- Shapiro reaction
The Shapiro reaction or tosylhydrazone decomposition is an
organic reaction in which aketone oraldehyde is converted to analkene through an intermediatehydrazone 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 byRobert H. Shapiro in 1975.Reaction mechanism
In a prelude to the actual Shapiro reaction a
ketone or analdehyde is reacted withp-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 animine orhydrazone . 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 acarbanion . The carbanion proceeds in anelimination reaction creating the carbon to carbondouble bond and converting the hydrazone group into a lithium diazonium group. This group expelsnitrogen leaving a vinyllithium compound.cope
Although many secondary reactions exist for this
functional group , in the Shapiro reaction in particularwater is added resulting in hydrolysis to thealkene . [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 instancealkyl halide s. ["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. TheBamford-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 thereaction mechanism now involves acarbene or in aprotic solvent acarbocation .ee also
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Hydrazone iodination References
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