Ch_322b_16.10

Ch_322b_16.10 - The Addition of Ylides: The Wittig Reaction...

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Unformatted text preview: The Addition of Ylides: The Wittig Reaction One of the most useful and general synthetic reactions in organic chemistry is the conversion of aldehydes and ketones into alkenes by the Wittig reaction , discovered by Georg Wittig in 1954 (co-recipient of Nobel Prize in Chemistry in 1979). C = O R R' aldehyde or ketone + (C 6 H 5 ) 3 P C R'' R''' +- a phosphorous ylide R R' C = C R'' R''' alkene + (C 6 H 5 ) 3 P=O triphenylphosphine oxide The classic reaction involves addition of a phosphorous ylide to the carbonyl compound. An ylide is a neutral compound with a carbanion center adjacent to cationic heteroatoms such as P or S. Synthesis of Ylides The ylides required for the Wittig reaction are generally readily available by a two-step synthesis beginning with triphenylphosphine. Step 1. (C 6 H 5 ) 3 P: nucleophile + R''CH-X R''' S N 2 (C 6 H 5 ) 3 P-CH R'' R''' + + X- an alkyltriphenylphosphonium salt Triphenylphosphine reacts with sterically unhindered alkyl halides by an S N 2 mechanism yielding alkyltriphenylphosphonium halide salts , which are analogous to quaternary ammonium halide salts, R 4 N + X- . Step 2....
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Ch_322b_16.10 - The Addition of Ylides: The Wittig Reaction...

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