Olefin metathesis metal carbene chain

The understanding of the reaction mechanism is directly related to the role of the catalyst, i.e., the transition metal. It is universally accepted that olefin metathesis proceeds via the so-called metal carbene chain mechanism, first proposed by Herisson and Chauvin in 1971 [25]. The propagation reaction involves a transition metal carbene as the active species with a vacant coordination site at the transition metal. The olefin coordinates at this vacant site and subsequently a metalla-cyclobutane intermediate is formed. The metallacycle is unstable and cleaves in the opposite fashion to afford a new metal carbene complex and a new olefin. If this process is repeated often enough, eventually an equilibrium mixture of alkenes will be obtained. 

The kinetics of the olefin metathesis of 2-pentene by (pyri-dine)2Mo(NO)2Cl2 and organoaluminum halides have been measured (56) as first order in the metal and variable order in olefin (seemingly first order at high olefin concentration and up to order 1.7 at low olefin concentration) and were originally interpreted to support the conventional mechanism, but they now also seem in accord with the metal-carbene chain mechanism. 

Catalyzed olefin metathesis reactions are chain reactions with high turnover numbers. In some cases a metal carbene complex can exchange its alkylidene... 

The elucidation of the mechanism for olefin metathesis reactions has provided one of the most challenging problems in organometallic chemistry. In Volume 1 Rooney and Stewart concluded that the carbene chain mechanism is now generally accepted for olefin metathesis reactions, but much remains to be learned about the formation and reactivity of metal-carbene intermediates, metallocycles, and especially the mechanistic aspects of chain initiations. Since that report, systems have been designed that begin to reveal the important mechanistic features of olefin metathesis. 

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). 

In the 1980s, well-defined metal alkylidenes were introduced as catalyst precursors for olefin metathesis [99, 109-111]. Especially for aqueous ROMP, ruthenium alkylidenes represent readily activated, well-defined, easy to handle catalyst precursors respectively initiators (for a living ROMP without chain-transfer, the term initiator appears more appropriate). Whereas in initial work vinyl-substituted carbenes (cf. 16a) were employed [112], more straightforward routes to aryl-substituted carbenes (16b) were soon developed [113]. Today, vinyl-substituted carbenes are also accessible in one-pot procedures [114], and 16a and 16b are both commercially available. 

A significant characteristic of the polymer besides its translational invariance and its stereochemistry is its absorption in the ultraviolet spectrum (after purification by thin-layer chromatography) with a maximum at 245 nm, the same wavelength as that at which 1,1-diphenyl-1-propene exhibits its ultraviolet maximum. This should be expected if the diphenylcarbene moiety of the initiator [Eq. (24)] were attached to the beginning of the polymer chain, as it would be if the mechanism of initiation and metathesis involved the addition in Eq. (4) of a metal-carbene to an olefin. 

Actual metal carbene complex catalysts can be divided into two broad classes, Fischer-type and Schrock-type . The Fischer-type carbene complexes are low-valent and generally characterized by the presence of one or two heteroatoms (O, N, or S) bonded to the carbene carbon. Such complexes do not normally initiate the chain metathesis of olefins, since they are both coordinatively and electronically (18e) saturated. However, they can sometimes be activated for metathesis by heating, or by reaction with a cocatalyst, or photochemically. Some examples are listed in Table 2.1. 

Although acyclic olefins do not generally undergo metathesis with Ir-based catalysts, they can act as chain-transfer agents in the ROMP of norbomene (Rinehart 1965 Porri 1974 Ivin 1979c) also see Ch. 15. It appears therefore that the failure of most Ir-based catalysts to induce self-metathesis of acyclic olefins lies in their inability to generate an initial metal carbene from such olefins. 

In metathesis polymerization, the catalyticaUy active species is a stable metal-carbene bond that is formed between the metal and the alkene. Upon reaction with cycloalkane, a living moiety capable of chain growth is formed. The olefin metathesis reaction mechanism is shown in Scheme 3.18. 

In 1964 Fischer prepared the first stable metal carbene complexes [11], such as W[=C(OMe)Me](CO)5. Six years later Herisson and Chauvin proposed that olefin metathesis reactions were initiated and propagated by complexes of this type in a chain reaction involving the intermediate formation of metallacyclobutane complexes, eq. (4) where [Mt] is a transition-metal atom surrounded by various ligands [12]. 

A major advance in polymerization via metathesis occurred when Schwab et al7 and Schrock unveiled well-defined transition metal catalysts. Molybdenum- and rathenium-based catalysts are highly active in olefin metathesis reactions. The general mechanism that underlies their reactivity is outlined in Figure 2. The catalyst reacts with the alkene via a (2 + 2] cycloaddition reaction to afford a metallocyclobutane intermediate. Cycloreversion with concomitant ring opening relieves strain and unleashes a new metal carbene that reacts with subsequent monomers to yield the polymer chain. 

It is now generally accepted that the chain propagation steps in transition-metal-catalyzed metathesis of olefins involve reaction of an olefin with an intermediate carbene... 

A chain termination with aldehydes as with the Schrock systems is not possible if using Grubbs catalysts. Vinylethers are used to cleave the polymer chain from the metal via formation of Fischer-type carbene complexes, which are metathesis inactive as reported by Grubbs, and an olefinic end group [189,190]. 

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