Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Named Reactions/Shapiro Reaction
This page was imported and needs to be de-wikified. Books should use wikilinks rather sparsely, and only to reference technical or esoteric terms that are critical to understanding the content. Most if not all wikilinks should simply be removed. Please remove {{dewikify}} after the page is dewikified. |
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] The reaction was discovered by Robert H. Shapiro in 1975. The Shapiro reaction was used in the Nicolaou Taxol total synthesis.
Reaction mechanism
[edit | edit source]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
[edit | edit source]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.
References
[edit | edit source]- ↑ 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)
- ↑ Organic Syntheses Coll. Vol. 5, p.1055 (1973); Vol. 40, p.93 (1960) (Article)
- ↑ Shapiro, R. H.; Duncan, J. H. Organic Syntheses Coll. Vol. 6, p.172 (1988); Vol. 51, p.66 (1971). (Article)
- ↑ Organic Syntheses Coll. Vol. 7, p.77 (1990); Vol. 61, p.141 (1983). (Article)