Platinum and ruthenium chloride-catalyzed cycloisomerization of 1-alkyl-2-ethynylbenzenes: Interception of π-activated alkynes with a benzylic C-H bond

(Chemical Equation Presented) Air-stable and commercially available alkynophilicmetal salts, such as PtCl₂, PtCl₄, and [RuCl₂(CO)₃]₂, catalyze the cycloisomerization of 1-alkyl-2-ethynylbenzenes to produce substituted indenes even at an ambient temperature. Electrophilically activated alkynes can be intercepted by simple benzylic C-Hbonds at primary, secondary, and tertiary carbon centers. Mechanistic studies, such as labeling studies and kinetic isotope and substituent effects, indicate that the cycloisomerization proceeds through the formation of a vinylidene intermediate and turnover-limiting 1,5-shift of benzylic hydrogen.