Ruthenium-Catalyzed Regioselective Cyclization of Aromatic Ketones with Alkynes: An Efficient Route to Indenols and Benzofulvenes

The reactions of substituted acetophenones with diphenylacetylene in the presence of [{RuCl2(p-cymene)}(2)] (2 mol-%), AgSbF6 (8 mol-%), and Cu(OAc)(2)center dot H2O (25 mol-%) in 1,2-dichloroethane at 120 degrees C for 10 h provided substituted indenol derivatives in good-to-excellent yields. Under similar reaction conditions, unsymmetrical alkynes such as 1-phenyl-1-propyne, 1-phenyl-1-butyne, 1-phenyl-2-(trimethylsilyl)acetylene, and a substituted enyne also reacted efficiently with substituted acetophenones to afford the corresponding indenol derivatives in a highly regioselective manner. The amount of silver salt plays a key role in the reaction. When the amount of silver salt exceeded more than 8 mol-% in the presence of 2 mol-% [{RuCl2(p-cymene)}(2)], a different type of dehydration product, namely a benzofulvene derivative, started to appear. In the presence of 20 mol-% AgSbF6, substituted acetophenones readily reacted with alkynes in the presence of [{RuCl2(p-cymene)}(2)] (2 mol-%) to give benzofulvene derivatives in excellent yields. A plausible reaction mechanism is proposed to account for the cyclization reaction.