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. The reaction was discovered by Robert H. Shapiro in 1967. The Shapiro reaction was used in the Nicolaou Taxol total synthesis.

In a prelude to the actual Shapiro reaction a ketone or an aldehyde is reacted with p-toluenesulfonylhydrazide to a p-toluenesulfonylhydrazone (or tosylhydrazone) which is an imine or hydrazone. Two equivalents of a strong base, such as n-butyllithium, then abstract first the proton from the hydrazone and then the less acidic proton α to the hydrazone carbon, leaving a carbanion. The carbanion undergoes an elimination reaction producing a carbon–carbon double bond. This tosyl acts as a leaving group in the formation of a diazonium anion, from which a nitrogen molecule is then lost. The result is a vinyllithium at the position where the nitrogen had been attached. This organolithium carbon is both nucleophilic and basic. It can be reacted with various electrophiles or simply neutralized with water or an acid.