Disproving the pairwise mechanism for metathesis

In fact, the (C2)-[(C5)n]-(C2) and (C3)-[(C5) ]-(C3) telomers were produced inparallel with the C4 and Cs alkenes, with the constant statistical telomer ratio of 1 2 1. The result was inconsistent with the pairwise mechanism and suggests that the two ends of the telomer arise from two different alkene units thus, even in the early stages of the reaction, the pent-2-ene can generate all three telomer forms. In other words, a sequential rather than pairwise mechanism seems to have been operative. It was proposed, therefore, that the catalyst acts as a carrier for one half of the alkene and thereby allows sequential reactions of one alkene with 

There thenfollowed reports by Katz [13] and Grubbs [14] and their co-workers on studies that aimed to simplify and confirm the analysis. The key remaining issue was whether a modified pairwise mechanism, in which another alkene can coordinate to the metal and equilibrate with the product prior to product displacement, would also explain the appearance of the anomalous cross-over products early in the reaction evolution. However, a statistical kinetic analysis showed that for a 1 1 mixture of equally reactive alkenes, the kinetic ratio of cross-metathesis should be 1 1.6 1 for the pairwise mechanism and 1 2 1 for the Chauvin mechanism. Any equilibration (substrate or product) would, of course, cause an approach towards a statistical distribution (1 2 1) and thus allow no distinction between the mechanisms. 

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