[OrganicSyn1.0]

So...lets open pandora's box of chemistry here. lol I am curious to hear individuals opinions (considering the huge amount of literature) on the Wittig reaction mechanism (concerted cycloaddition vs betaine intermediary or both). Also I'm curious to see what people have found in terms of salts effects on stereoselectivity. Lets start!

[OC pro]

I don´t think there is much to discuss. Both mechanisms have their charme. The only thing which counted for me in the past that this rxn worked out pretty well in most cases.

[OrganicSyn1.0]

Ok. Well in all fairness the Wittig rxn is pretty complex when trying to determine a mechanistic explanation for product stereoselectivity (E:Z ratios). So many factors contribute to final product stereochemistry such as: solvent, salt content (ion size as well), electron donating groups, electron withdrawing groups, sterics of aldehyde/ketone, electrophilicity of carbonyl carbon, thermodynamics, kinetics, ylide stability, enthalpy etc. Here's some journal articles discussing  various aspects of the wittig that are still under debate:
http://www.t-kougei.ac.jp/research/pdf/vol1-29-08.pdf
http://pubs.acs.org/doi/abs/10.1021/ja00166a026
http://pubs.acs.org/doi/abs/10.1021/jo00251a032
There are many many more articles investigating stereoselectivity of the wittig reaction.