Expansion of an N^C Ligand Aromatic System With Terminal Alkynes in [(pyridyl-benzothiophene)AuCl₂] Complex Promoted by Au(III)→Au(I) Conversion.

] complex (N^C-pyridyl-benzothiophene) toward a series of terminal alkynes has been investigated. The reactions afford products containing a C-coordinated benzo[4,5]thieno[2,3-a]quinolizin-5-ium aromatic fragment bound to an Au(I) center. The obtained aromatic systems can be isolated in the free state through reaction of the Au(I) complexes with a protonated phosphonium cation. The studied reaction sequence may be considered a novel strategy for expansion of condensed aromatic systems via 1,4-addition of alkynes to an orthometalated N^C ligand bound to an Au(III) center. The mechanism of this transformation in the gold coordination sphere was investigated theoretically by application of the Double Anharmonic Downward Distortion Following methodology, revealing the details of the coordinated ligands transformations and confirming the key role of the solvent in determining the reaction pathway. Isolation and characterization of the Au(I) complex shed light on the mechanism of related catalytic reactions. The results obtained are of a general nature and pave the way for further development of this chemistry with the wider range of N^C and alkyne ligands, enabling the directed synthesis of new condensed aromatic systems.