Stereochemical Aspects of the Olefin Metathesis Reaction

One of the most intriguing features of olefin metathesis, and an aspect which continues to receive increasing attention, is the stereochemistry of the reaction. Rationalizations of stereochemical peculiarities have undergone a metamorphosis which parallels advances of mechanistic theories from the pairwise scrambling schemes to the now-popular car-bene-to-metallocycle scheme. As yet there exists no unified stereochemical model which can adequately account for all the observed results. 

Substantial data exist in the literature to demonstrate that the metathesis reaction itself is a means for cis trans interconversions. For the purpose of this discussion, it is assumed that metathesis is the only pathway for cis trans isomerization. 

Stereospecificity as applied to olefin metathesis may be considered in two ways (a) How does the cis/trans isomer ratio of a product olefin compare with its equilibrium ratio, or (b) how does this cis/trans value differ from 1.0, which is the statistically expected value in terms of probabilities. In the present discussion, the latter definition applies. 

With but few notable exceptions (75-77), an inherent characteristic of the metathesis of acyclic olefins with both homogeneous and heterogeneous catalysts is the tendency for attainment of thermodynamic equilibrium in the composition of cis and trans isomers in reactions carried to high conversion. Therefore, any inherent stereospecificity can only be evaluated by extrapolating compositional data to zero percent reaction. 

Any acceptable account of the steric course of olefin metathesis must also take into consideration the following experimentally observed facts  

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