Wittig - Wittig Reaction 49 The Wittig Reaction Synthesis...

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Wittig Reaction 49 The Wittig Reaction: Synthesis of Alkenes O H Ph 3 P H H H H + Benzyltriphenyl- phophonium chloride mw = 389 g/mol 1 2 4 9-Anthraldehyde mw = 206 g/mol 9-(2-Phenylethenyl)anthracene mw = 280 mp = 100-150º NaOH CH 2 Cl 2 , H 2 O Ph 3 P H 3 The "Wittig Reagent" an "ylide" Cl Intro The “Wittig Reaction” is one of the premier methods for the synthesis of alkenes. It uses a carbonyl compound as an electrophile, which is attacked by a “phosphorus ylide” (the “Wittig reagent”.) While many other routes to alkenes can proceed via elimination reactions (E1 or E2 reactions from alcohols or alkyl halides, for example), in elimination reactions the carbon skeleton is already pre-assembled. In the Wittig reaction, however, two smaller carbon units are conjoined to make the alkene double bond. Thus molecules of increasing size and complexity can be quickly assembled. In addition, there is no ambiguity regarding the site of the double bond. (In contrast to elimination reactions, which often give mixtures of “more substituted” and “less substituted” structural isomers.) The Wittig reaction is nicely complementary to the aldol condensation, in which carbonyl compounds are attacked not by a phosphorus ylide but by an enolate. Aldol condensations always result in “enones”, alkenes with a carbonyl attached. Wittig reactions are more general in that the product carbonyl does not need to have an attached carbonyl. The alkene product 4 that you make today is the one that was used a few weeks ago as the colorizer for the chemiluminscence experiment (it gave the green solution.) R 3 R R 3 O R 2 Ph 3 P R 1 H R 2 R 1 R Br R 1 H R PPh 3 S N 2 Phosphonium Salt Aldehyde or Ketone Alkene General Wittig Reaction: Synthesis of Alkenes R Ph 3 P R 1 The "Wittig Reagent" an "ylide" Base (usually BuLi) Br Mechanism The general mechanism of the Wittig reaction is shown above. The phosphonium ion is deprotonated by base. The positively charged phosphorus atom is a strong electron-withdrawing group, which activates the neighboring carbon atom as a weak acid. For many phosphonium ions, a very strong base (commonly butyl lithium) is required in order to do the deprotonation. The use of such strong base requires moisture-free conditions such as were required for doing the Grignard reaction. In today’s experiment, however, very concentrated
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Wittig Reaction 50 H H Ph 3 P H H 1 OH Ph 3 P H Ph 3 P H 3 O H O Ph 3 P O Ph 3 P O PPh 3 + Triphenylphosphine Oxide 2 4 5 6 7 3' Wittig Mechanism sodium hydroxide is strong enough to do the deprotonation. This is because the carbanion 3 that is produced is stabilized not only by the positive phosphorus, but also by conjugation with the
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Wittig - Wittig Reaction 49 The Wittig Reaction Synthesis...

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