Non-pairwise Mechanisms

Mechanism 3 shows a pathway that was strongly influenced by the results of Herisson and Chauvin and is outlined in Scheme 11.2. Two key intermediates in this pathway are an alkene-metal carbene complex (5) and a metallacyclobu-tane (6), formed through concerted cycloaddition of the M=C and C=C bonds. A highly significant feature of the mechanism, caused by the unsymmetrical structure of 6, is its explanation of randomization early in the course of reaction. The Herisson-Chauvin mechanism does not require a specific pair of alkenes to interact directly for metathesis to occur, hence the name non-pairwise mechanism. 

J. L. Herisson and Y. Chauvin, Makromol. Chern., 1971, 141, 161. This paper does not propose the discrete intermediates, 5 and 6, but it does suggest that carbene complexes could interact with alkenes separately in a non-pairwise manner such that a new alkene and a new carbene complex could form after a bond reorganization. The mechanism shown in Scheme 11.2 is the non-pairwise mechanism that was elucidated after much work by other chemists, and it is discussed later in this section and in Section 11-1-2. 

The early development of Mechanism 3 was bold for its day because Fischer carbene complexes had just been discovered a few years earlier, and alkylidenes were not yet known. The carbene complexes prepared before 1971 also did not catalyze olefin metathesis. With the discovery of Schrock carbene complexes and the demonstration that some alkylidenes could promote metathesis, the non-pairwise mechanism became more plausible (Section 11-1-2). It was, however, the elegant work of Katz and co-workers that provided early substantial support for the Herisson-Chauvin mechanism. 

Despite this strong evidence supporting the non-pairwise mechanism, others objected that the pairwise mechanism could explain the results of Katz s experiment if another step in the pairwise mechanism were rate determining. Consider Scheme 11.4, in which initial single cross metathesis is rapid to form the C12 alkene (7). If 7 sticks to the metal and 4-octene attacks in a rate-determining step... 

Since the definitive experiments of Katz and Grubbs appeared in the literature, the non-pairwise mechanism has become the accepted pathway for metathesis. Subsequent investigations have supported this pathway.1718 Key intermediates in the non-pairwise mechanism are metal-carbene and metallacyclo-butane complexes. Both have been prepared and shown to catalyze metathesis, and more recently both species have actually been observed in the same reaction mixture and shown to interconvert during it, thus offering additional support for Mechanism 3.19 The next section will cover the discovery of discrete metal-carbene complexes that do serve as metathesis catalysts. 

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. 

A variety of experimental evidence showed results consistent with a non-pairwise mechanism involving carbene complexes and a metallacyclobutadiene intermediate. For example, in the Figure 14.28 reaction, the gaseous products H2C=CH2, D2C=CD2, and H2C=CD2 formed in the expected statistical ratio early in the reaction, with no indication that the mixed product H2C=CD2, the first product by the pairwise mechanism, forms before the others." The first direct demonstration that a carbene complex could engage in metathesis involved the carbene complex in Figure 14.29, which was able to replace its carbene ligand with a terminal alkene in a reaction consistent with the non-pairwise mechanism. ... 

Althongh alternatives to the non-pairwise mechanism were proposed, the preponderance of evidence has strongly snpported the role of carbene complexes as catalysts for olefin metathesis. This mechanism, now known as the Chauvin mechanism, is believed to be the pathway of the majority of transition metal-catalyzed olefin metathesis reactions. ... 

Terminal tungsten nitride complexes containing W N bonds have been reported to catalyze nitrile-alkyne cross-metathesis (NACM) by a mechanism that has parallels with the non-pairwise mechanism for olefin metathesis (A. M. Geyer, E. S. Weidner, J. B. Gary, R. L. Gdula, N. C. Kuhlmann, M. J. A. Johnson, B. D. Dunietz, J. W. Kampf, J. Am. Chem. Soc., 2008, 130, 8984.) When p-methoxybenzonitrile and 3-hexyne were mixed in the presence of catalyst candidate N=W(OC(Cp3)2Me)3(DME), two principal products attributable to metathesis were observed. 

Although it is now fully accepted that olefin metathesis ocurrs by this sequence of steps, the mechanism of olefin metathesis was not clear when it was first discovered. A series of papers were published with clever experiments to distinguish between "pairwise" and "non-pairwise" mechanisms. Pathway A of Scheme 21.3 is a "non-pairwise" mechanism because the olefin reacts with a metal complex containing only half of the second olefin reactant. In contrast, paths B and C contain all of both olefins within the coordination sphere of the metal. Several experiments to distinguish between these paths were reported by Grubbs, Casey, and Katz. ... 

If a non-pairwise mechanism occurs, then ail products are formed at the beginning of the reaction. 

Chauvin and Herisson found in 1970, that the initial product distribution in the cross metathesis of cyclopentene and 2-pentene is not in accordance with such a simple pairwise mechanism [30,50]. Therefore, they proposed a novel non-pairwise mechanism with metal carbene complexes as intermediates (5) [50]. 

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