Anionic N-Heterocyclic Carbene Complexes of Gold(I) as Precatalysts for Silver-Free Cycloisomerization of Enynes

The anionic malonate-derived N-heterocyclic carbene maloNHC readily and efficiently reacts with AuCl(tht) to generate the corresponding anionic gold(I) complexes of type [(maloNHC)AuCl](M). A stoichiometric reaction of these complexes with triphenylphosphine affords the zwitterionic complex [(maloNHC)Au(PPh3)]. Although totally insoluble in the reaction medium, the [(maloNHC)AuCl](M) salts were shown to be suitable precatalysts for the cycloisomerization of 1,6-enynes through the in situ generation of soluble zwitterionic catalytic species of the type [(maloNHC)Au].