Rutheniun-Catalyzed Cycloisomerization of o-(Ethynyl)phenylalkenes to Diene Derivatives via Skeletal Rearrangement

Abstract
Treatment of a series of 2‘,2‘-disubstituted (o-ethynyl)styrenes with TpRu(PPh3)(CH3CN)2PF6 (10 mol %) in benzene (80 °C, 12−18 h) efficiently gave 2-alkenyl-1H-indene derivatives. This catalytic reaction represents an atypical enyne cycloisomerization with skeletal rearrangement of starting enyne, where the CC bond is completely cleaved and inserted by the terminal alkynyl carbon. The reaction mechanism was elucidated by a series of deuterium and 13C labeling experiments, as well as by changing the substituents at the phenyl moieties. The mechanism is proposed to involve the following key steps: 5-endo-dig cyclization of ruthenium-vinylidene intermediate, a nonclassical ion formation, and the “methylenecyclopropane-trimethylenemethane” rearrangement.