Shapiro reaction

The Shapiro reaction or tosylhydrazone decomposition is an organic reaction in which a ketone or aldehyde is converted to an alkene through an intermediate hydrazone in the presence of 2 equivalents of strong base.^[1][2][3]

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Reaction mechanism

In a prelude to the actual Shapiro reaction a ketone or an aldehyde is reacted with p-toluenesulfonylhydrazide^[4] to a p-toluenesulfonylhydrazone (or tosyl hydrazone) which is an imine or hydrazone. Two equivalents of a strong base such as n-butyllithium then abstract first the hydrazone proton and then the less acidic α carbonyl proton leaving a carbanion. The carbanion proceeds in an elimination reaction creating the carbon to carbon double bond and converting the hydrazone group into a lithium diazonium group. This group expels nitrogen leaving a vinyllithium compound.

Scope

Although many secondary reactions exist for this functional group, in the Shapiro reaction in particular water is added resulting in hydrolysis to the alkene.^[5] Other reactions of vinyllithium compounds include alkylation reactions with for instance alkyl halides.^[6] In general the least substituted alkene is the preferred reaction product.

The Bamford-Stevens reaction is also a tosylhydrazone decomposition but in this reaction a much weaker base can only abstract the hydrazone proton and not the α carbonyl proton and the reaction mechanism now involves a carbene or in a protic solvent a carbocation.

See also

• Hydrazone iodination

References