Cis-trans Selectivity in Wittig Reactions

[brettlawrie]

With regards to Wittig reactions, how does one know whether the cis or trans isomer is favoured? More specifically, can cis-stilbene be prepared in the attached Wittig reaction? Experimentally, the trans isomer was recovered.

I realize that trans isomers are generally more stable, but am confused as to why certain isomers are produced over their counterpart.

Thank you for your help.

 

[chemisttree]

Make a model of that transition state... the one shown on the lower right. The cis isomer would require the phenyl groups to arrange themselves in a certain way in that structure. Do you think that would be favorable?

 

[brettlawrie]

Make a model of that transition state... the one shown on the lower right. The cis isomer would require the phenyl groups to arrange themselves in a certain way in that structure. Do you think that would be favorable?

First of all, thank you for your help. I've taken your lead (see attachment) and I am lead to believe that the cis transition state arrangements are not favourable. I am still not precisely sure why though (I have never gotten a good grasp of stability/stereochemistry/whatever word describes the genre of this problem).

Having all those big phenyl groups close together on the cis (A), cis (B), and even trans (B) diagrams in my attachment can't be good though...I've got a feeling my question has something to do with steric hindrance or delocalization. If so, could you help me understand how an isomer being sterically hindered or less delocalized makes these isomers less stable?