition metals there is no other mechanism than o-bond metathesis at hand. Many similar reactions such as the reaction of metal alkyls with other HX compounds could be described as if they would follow this pathway, but the use of the term o-bond metathesis is restricted to those reactions in which one reacting species is a metal hydrocarbyl or metal hydride and the other reactant is a hydrocarbon or dihydrogen. In Figure 2.30 the reaction has been depicted. 

In the last decade an enormous revival of late transition catalysts for the polymerisation of alkenes has taken place [45] (remember that the first discovery of Ziegler for ethene polymerisation also concerned nickel and not titanium). The development of these catalysts is due to Brookhart in collaboration with DuPont (Figure 10.28) [46], Detailed low-temperature NMR studies have revealed the mechanism of the reaction [47], Interestingly, the resting state of the catalyst is the ethene-metal-alkyl complex and not the metal-alkyl complex as is the case for the ETM catalysts. For ETM catalysts the alkene complex intermediates are never observed. Thus, the migratory insertion is the rate-determining step (the turnover limiting step , in Brookhart s words) and the reaction rate is independent of the ethene concentration. 

This difference in behavior suggests different mechanisms. As already observed with cyclic alkanes, the initiahon is a C-H bond achvahon (likely via sigma bond metathesis), leading to the corresponding metal-alkyls. With group 4 metals, a... 

Some of these intermediates are analogous to those proposed by Chauvin in olefin metathesis ( Chauvin s mechanism ) [36]. They can be transformed into new olefins and new carbene-hydrides. The subsequent step of the catalytic cycle is then hydride reinsertion into the carbene as well as olefin hydrogenation. The final alkane liberation proceeds via a cleavage of the Ta-alkyl compounds by hydrogen, a process already observed in the hydrogenolysis [10] or possibly via a displacement by the entering alkane by o-bond metathesis [11]. Notably, the catalyst has a triple functionality (i) C-H bond activation to produce a metallo-carbene and an olefin, (ii) olefin metathesis and (iii) hydrogenolysis of the metal-alkyl. 

The most important difference between Chauvin s mechanism for olefin metathesis and the mechanism for alkane metathesis is that the latter applies itself to the reverse reaction of cleavage of alkanes by methane (which has no single C-C bond, see below) and, especially, it is based on a metal hydrido-carbene in equi-Ubrium with a metal-alkyl. 

As has already been indicated, a thorough discussion of the mechanism of metal, metal alkyl-, oxygen- or peroxide-induced polymerizations is beyond the scope of this article. Chiefly for purposes of comparison, it seems worthwhile to present a brief outline of the free radical type of mechanism which best explains these reactions and which probably is valid also for the photochemical and thermal polymerizations, particularly in the lower temperature range. 

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