Shapiro reaction

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. [Robert H. Shapiro ; Lipton, M.F.; Kolonko, K.J.; Buswell, R.L.; Capuano, L.A. "Tetrahedron Lett.", 1975, 1811. doi|10.1016/S0040-4039(00)75263-4 ] [Shapiro, R. H. "Org. React.", 1976, "23", 405. (Review)] [Adlington, R.M.; Barret, A.G.M. "Acc. Chem. Res.", 1983, "16", 55. (Review)] The reaction was discovered by Robert H. Shapiro in 1975.

Reaction mechanism

In a prelude to the actual Shapiro reaction a ketone or an aldehyde is reacted with p-toluenesulfonylhydrazide ["Organic Syntheses" Coll. Vol. 5, p.1055 (1973); Vol. 40, p.93 (1960) ( [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv5p1055 Article] )] 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.

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Although many secondary reactions exist for this functional group, in the Shapiro reaction in particular water is added resulting in hydrolysis to the alkene. [Shapiro, R. H.; Duncan, J. H. "Organic Syntheses" Coll. Vol. 6, p.172 (1988); Vol. 51, p.66 (1971). ( [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0172 Article] )] Other reactions of vinyllithium compounds include alkylation reactions with for instance alkyl halides. ["Organic Syntheses" Coll. Vol. 7, p.77 (1990); Vol. 61, p.141 (1983). ( [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv7p0077 Article] )] 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.

ee also

*Hydrazone iodination

References


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