Metal carbene/metallacyclobutane mechanism

The metal carbene/metallacyclobutane mechanism of olefin metathesis, as outlined in Section 1.3, was first proposed by Herisson and Chauvin in 1971. By 1975 the evidence in its favour had become so compelling that the earlier pairwise mechanism had been totally discarded. From 1980 onwards well-defined carbene complexes of Ta, Mo, W, Re, and Ru were discovered which would act as initiators without the need for activation by heat, light, or cocatalyst. This in turn led to the spectroscopic detection of the propagating metal-carbene complexes in many systems, to the detection of the intermediate metallacyclobutane complexes in a few cases, and in one case to the detection of the metal-carbene-olefin complex that precedes the formation of the metallacyclobutane complex. In no individual case have all three intermediates been detected at most two have been observed, sometimes one, more often none. After 1980 metallacyclobutane complexes of Ti and Ta were found which would act as initiators at 60°C, but where the intermediate metal carbene complexes could not be detected. 

In this chapter we present the historical development of the metal carbene/ metallacyclobutane mechanism of olefin metathesis. 

The Metal Carbene/Metallacyclobutane Mechanism 3.2 Evidence from cross-metathesis reactions... 

With experimental support for the metal-carbene-mediated mechanism of olehn metathesis, a number of groups initiated studies with isolated metal-carbene and metallacyclobutane complexes. Early work by Chauvin and Katz on the polymerization of strained olefins using Fischer-type carbenes demonstrated the success of such an approach [56], The introduction of high oxidation state alkylidene complexes led to well-defined catalyst in which the propagating species could be observed and studied, such as the tungsten-based systems developed by Osborn, Schrock, and Basset [59,60], The best-studied and useful of these have been the Schrock arylimido alkylidene complexes, and we will return to these later in this chapter. 

Over the past 15 years the understanding of the mechanism of these reactions has been greatly enhanced through the preparation of metal carbene complexes, particularly of Mo, W and Ru, that are both electronically unsaturated (<18e) and coordinatively unsaturated (usually <6 ligands), and which can act directly as initiators of olefin metathesis reactions. The intermediate metallacyclobutane complexes can also occasionally be observed. Furthermore, certain metallacyclobutane complexes can be used as initiators. 

Metathesis of alkene 6 to give the new alkenes 11 and 15 is explanined by the following mechanism. The first step is [2+2] cycloaddition between metal carbene 5 and alkene 6 to generate the metallacyclobutane 7 as an intermediate. The real catalyst 8 is generated by retrocycloaddition of the metallacyclobutane 7. Reaction of 8 with alkene 6 generates the metallacyclobutanes 9 and 10 as intermediates. The intermediate 10 is a nonproductive intermediate, which reproduces 6 and 8, while 9 is a productive intermediate and yields the new alkene 11 and the real catalyst 12. Cycloaddition of 12 to alkene 6 produces the productive intermediate 14, from which the new alkene 15 and the active catalytic species 8 are formed. The intermediate 13 is a nonproductive one. 

The current general understanding of the mechanism operating in cycloolefin metathesis polymerisation leads us towards the acceptance of the structure of active sites in systems with catalysts belonging to the aforesaid three major groups as one that alternates between metal carbene complexes and metallacyclobutanes. 

In this mechanism, a metal carbene complex first reacts with an alkene to form a metallacyclobutane Intermediate. This intermediate can either revert to reactants or form new products because all steps in the process are equilibria, an equilibrium mixture of alkenes results. 

It was also found that the recycled aqueous catalyst solutions were actually more active than the original solutions. This increase in activity was attributed to the in situ formation of ruthenium(II) olefin complexes that were shown, after isolation, to be highly active ROMP catalysts. The mechanism involved in converting these olefin complexes to either metallacyclobutane or metal carbene species remains unknown. 

Green and Rooney81 proposed an alternative mechanism (Scheme 11.13b) that also accounted for Z-N catalysis. The mechanism resembles a metathesis-like pathway by starting with a-elimination to give a metal-carbene hydride followed by cycloaddition with the alkene monomer to form a metallacyclobutane. Reductive elimination finally yields a new metal alkyl with two more carbon atoms in the growing chain. The Green-Rooney mechanism, although plausible overall, requires an a-elimination, a process that is difficult to demonstrate. 

