Mechanistic examination of Au super(III)-mediated 1,5-enyne cycloisomerization by AuBr sub(2)(N-imidate)(NHC)/AgX precatalysts - is the active catalyst Au super(III) or Au super(I)?

Gold(iii) catalysts mediate 1,5-enyne cycloisomerization or tandem nucleophilic substitution-1,5-enyne cycloisomerization processes in an efficient manner. This study examines the reaction kinetics of 1,5-enyne cycloisomerization, mediated by AuBr sub(2)(N-imidate)(NHC) catalysts {where N-imidate = N-tetrafluorosuccinimide (N-TFS) or N-phthalimide (N-phthal) and NHC = N,N'-di-tert-pentylimidazol-2-ylidene (I super(t)Pe)}, in the presence of AgOTf, in comparison with Au super(III)Br sub(3)(NHC) and Au super(I)Br(NHC). The nature of N-imidate anion influences catalyst efficacy. NMR spectroscopic investigations have allowed the ease of reduction of AuBr sub(2)(N-TFS)(NHC) to Au super(I)X(NHC) (where X = N-TFS or Br) to be examined. Br sub(2) is liberated from Au super(III), which has been trapped by a sacrificial alkene. Under working catalyst conditions cationic Au super(III) is reduced to Au super(I).