The Banert cascade is an organic reaction in which an NH-1,2,3-triazole is prepared from a propargyl halide or sulfate and sodium azide in a dioxane- water mixture at elevated temperatures.[1] It is named after Klaus Banert, who first reported the process in 1989.[2] This cascade reaction is unusual because it consists of two consecutive rearrangement reactions.
![Banert cascade](https://upload.wikimedia.org/wikipedia/commons/thumb/3/3d/BanertCascade.png/600px-BanertCascade.png)
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 6 pi electrocyclization. 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 second azide ion on this alkene with more double bond rearrangements and proton abstraction from a proton source.
References
[edit]- ^ Loren, Jon C.; Sharpless, K. Barry (2005). "The Banert Cascade: A Synthetic Sequence to Polyfunctional N H-1,2,3-Triazoles". Synthesis (9): 1514–1520. doi:10.1055/s-2005-869892.
- ^ Banert, Klaus (May 1989). "Reactions of Unsaturated Azides, 6. Synthesis of 1,2,3‐Triazoles from Propargyl Azides by Rearrangement of the Azido Group. – Indication of Short‐Lived Allenyl Azides and Triazafulvenes". Chemische Berichte. 122 (5): 911–918. doi:10.1002/cber.19891220520.