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17.12 The Wittig Reaction

17.12 The Wittig Reaction. Some reactions of aldehydes and ketones progress beyond the nucleophilic addition stage. Acetal formation Imine formation Compounds related to imines Enamines The Wittig reaction.

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17.12 The Wittig Reaction

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  1. 17.12The Wittig Reaction

  2. Some reactions of aldehydes and ketones progressbeyond the nucleophilic addition stage Acetal formation Imine formation Compounds related to imines Enamines The Wittig reaction

  3. Some reactions of aldehydes and ketones progressbeyond the nucleophilic addition stage Acetal formation Imine formation Compounds related to imines Enamines The Wittig reaction

  4. The Wittig Reaction Synthetic method for preparing alkenes. One of the reactants is an aldehyde or ketone. The other reactant is a phosphorus ylide.

  5. A A + – (C6H5)3P (C6H5)3P C C •• B B Phosphorus ylides R is usually C6H5 (phenyl) key point is that carbon is negatively polarized and nucleophilic

  6. Figure 17.9 Charge distribution in a ylide

  7. R A + – •• C O (C6H5)3P C •• •• B R' R A + – •• C C (C6H5)3P O •• •• B R' The Wittig Reaction + +

  8. + – •• O (C6H5)3P CH2 •• •• + – •• CH2 (C6H5)3P O •• •• Example + dimethyl sulfoxide (DMSO) or tetrahydrofuran (THF) is the customary solvent DMSO + (86%)

  9. R R •• •• C O R' •• •• C O R' A A C P(C6H5)3 + •• P(C6H5)3 C B – B Mechanism Step 1

  10. R •• R – •• R' R' O •• C O •• •• C P(C6H5)3 C A C P(C6H5)3 + B A B Mechanism Step 2 +

  11. 17.13Planning an Alkene Synthesis viathe Wittig Reaction

  12. R A C C B R' Retrosynthetic Analysis There will be two possible Wittig routes toan alkene. Analyze the structure retrosynthetically. Disconnect the doubly bonded carbons. One will come from the aldehyde or ketone, theother from the ylide.

  13. CH2 C6H5CH O + – C6H5CH (C6H5)3P CH2 •• Retrosynthetic Analysis of Styrene +

  14. CH2 C6H5CH O + – HCH (C6H5)3P CHC6H5 •• Retrosynthetic Analysis of Styrene +

  15. Retrosynthetic Analysis Both of the routes are acceptable.

  16. Preparation of Ylides Ylides are prepared from alkyl halides by atwo-stage process. The first step is a nucleophilic substitution.Triphenylphosphine is the nucleophile.

  17. A (C6H5)3P (C6H5)3P CH •• B Preparation of Ylides Ylides are prepared from alkyl halides by atwo-stage process. The first step is a nucleophilic substitution.Triphenylphosphine is the nucleophile. A + + X CH B + X–

  18. Preparation of Ylides In the second step, the phosphonium salt istreated with a strong base in order to removea proton from the carbon bonded to phosphorus.

  19. A + (C6H5)3P C H B base H Preparation of Ylides In the second step, the phosphonium salt istreated with a strong base in order to removea proton from the carbon bonded to phosphorus. A + – (C6H5)3P C •• B – base ••

  20. A + (C6H5)3P C H B base H Preparation of Ylides Typical strong bases include organolithium reagents (RLi), and the conjugate base of dimethyl sulfoxide as its sodium salt[NaCH2S(O)CH3]. A + – (C6H5)3P C •• B – base ••

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