Gold(I)-Catalyzed Hydroammoniumation and Methylamination of Alkynes to Indoles

Significance

Based on the developed intermolecular hydroamination of alkynes with amines catalyzed by cationic gold(I) complexes supported by CAAC ligands (G. Bertrand and co-workers J. Am. Chem. Soc. 2009, 131, 8690), Bertrand and co-workers now report the intramolecular hydroammoniumation and carboamination of C-C triple bonds to afford indoles. Studies by multinuclear NMR spectroscopy showed that complex 2 is formed from the reaction of 1 with complex B, rather than the suggested tricoordinated gold(I) complex of type I. Single-crystal X-ray diffraction of complex 4 revealed a gold(I) η^¹-alkene complex resulting from the addition of the tertiary amino group to the coordinated alkyne. Several indolium triflates 5 were synthesized in very good yields ­using A/AgOTf in combination with triflic acid. ­Methylamination to give 6 was also realized in good yield using A/KB(C₆F₅)₄ at high temperature.