3BL-Lecture06

3BL-Lecture06 - Welcome to Chemistry 3BL Lecture 6 David...

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Welcome to Chemistry 3BL Lecture 6 David Rabuka
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Wittig Lab Reflections • Questions for the Wittig lab are posted on the Chem 3BL Blackboard site. • Let’s examine the experiment in more detail.
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Wittig Reaction Review • A carbon-carbon bond forming reaction between a carbonyl electrophile (aldehyde or ketone) and an “ylide” nucleophile. • The “ylide” nucleophile is stabilized by resonance (sometimes more than one way.) • Produces an alkene product - possible cis / trans or E / Z isomers. • Products are difficult to purify by recrystallization, so you learned a new purification technique: chromatography. • Georg Wittig (1897-1987), at the University of Heidelberg, won the Nobel Prize in 1979.
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The Wittig Reaction O H Carbonyl Electrophile + Ylide Nucleophile O H H + O P O P O 2 N O O 2 N O O H O P O 2 N O O H O P OCH 3 O
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The Wittig Reaction : Ylide Formation P O Br SN 2 + O P Br H H O O O P O P Br H H – 78 °C + THF + + LiBr O O Si N Si Li Si N Si H (LiHMDS)
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Ylides Can Be “Stabilized” P P "Unstabilized" Ylide O P O P "Stabilized" Ylide O O O P O Ylide stabilization allows us to use a commercial ylide rather than making it in situ or producing it immediately before use.
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Alkene Product - Can Be cis / trans or E/Z Isomers O H H O H H H H O Or cis trans Or a mixture of the two . . . Means it could be: O 2 N O O 2 N O 2 N O O
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Hints for Reading the 1 H-NMR • If there is a mixture of products, then you will see two “singlets” for the methyl group of the methyl-ester. The ratio of the intensities of those singlets is approximately equal to the ratio of the two different products. • Similarly, if there is a mixture of products, then you would see both the cis and corresponding trans protons not too far from each other. Again, the ratio of intensities will be approximately equal to the ratio of the two different products.
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This note was uploaded on 01/26/2009 for the course CHEM 3BL taught by Professor Chunmei during the Fall '08 term at Berkeley.

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3BL-Lecture06 - Welcome to Chemistry 3BL Lecture 6 David...

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