The Stereochemistries of Olefin Metatheses

If the conformational analyses above are correct, similar analyses should also account for why 1-methylcyclobutene (67) gives polyiso-prene that has largely the Z configuration (it is 85% Z, 15% ). The analysis for this trisubstituted olefin metathesis focuses on whether reaction proceeds according to Eq. (27) or Eq. (28) and whether R is the 

Opposed to another that is unsubstituted, the m or product should be, as it is, the Z isomer. 

The reason the stereospecificities often are low may be that a carbon-metal bond in the metallocyclobutane cleaves heterolytically forming a 3-metallopropyl cation, the remaining bonds rotate, and the four-mem-bered ring then reforms (63). If the heterolytic cleavage is facilitated by Lewis acids, their presence in quantity will diminish stereospecificity. In accord with this, stereospecificities do diminish with the A1 W ratio when (48) cyclopentene is metathesized at -30°C by WFg + (CaHslgAlaClg. The decrease in the amount of cis double bonds when the temperature is raised in two cyclopentene metatheses might also be a manifestation of this reaction (85, 101). 

I have incorporated into this review ideas and experiments that are the products of my students insights, labor, and skill. James McGinnis, Steven J. Lee, Nancy Acton, Robert Rothchild, and William Hersh did the bulk of the research with me, and Craig Altus, Samuel Hurwitz, Leslie Lobel, and David Treatman, as undergraduates, helped with the laboratory work and the algebra. I am grateful to all of them. 

The National Science Foundation provided financial support for the early part of our work.  

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The stereochemistries of olefin metatheses are analyzed further in Section V. 

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