Gold catalysis: catalyst oxidation state dependent dichotomy in the cyclization of furan-yne systems with aromatic tethers.

Four different synthetic strategies led to a variety of furan-yne systems that contained an aryl system in the tether. Due to the short routes to these systems (four steps or less), a small library of substrates could easily be prepared. These were treated with AuCl(3) or with the Gagosz's catalyst Ph(3)PAuNTf(2) complex. The AuCl(3)-catalyzed reactions delivered highly substituted fluorene derivatives, a class of compounds of great importance as precursors for luminophores with extraordinary abilities. Conversely, a different mechanistic pathway was observed with the cationic gold(I) catalyst. In the latter case, a mechanistically interesting reaction cascade initiated a formal alkyne insertion into the furyl-sp(3)-C bond, which gave indene derivatives as the final products. This new reaction pathway depends on the aromatic moiety in the tether, which stabilizes a crucial cationic intermediate as a benzylic cation.