Gold-Catalyzed Ascorbic Acid-Induced Arylative Carbocyclization of Alkynes with Aryldiazonium Tetrafluoroborates

We
describe, herein, arylative carbocyclization of alkynes catalyzed
by gold. In this process, Au­(I) is oxidized to Au­(III) with aryldiazonium
tetrafluoroborates following a photosensitizer-free and irradiation-free
protocol. Ascorbic acid acts as a radical initiator, generating aryl
radicals. According to DFT calculations, these radicals are added
to Au­(I), leading to a Au­(II) species that is further oxidized to
Au­(III) with the assistance of a tetrafluoroborate anion. The overall
arylative carbocyclization process is very energetically favorable,
transforming arylpropargyl ethers into valuable 3,4-diaryl-2H-chromenes in a completely regio- and stereoselective fashion.
Furthermore, we show that one of the synthesized 3,4-diaryl-2H-chromenes exhibits polymorphism with marked differences
in the color of its crystals, a property that could lead to the development
of colored derivatives in the future.