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Hydrazone iodination is anorganic reaction in which ahydrazone is converted into avinyl iodide by reaction ofiodine and a non-nucleophilic base such asDBU.[1][2] First published byDerek Barton in 1962 the reaction is sometimes referred to as theBarton reaction (although there are many different Barton reactions) or, more descriptively, as theBarton vinyl iodine procedure.
The reaction has earlier roots with the 1911 discovery by Wieland and Roseeu that the reaction of hydrazones with iodine alone (without base) results in theazine dimer (structure2 inscheme 1).
In the original Barton publication[3] the reaction was optimized by using a strongguanidine base, the inverse addition of the hydrazone to an iodine solution, and by exclusion of water.
When iodine as anelectrophile is replaced byaromaticselenyl bromides, the corresponding vinyl selenides are obtained:[4]
Thereaction mechanism proposed in the original Barton publication is outlined as follows:
The hydrazone isoxidized by iodine into adiazo intermediate. In the next step, iodine reacts as an electrophile; displacement of nitrogen then generates aniodocarbonium ion. When the reaction site is not sterically hindered, a second iodide can recombine to form thegeminal di-iodide; otherwise anelimination reaction leads to the vinyliodide. When water is present, the reaction product can revert to theketone.
This reaction is related to theShapiro reaction.
An example of this procedure is the reaction of2,2,6-trimethylcyclohexanone to the hydrazone by reaction withhydrazine andtriethylamine inethanol atreflux followed by reaction of the hydrazone withiodine in the presence of2-tert-butyl-1,1,3,3-tetramethylguanidine (cheaper than DBU) indiethyl ether atroom temperature.[5] Another example can be found in theDanishefsky Taxol total synthesis.
In one study[6] it is attempted to trap anyreactive intermediate of this reaction with an internalalkene. When the hydrazone1 inscheme 5 is reacted with iodine andtriethylamine intoluene, the expected reaction product is not the di-iodide10 through path B in afree radical mechanism. Reaction sequence starting from1:halogen addition reaction to di-iodide intermediate2 followed byelimination reaction with loss ofHydrogen iodide to3. In path B another equivalent of iodine reacts to the azo double bond followed by loss of HI and formation of6. The nitrogen to iodine bond is weak andhomolysis gives the nitrogenfree radical7. Loss of nitrogen results in radical species8. The radical position gets transferred to the alkene in9 which later recombines with iodide to10. Note that in absence of the alkene8 would accept an iodide radical and thegeminal di-iodide then loses HI to form the vinyl iodide. The actual process taking place is path A withelimination of HI to thediazo compound4 followed by adiazoalkane 1,3-dipolar cycloaddition to the pyrazoline5 in 85% yield.
