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#1 2012-02-26 09:54:51

orgopete
Administrator

Stereochemistry of Wittig reaction

In a discussion of the preference for cis product formation in a forum, I suggested the following scheme could be used. In retrospect, I think selection of the addition product should follow an appropriate addition scheme, Cram, Cornforth, Felkin, or others. Therefore, describing the intermediate as more stable may simply be a useful device to remember which product should be the intermediate rather than why it formed.
     http://www.curvedarrow.com/chem/ZZ355D061F.jpg

The Horner-Wadsworth-Emmons variation of the Wittig reaction may follow a different kinetic path. If the cyclization is slower than addition, then the addition reaction may revert back to starting materials and the product could be the result of the more selective cyclization/elimination reaction.
     http://www.curvedarrow.com/chem/ZZ2A1DCDFF.jpg

As noted, these reactions may be more complicated than I have indicated. Many reactions that give a selective stereochemical outcome are often subjected to scrutiny to try to find conditions that favor the opposite outcome. The challenge for chemists is to understand what the controlling factors are in a reaction and possibly change them to our advantage. It should be clear that I am describing relative rate results. Therefore, if the cyclization of a Wittig reaction can be slowed, it may result in the kinetic outcome I have suggested for the Horner-Wadsworth-Emmons reaction or visa versa.

This examples are excerpted from A Handbook of Organic Chemistry Mechanisms, available at http://www.curvedarrowpress.com/ or Amazon.com

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