1. Witting Reaction Presented by
IRSHAD AHMAD SHAH
Department of Chemistry
Govt. P.G. College Rajouri

2. Addition of carbon-hetro multiple bonds The Wittig Reaction20

3. Compounds related to imines Enamines 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 22

4. Synthetic method for preparing alkenes.
The Wittig Reaction
Synthetic method for preparing alkenes.
One of the reactants is an aldehyde or ketone.
The other reactant is a phosphorus ylide. 22

5. R is usually C6H5 (phenyl)
Phosphorus ylides
R is usually C6H5 (phenyl)
key point is that carbon is negatively polarized and nucleophilic
(C6H5)3P C + A B
• • –
(C6H5)3P C A B 22

6. Figure 17.9 Charge distribution in a ylide22

7. The Wittig Reaction C O R R' (C6H5)3P C + A B – + C R R' A B (C6H5)3P••
(C6H5)3P C + A B
• • – + C R R' A B
(C6H5)3P O +
• • – •• + 22

8. Example + – O + (C6H5)3P CH2 DMSO + – CH2 + (C6H5)3P O (86%)••
(C6H5)3P
CH2 + – •• +
DMSO
CH2
(C6H5)3P O +
• • – •• +
(86%)
dimethyl sulfoxide (DMSO) or tetrahydrofuran (THF) is the customary solvent 22

9. Mechanism Step 1 C O R R' O C P(C6H5)3 R R' B A P(C6H5)3 + C A B – ••
• •
P(C6H5)3 + C A B – •• 22

10. Mechanism Step 2 O C P(C6H5)3 R R' B A R' R A B C P(C6H5)3 + – O + ••
• • R' R A B C
P(C6H5)3 + – O ••
• • + 22

11. Planning an Alkene Synthesis via the Wittig Reaction20

12. Retrosynthetic AnalysisB
There will be two possible Wittig routes to an alkene.
Analyze the structure retrosynthetically.
Disconnect the doubly bonded carbons. One will come from the aldehyde or ketone, the other from the ylide. 22

13. Retrosynthetic Analysis of Styrene
C6H5CH
CH2
C6H5CH O
(C6H5)3P
CH2 + – •• + 22

14. Retrosynthetic Analysis of Styrene
C6H5CH
CH2
HCH O
(C6H5)3P
CHC6H5 + – •• + 22

15. Retrosynthetic Analysis
Both of the routes are acceptable. 22

16. Ylides are prepared from alkyl halides by a two-stage process.
Preparation of Ylides
Ylides are prepared from alkyl halides by a two-stage process.
The first step is a nucleophilic substitution. Triphenylphosphine is the nucleophile. 22

17. Ylides are prepared from alkyl halides by a two-stage process.
Preparation of Ylides
Ylides are prepared from alkyl halides by a two-stage process.
The first step is a nucleophilic substitution. Triphenylphosphine is the nucleophile.
(C6H5)3P CH A B A +
(C6H5)3P
• • + CH X B + X– 22

18. Preparation of Ylides
In the second step, the phosphonium salt is treated with a strong base in order to remove a proton from the carbon bonded to phosphorus. 22

19. Preparation of Ylides
In the second step, the phosphonium salt is treated with a strong base in order to remove a proton from the carbon bonded to phosphorus.
(C6H5)3P C A B + H A + –
(C6H5)3P C
• • B – H
base
base
• • 22

20. Preparation of Ylides
Typical strong bases include organolithium reagents (RLi), and the conjugate base of dimethyl sulfoxide as its sodium salt [NaCH2S(O)CH3].
(C6H5)3P C A B + H A + –
(C6H5)3P C
• • B – H
base
base
• • 22

21. Thanking you
Wish you all the best