Nonpairwise mechanism

The mechanism of the reaction remained mysterious for many years. Several early papers, the first by Chauvin, suggested the correct solution, but this was not generally accepted until much later. The question was whether the two alkenes bound to the metal and underwent rearrangement (called the pairwise mechanism), or whether the alkenes reacted one at a time (the nonpairwise mechanism). The Chauvin Mechanism, equation (41), is now the accepted pathway, and was a particularly imaginative suggestion at a time when both the required metalacyclobutane formation and fragmentation reactions and nonhetroatom substituted carbenes were unknown. 

In principle, cyclopentene might metathesize to 1,6-cyclodecadiene (cdd). In fact, a polymer is observed. W hat is the structure of the polymer, and how does its formation, rather than that of cdd, relate to the question of pairwise versus nonpairwise mechanisms ... 

A pairwise mechanism would give cdd, but a nonpairwise mechanism should give polymer. 

The criticai experiment to decide between these two routes, the double cross shown in Eq. 12.10, is a more elaborate form of the crossover experiment. If the reaction is pairwise, then at the beginning of the reaction we will see products from only two of the alkenes (e.g., the C12 and Cic products in Eq. 12.10), not the double-cross product containing fragments of all three alkenes (C14 in Eq. 12.10), which would be expected on the nonpairwise mechanism. The pairwise inedi-anism requires that no Cj4 form initially later on in the reactiiM), dotiUe-cioss products are bound to form, whatever the mechanism, by subsequent m athesis of C12 with C16-... 

The production of C12, C, and Cie was followed over time, and the [Ci4]/[Ci2] and [Ci4]/[Ci6] ratios extrapolated back to time zero. These ratios should be zero for a pairwise pathway, since no C14 should be formed initially. The results instead showed that a nonpairwise mechanism operates because [Ci4]/[Ci2] extrapolated to 0.7 and, more impressively, [Ci4]/[Ci6] was 8.35 for a standard catalyst, MoCl2(NO)2(PPh3)2/ le3Al2Cl3. 

The first report of using RCM to synthesize aromatic compounds seems to be the 1976 report by Katz and Rothchild, who investigated the mechanism of olefin metathesis [3], They synthesized phenanthrene (7) from a 1 1 mixture of 2,2 -divinylbiphenyl (5) and the deuterated derivative 6 by RCM using a Fischer carbene complex (molybdenum catalyst or tungsten catalyst) in one of the experiments to reveal whether metathesis occurred by a pairwise or a nonpairwise mechanism [4] (Scheme 26.1). Although the yield of 7 was only 1 to 2% [5], this experiment clearly demonstrated the possibility of using RCM to synthesize aromatic compounds. 

At present the accepted mechanism is a nonconcerted, nonpairwise process involving metallocarbenes that reversibly react with alkenes to yield metallacyclo-butane intermediates 48,64,68... 

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. 

Unlike all previous mechanisms for olefin metathesis, the metallocyclo-butane mechanism does not involve the pairwise exchange of alkylidene units between two alkenes undergoing dismutation. The nonpairwise exchange of alkenes in the olefin dismutation reaction has now been established (Grubbs et al., 1975 Katz and McGinnis, 1975 Katz and Rothchild, 1976). 

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