Rhodium-Catalyzed Synthesis of Pyrroles by [4+1] Cycloaddition

Significance

Iwasawa and co-workers report the rhodium(I)-catalyzed [4+1] cycloaddition of α,β-unsaturated imines with terminal alkynes to afford pyrrole derivatives. The best results were obtained using [{RhCl(coe)₂}₂] as catalyst and Cy_3P as ligand. A mechanism involving nucleophilic addition of the imine nitrogen to the carbene carbon of the rhodium vinylidene complex A is proposed. In support of this mechanism, the isolated vinylidene complex {RhCl[=C=C(H)n-Hex](PCy₃)₂} also catalyzed the reaction under similar conditions. Further mechanistic studies have been promised.