Mechanistic examination of AuIII-mediated 1,5-enyne cycloisomerization by AuBr2(N-imidate)(NHC)/AgX precatalysts – is the active catalyst AuIII or AuI?

Abstract

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 AuBr₂(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^tPe)}, in the presence of AgOTf, in comparison with Au^IIIBr₃(NHC) and Au^IBr(NHC). The nature of N-imidate anion influences catalyst efficacy. NMR spectroscopic investigations have allowed the ease of reduction of AuBr₂(N-TFS)(NHC) to Au^IX(NHC) (where X = N-TFS or Br) to be examined. Br₂ is liberated from Au^III, which has been trapped by a sacrificial alkene. Under working catalyst conditions cationic Au^III is reduced to Au^I.