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

Abstract

The reactions of substituted acetophenones with diphenylacetylene in the presence of [{RuCl2(p‐cymene)}2] (2 mol‐%), AgSbF6 (8 mol‐%), and Cu(OAc)2·H2O (25 mol‐%) in 1,2‐dichloroethane at 120 °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.