Mn(III)-Catalyzed Synthesis of Pyrroles from 1,3-Dicarbonyls and Vinyl Azides

Significance

After the publication of a limited Cu(II)-catalyzed synthesis of pyrroles from alkoxy-vinylazides and ethyl acetoacetate (Org. Lett. 2008, 10, 313), this group developed a more general and efficient methodology for the synthesis of tri- and tetrasubstituted pyrroles using Mn(III) catalysis. After addition of Mn(III) enolate 1 to the ­vinyl azide, the resulting iminyl radical 2 is suggested to undergo the formation of the five-membered ring in two ways: a) intramolecular addition on the carbonyl group followed by a reduction of the resulting alkoxy radical 3; b) reaction of the radical 2 with [Mn(II)] to get the alkylideneamino­manganese(III) intermediate 5 which attacks the carbonyl group to yield 4. Finally, protonation of 4 followed by dehydration affords the polysubstituted pyrrole. Evidence for these mechanistic proposals was not provided.