The currently accepted mechanism-- -- - involves the initial formation of a metal-carbene (alkylidene) complex that reacts with an alkene to form a three-center bound n complex. The three-center bound n complex subsequently rearranges to a metallacyclobutane intermediate (Scheme 6.41). 

The preferences of the various pathways are dependent on the catalyst used, specifically the electronic and steric factors involved. The electronic contribution is based on the preference of the metallacycle to have the electron-donating alkyl groups at either the a or the carbon of ftie metallacycle [23]. The steric factors involved in the approach of the olefin to the metal carbene also determine the re-giochemistry of the metallacyclobutane formed. These factors include both steric repulsion of the olefin and carbene substituents from each other and from the ancillary ligands of the metal complex. Paths (b), (c), and (e) in Scheme 6.10 are important to productive ADMET. The relative rates of pathways (c) and (e) will determine the kinetic amount of cis and trans double bonds in the polymer chain. Flowever, in some cases a more thermodynamic ratio of cis to trans olefin isomers is attained after long reaction times, presumably by a trans-metathesis olefin equilibration mechanism [31] (Scheme 6.11). 

The cyclopropanation is initiated by the interaction of the electrophilic metal-carbene species with the jr-system of the olefin (Scheme 4). Two different mechanisms have been proposed for the formation of the cyclopropane ring a concerted pathway (a) or a two-step process via a metallacyclobutane (b). The first pathway (a) resembles the mode of addition of free carbenes to (C=C) double bonds [33] and has been proposed for reactions of metal carbenoids by various authors [7,11]. The principal bonding interaction in this case initially develops between the electrophilic carbenoid C-atom and the Ti-system of the C-C double... 

The reaction is of great interest because the strongest bond in alkene, the C=C bond, is broken during the reaction. The widely accepted so-called Chauvin mechanism [42] suggests that transition-metal carbene complex acts as catalyst by undergoing a [2 -I- 2J cycloaddition reaction with olefin, via a metallacyclobutane intermediate ... 

The most important advance over the past 15 years has been the preparation of numerous well-defined metal carbene complexes which can act directly as initiators of all types of olefin metathesis reaction. These second-generation catalysts allow much closer control and better understanding of the mechanism of the olefin metathesis reaction. The initiating and propagating species can be closely monitored and in some cases the intermediate metallacyclobutane complexes can also be observed. Well-defined metallacyclobutane complexes also can sometimes be used as initiators. 

Cossee type. The mechanism comprised of olefin metathesis type elementary processes involving a metal carhene complex and a metallacyclobutane was established only recently after the development of the chemistry of metal-carbene complexes [129], Understanding of the novel kind of reaction mechanism opened a new horizon in polymerization. Various new types of polymerization of cyclic monomers have been realized by ROMP. An advantage of the process is that the processes are tolerant to polar reactants and solvents enabling ready incorporation of polar substituents into polymers. Another advantage is that the ROMP is living in nature and polymers of narrow molecular weight distributions are available by the method. 

The factors that direct a metallacyclobutane intermediate along one of these pathways are only partially understood. For example, in the case shown in Fig. 4.10, both metathesis and cyclopropanation are mediated by the same tungsten-carbene complex when the reaction is performed in a non-coordinating solvent [31]. These conditions are consistent with reaction via a metallacyclobutane because they allow the olefin to coordinate to the metal center, a prerequisite for [2 -I- 2] cycloaddition. In contrast, only cyclopropanation occurs when the reaction is performed in a coordinating solvent, presumably because formation of a metal-solvent adduct prevents olefin coordination and leaves only a direct carbene transfer mechanism available. 

Herisson and Chanvin proposed that metathesis reactions are catalyzed by carbene (alkyl-idene) complexes that react with alkenes via the formation of a cyclic intermediate, a metallacyclobutane, as shown in Figure 14.24c. In this mechanism, a metal carbene complex first reacts with an alkene to form the metallacyclobutane. This intermediate can either revert to reactants or form new products because all steps in the process are in equilibria, an equilibrium mixture of alkenes results. This non-pairwise mechanism would enable the statistical mixture of products to form from the start by the action of catalytic amounts of the necessary carbene complexes, with both R and R groups, as shown in Figure 14.24c. 

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