Banert cascade

The Banert cascade is an organic reaction in which an NH-1,2,3-triazole is prepared from a propargyl halide or sulphate and sodium azide in a dioxane- water mixture at elevated temperatures. This cascade reaction is unusual because it consists of two consecutive rearrangement reactions.

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The starting material is prepared from propargyl chloride and an aldehyde or ketone such as acetaldehyde. In the first step an azido compound is formed in situ in a nucleophilic displacement of chloride by the azide ion. A (3,3)Sigmatropic reaction takes place between the azide and the alkyne to the allenyl azide. This allene rearranges to the triazafulvene in a 1,3-dipolar cycloaddition. The exocyclic alkene in this intermediate is very electrophilic because the triazole group has a dipole moment of 5 debye. The reaction sequence concludes with nucleophilic attack of a seconde azide ion on this alkene with more double bond rearrangements and proton abstraction from a proton source.

References

• Banert, K. Chem. Ber. 1989, 122, 911.
• Loren, J. C.; Sharpless, K. B. Synthesis 2005, 1514-1520. (doi:10.1055/s-2005-869892)