Degenerate mechanistic pathways

Since it was difficult to make an exclusive choice between bridged (involving 157) and unbridged (involving 158) mechanistic pathways for the degenerate rearrangement and thus the structure of 155 on the basis of NMR data alone, further evidence were derived from a solvolytic study, and the mechanism involving 157 became the preferred explanation for the behavior of 155 in superacid. 

Two of these are the cycloaddition of the methyhdene with ethylene (path E, non-productive), reaction of the methylidene with an internal olefin such that the alkyl substituent on the metallacyclobutane is in the j9-position (path H, non-productive). The other two pathways are the cycloaddition of the alkylidene with an internal olefin to give the trisubstituted metallacyclobutane (path G, frans-metath-esis, non-productive) and the reaction of the alkylidene with a terminal olefin to give the a,a -disubstituted metallacyclobutane (path F), which can be looked at as a chain transfer-type event, albeit not in the sense of a chain polymerization. In this case, the alkylidene is shifted from the end of one chain to the end of another chain. So, assuming that all pathways have somewhat similar rates, the elimination of ethylene will drive the reaction to high polymer. In the case of ADMET, these additional mechanistic pathways do not prevent the polymerization reaction, since these additional pathways are either degenerate or represent processes that do not affect the overall molecular weight distribution of the polymer. 

In mechanistic terms, the PMR states that the pathways for forward and reverse reactions at equilibrium are described by the same energy surface it does not state that the profile of such a surface must be symmetrical with respect to the reaction path 2 Application of the PMR to displacement reactions at phosphorus is aided by Fig. 9, which depicts all of the possible reaction pathways for degenerate ligand exchange at tetracoordinate phosphorus that proceed either via pentacoordinate transition states or via phosphorane intermediates capable of pseudorotation. The letters a and e in Fig. 9... 

The overall mechanistic picture that these experiments paint is summarized in Scheme 10.25. For clarity, the processes involved in catalyst activation (i.e., reactions of neophylidene 11) have been omitted. Ethylene opens two pathways that result in isomerization of the metal center direct epimerization of the metal-methylidene species via an unsubstituted metallacycle and interception of the substrate-bound intermediate via cross metathesis to generate a metal methylidene. At low conversions, when the concentration of ethylene in the system is low, these degenerate processes are not kinetically significant, and the initial enantioselectivity is low. As RCM proceeds and ethylene is generated, the rate of epimerization is increased, which, in turn, increases the enantiomeric excess of the cyclized product. These processes also provide an explanation for why the ultimate stereochemical outcome is not dependent on the diastereomer of the catalyst used. 

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