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Chapter 16-2 - CHEM 262 Wei You Chapter 16 Aldehydes and...

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11 CHEM 262 Wei You Chapter 16. Aldehydes and Ketones (2)
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22 The Wittig Reaction The Wittig reaction is a very versatile synthetic method for the synthesis of alkenes from aldehydes and ketones Triphenyl- phosphine oxide Methylene- cyclohexane A phosphonium ylide + + - + CH 2 Ph 3 P= O Ph 3 P- CH 2 Cyclohexanone O
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33 Georg Wittig (1897-1987) Nobel Prize in Chemistry 1979 "for their development of the use of boron- and phosphorus-containing compounds, respectively, into important reagents in organic synthesis"
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44 Phosphonium ylides are formed in two steps: Step 1: nucleophilic displacement of iodine by triphenylphosphine Step 2: treatment of the phosphonium salt with a very strong base, most commonly BuLi, NaH, or NaNH2 Ph 3 P CH 3 -I Ph 3 P-CH 3 I Triphenylphosphine + + S N 2 Methyltriphenylphosphonium iodide (an alkyltriphenylphosphine salt) CH 3 CH 2 CH 2 CH 2   Li  + H-CH 2 -PPh 3   I - -  CH 2 -PPh 3 CH 3 CH 2 CH 2 CH 3 LiI A phosphonium ylide Butane Butyllithium + + + + + Wittig Reaction - Phosphonium Ylides
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55 Wittig Reaction Phosphonium ylides react with the C=O group of an aldehyde or ketone to give an alkene
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66 Wittig Reaction Examples: O Ph 3 P + Ph 3 P=O 2-Methyl-2-heptene + Acetone Ph O H Ph 3 P Ph Ph Ph 3 P=O Phenyl- acetaldehyde + + + ( Z )-1-Phenyl-2- butene (87%) ( E )-1-Phenyl-2- butene (13%) Ph O H OEt Ph 3 P O Ph OEt O Ph 3 P=O Ethyl ( E )-4-phenyl-2-butenoate (only the E isomer is formed) + + Phenyl- acetaldehyde
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Synthesis Tips: Use the least sterically hindered alkyl halide to make the ylide
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88 Wittig Reaction: Selectivity Wittig reagents with an anion-stabilizing group, such as a
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