Reactivities of Indenylruthenium Complex toward Internal Alkynes: Formation of Disubstituted Vinylidene Complexes and Indenyl–Alkyne Coupling

Reaction of the indenylruthenium complex [(η5-C9H7)­RuCl­(dppe)] with internal alkynes (MeCCR, R = Et, Ph) in the presence of NaBArF4 gives rise to two types of C–C bond activation, i.e., alkyne insertion/β-carbon elimination and internal alkyne/disubstituted vinylidene rearrangement, as reversible processes. At 70 °C, regioisomeric complexes [Ru­{C­(Me)C­(R)–(η6-C9H7)}­(dppe)]­[BArF4] (2) and [Ru­{C­(R)C­(Me)–(η6-C9H7)}­(dppe)]­[BArF4] (2′) are formed through insertion of the alkyne into the Ru–indenyl bond followed by haptotropic rearrangement. Complexes 2′ isomerize completely to 2 at this temperature after longer reaction time. At 130 °C, 2 are further converted into the disubstituted vinylidene complexes [(η5-C9H7)­Ru­{CC­(R)­Me}­(dppe)]­[BArF4] (3) through alkyne–vinylidene rearrangement of the η2-alkyne intermediate. This provides a rare example of direct observation of the β-carbon elimination from an unstrained transition metal alkenyl complex.