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Chapter 19 October 19

Chapter 19 October 19 - reactivity and allows for an...

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1 Wittig Reaction: Mechanism of Addition Symmetrical Ketone:
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2 Wittig Reaction: Mechanism of Addition Aldehydes and Unsymmetrical Ketones:
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3 Wittig Reaction Practice Problems:
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4 Wittig reaction E/Z selectivity: General Rule : Wittig reagents with anion­stabilizing substituents, such as a carbonyl group, adjacent to the negative charge are called "stabilized ylides." These are E selesctive. Unstabilized ylides (without adjacent anion stabilizing groups) are Z selective Stabilization of the ylide through resonance decreases its
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Unformatted text preview: reactivity, and allows for an equilibrium to be established during the product determining step that favors the more stable E isomer. 5 Horner­Emmons­Wadsworth Modification: Uses a phosphonate ester derived from an α ­haloester or ketone to generate the Wittig carbanion Treatment of the phosphonoester with a strong base followed by an aldehyde or ketone gives an alkene, in this case an α , β ­ unsaturated ester, ketone, or aldehyde The resulting alkene is almost entirely the E isomer:...
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