Mechanism of alkene metathesis

The mechanism described in Scheme 1 (the Chauvin mechanism) is the accepted mechanism of alkene metathesis, and its validity has been demonstrated in two ways. First, classical kinetic studies, including isotopic labeling and crossover experiments performed using poorly defined catalysts, conclusively demonstrated that the carbene mechanism was consistent with the experiments, while the pairwise mechanism was not. More recently, the synthesis of isolable carbene complexes that catalyze the reaction has allowed a more direct observation of the reaction. Each individual step in the Chauvin mechanism has now been observed spectroscopically for several of the well-defined catalyst systems. 

The mechanism of alkene metathesis has been well studied. The generally accepted mechanism is depicted in Scheme 3, using the hypothetical dimerization of 1-butene to 3-hexene as an example, and using the undefined... 

Fig. 27.5 Initial steps in the mechanism of alkene metathesis involving first and second generation Grubbs catalysts. Two possibilities for the formation of the metallocyclobutane intermediates are shown.

The mechanism of alkene metathesis occurs by a series of [2+2] cycloadditions to form metallacyclobutanes from the metal-carbene and the alkene, followed by retro [2+2]... 

AddititMi of PMc3 or quinnuclidine to alkylidene complexes M(OR)2(CHBuO(NAr) [M ss Mo, W Ar = QH3-Pr 2-2,6 R = CMe(CF3)2, CMe2CF3] affords five-coordinate adducts whose behaviour is relevant to the mechanism of alkene metathesis.340 The complexes... 

Many of the leading workers in this field participated in the Lyon symposium on the subject.The mechanism of alkene metathesis proceeds through a metallocyclobutane derivative formed by reaction of an alkene with a metal-carbene complex. Retro-carbene addition yields a product molecule and a new carbene... 

In Section III.D this reaction will be discussed further, particularly in connection with the mechanism of the metathesis of alkenes. 

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

Figure 3.38. Mechanism of olefin metathesis and strategies for the cleavage of alkenes from polymeric supports by olefin metathesis.

Studies involving the specific exchange mechanisms," as well as theoretical calculations" " of alkene metathesis reactions, generally agree that the mechaiusm begins with jt-coordination of the alkene to the electrophihc metal as shown for a general diene in Scheme 5. 

Based upon your knowledge of alkene metathesis, what is the mechanism Exercise 11-4... 

We first encountered alkyne metathesis in Chapter 10 in connection with reactions of metal-carbyne complexes. The mechanism of alkyne metathesis, first proposed by Katz,64 is analogous to that for alkenes, and it is shown in Scheme 11.9. 

In Section 24.12, we introduced alkene (olefin) metathesis, i.e. metal-catalysed reactions in which C=C bonds are redistributed. The importance of alkene and alkyne metathesis was recognized by the award of the 2005 Nobel Prize in Chemistry to Yves Chauvin, Robert H. Grubbs and Richard R. Schrock for the development of the metathesis method in organic synthesis . Examples of alkene metathesis are shown in Figure 27.3. The Chauvin mechanism for metal-catalysed alkene metathesis involves a metal alkyli-dene species and a series of [2 + 2]-cycloadditions and cycloreversions (Figure 27.4). Scheme 27.6 shows the mechanism for alkyne metathesis which involves a high oxidation state metal alkylidyne complex, L M=CR. 

The mechanism of olefin metathesis was originally worked out in the early 1970s by H risson and Chauvin. The mechanism involves the nonpairwise cleavage of C=C bonds that occurs in a [2-I-2] cycloaddition reaction between a carbene and an alkene to form an intermediate metallacyclobutane, as shown in Figure 19.23. The metallo-cyclobutane can open in either direction, such that an equilibrium mixture of alkenes results with the product distribution dictated by the thermodynamic stabilities of the different alkenes. Two of the more important organometallic catalysts for olefin metathesis are shown in Figure 19.24. 

As mentioned earUer the mechanism of alkane metathesis occurs via (i) a C-H bond activation, followed by (ii) metal alkylidene and olefin formation, and finally (iii) the olefin metathesis step and alkene hydrogenation. Thus, it was of interest to commence this transformation with an alkylidene precursor rather than metal... 

During the past two decades, the alkene metathesis reaction has developed from its early appUcations in large-scale processes with heterogeneous and ill-defined catalyst systems to a standard technique in synthetic chemistry and polymer laboratories. The development of well-defined and often bench-stable precatalysts " has been key to the widespread use of alkene metathesis in modem target synthesis projects. The impact of this useful reaction was recognized in 2005 by the award of the Nobel Prize in Chemistry to Yves Chauvin, Robert Gmbbs, and Richard Schrock. Astmc has published an excellent article on the early history of the alkene metathesis reaction, which covers the determination of the mechanism and the rejection of alternative hypotheses, so this early history will not be discussed here. 

This chapter will focus almost exclusively on alkene metathesis catalyzed by well-defined homogeneous ruthenium-catalyst systems and is divided into three sections (1) a discussion of how precatalyst structure affects the rate and mechanism of initiation in two key series of metathesis precatalysts (2) a discussion of how substrate structure, both close to and remote from the alkene termini, affects the rate and selectivity of alkene metathesis in synthetic chemistry and (3) the tools that have been used by experimental and theoretical chemists to study alkene metathesis reactions. In each case, the discussion will be focused on the specific topics interested readers are referred to a recent article which covers a wider range of the mechanistic aspects of alkene metathesis with ruthenium complexes, albeit in less depth. 

The mechanisms in operation for some important classes of alkene metathesis catalyst have been studied density functional theory (DFT) studies have also provided useful insights into reaction mechanisms. In particular, the initiation mechanisms of Grubbs-type and Hoveyda-type complexes have been explored. In each case, insights into the effects of structure on initiation rate have been achieved. 

While studies of mechanism in alkene metathesis have been conducted by a number of researchers, the quantitative exploration of stmcture/activity relationships is relatively rare. 

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