Pyrazolidinones via Amination of Homoenolates Catalyzed by N-Heterocyclic Carbenes

Significance

A mild triazole carbene catalyzed synthesis of pyrazolidinones from cinnamaldehydes and diazenes is reported. Use of β-aryl-substituted cinnamaldehydes afforded lower yields of products compared to the corresponding β-alkyl-substituted aldehydes (60% vs 80%). For the diazenes, electron-rich and electron-poor substituents R³ gave comparable yields. However, a di­azine bearing an electron-poor substituent R² (4-BrC₆H₄) afforded a very low yield (25%) of product. A mechanism is proposed which involves addition of the triazole carbene to cinnamaldehydes to give intermediate D which, upon isomerization and cyclization with expulsion of the carbene catalyst, led to the product. In the presence of catalytic chiral triazolium salt B, pyrazolidinone C was obtained in 61% yield and 90% ee. The scope of this reaction was only modestly explored.