➠The conversion of tosyl hydrazone of an aldehyde or a ketone to an alkene with a strong base like LDA or n-BuLi is known as Shapiro Reaction.
➠This reaction is an elimination reaction accompanied by the H-transfer.
➠This reaction generates least substituted alkene.
➠E.g./Mechanism:-
➠All the intermediates have been trapped and the deuterium labelling has shown that hydrogen of alkene comes from water and not from adjacent carbon. Hence H-transfer is intermolecular. E.g.
➠In case of unsymmetrical acyclic ketones, least substituted alkene is formed as the major product. This regioselectivity always depends on the stereochemistry of C=N in the starting hydrazone.
➠There is a strong preference of abstraction of H syn to the aryl sulphonyl group, because it permits the chelation with Lithium ion.
E.g.
➲Note:- Jaha Carbonyl hai waha hi tosyl lagega➩ Jaha tosyl hoga waha hi Lithium lagega➩ Jaha Lithium hoga waha hi new group lagega.
➠With alpha-beta unsaturated hydrazone, addition of nucleophile leads to new alkene in which double bond has been migrated to the position occupied by the hydrazone group.
E.g.
➠Some more important examples of Shapiro Reaction are given below.
End
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