Bases to Form the Wittig Anion

Bases to Form the Wittig Anion - Non-Further Delocalized...

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Bases to Form the Wittig Anion (Phosphorus Ylide) What base strength is needed to form a phosphorus ylide? It depends on any additional stability of the anion provided by resonance forms. In the laboratory you used a benzyl halide to form the phosphonium salt and simple hydroxide to deprotonate it. The reason this worked is the anion produced is highly delocalized (multiple resonance forms), and hence, stable. Stable equates to easy to form. Further Delocalized Wittig Anion in Lab PPh 3 H H OH PPh 3 PPh 3 PPh 3 PPh 3 PPh 3 The benzyl anion has substantial delocalization into the aromatic ring that stabilizes it. Hence, the proton is far more acidic than a proton in a non-conjugated system.
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Unformatted text preview: Non-Further Delocalized Witting Anion in Class PPh 3 H H OH PPh 3 Li PPh 3 The methylene protons on this saturated molecule are not as acidic as the benzyl protons because the anion lacks further resonance forms. Hence, we need a stronger base (e.g., BuLi) for the deprotonation step). Do we need a base as strong as butyl lithium? Probably not, but; a) there is not a lot to choose from; and b) the byproduct of the butyl lithium deprotonation is butane, a hydrocarbon that cannot interfere with the chemistry....
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This note was uploaded on 09/25/2009 for the course CH 223 taught by Professor Brown during the Spring '08 term at N.C. State.

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