Silver- and Gold-Catalyzed Pyrrole Synthesis

Significance

The synthesis of substituted pyrroles from substituted β-alkynyl ketones using catalytic silver triflate or a mixture of gold(I) chloride, triphenylphosphine and silver triflate in moderate to good yields is reported. Reasonable tolerance towards functional groups is observed. Interestingly, while the reaction proceeds well in toluene, CH₂Cl₂ and i-PrOH, it fails in THF. Other silver sources (e.g., AgBF₄, AgOCOCF₃) gave comparable yields. For reactive amines, the two catalyst systems show comparable results, even though the AuCl/AgOTf/PPh₃ mixture seems to be thermally more stable. PtCl₂ and FeCl₃ give only traces of the pyrrole products. No pyrrole product was formed when tert-butylamine was used. Using the same reaction conditions, the cyclization of imines to bicylic pyrroles was achieved in moderate yields.