Catalytic annulation of heterocycles via a novel redox process involving the imidazolium salt N-heterocyclic carbene couple

Abstract

A novel atom-efficient catalytic reaction, which converts imidazolium salts, with N-butenyl, N-substituted butenyl, and N-pentenyl substituents, into five- and six-membered fused-ring imidazolium and thiazolium salts has been developed. The reaction proceeds through azolium, C2-H, oxidative addition to Ni(O) followed by intramolecular insertion of the N-alkenyl double bond into the Ni hydride to give an intramolecularly bound carbene-Ni-alkyl intermediate. Reductive elimination of the linked carbene and alkyl groups gives the fused-ring azolium products and regenerates the Ni(0) catalyst. Products are potential building blocks for the synthesis of pharmaceuticals and novel ionic liquids. For example, 1,7-dimethyl-6,7-dihydro-5H-pyrrole[l,2-α] imidazolium bromide (2f), a five-membered fused-ring imidazolium salt, is formed from the catalytic ring closing of l-butenyl-3-methylimidazolium bromide (1f). The reaction proceeds at moderate temperatures (50 °C) to give the products in high yield and selectivity. The catalyst was formed in situ from Ni(COD) 2 plus added ligand L (where L = IMes, SMes, IPr, SPr, 4,5-Me 2IPr, PPh3, PCy3, PCy2(Biphenyl), P'Bu3) in DMF. © 2008 American Chemical Society.