Cu-Mediated Synthesis of Pyrazoles via 1,3-Dipolar Cycloaddition

Significance

Reported is a one-pot Cu(I)-mediated synthesis of pyrazoles via a 1,3-dipolar cycloaddition reaction of lithium acetylides and α-diazocarbonyl compounds. As expected, the cycloaddition is regioselective, which is consistent with theoretical predictions for cycloadditions involving HOMO-LUMO interactions (e.g., J. Bastide, N. El Ghandour, O. Henri-Rousseau Tetrahedron Lett. 1972, 13, 4225; K. N. Houk Acc. Chem. Res. 1975, 8, 361). Electron-rich and electron-poor acetylenes performed well and the process tolerated many functional groups, for example halides, esters, and nitriles.