Shapiro Reaction
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MANA TV programme SHAPIRO REACTION P. Kiran Kumar Lecturer in Chemistry SGA Government Degree College Yellamanchili SHAPIRO REACTION Treatment of tosyl hydrazone of an aldehyde or a ketone with a strong base leads to the formation of vinyl anion which on hydrolysis given an olefin. Hydrazine Phenyl Hydrazine Tosyl hydrazide (p-Toluenesulfonyl hydrazide) Tosyl hydrazone Formed by Nucleophilic addition between aldehyde or ketone and Tosyl hydrazide (p-Toluenesulfonyl hydrazide) and subsequent loss of carbonyl oxygen Mechanism Deprotonation of Tosyl hydrazone with a strong base to form Hydrazone aza enolate. Elimination of aryl sulfinate gives an unstable anion. Loss of Nitrogen leads to vinyl anion Hydrazone aza enolate unstable anion Vinyl anion Vinyl anions can be trapped by number various electrophiles 1. Hydrolysis gives an Alkene 2. Reaction with D2O gives Deuterated Alkene 3. Reaction with CO2 gives α, β-unsaturated acid 4. Reaction with formaldehyde gives Primary alcohol Vinyl anionsanions can can be be trapped trapped by by number number various various electrophiles – –Cont’dCont’d 5. Reaction with DMF gives an α, β-unsaturated aldehyde 6. Reaction with Alkyl chloride gives an Alkyl substituted alkene 7. Reaction with (CH3)3SiCl gives a Vinyl Silane Shapiro reaction involving cyclic ketones Cyclohexanone Shapiro reaction involving cyclic ketones Mechanism Mechanisms Mechanisms Mechanism Shapiro reaction involving unsymmetrical ketones unsymmetrical ketones gives predominantly less substituted olefins Shapiro reaction involving unsymmetrical ketones Removal of proton from the more substituted carbon atom Not formed Stability of the carbanion: secondary versus tertiary Secondary carbanion Tertiary carbanion Secondary carbanion more stable than primary carbanion.