Metallalkenyl, Metallacyclopropene, or Metallallylcarbenoid? Ru-Catalyzed Annulation between Benzoic Acid and Alkyne

In this work, we present an alternative metallallylcarbenoid mechanism for the Ru-catalyzed annulation of benzoic acid with alkyne. Generally, this reaction was believed to proceed through the C-H bond metalation, the alkyne insertion, and the reductive elimination through the metallalkenyl species or the metallacyclopropene. However, a metallallylcarbenoid species containing both metallallyl and metal carbenoid is found to be crucial to promote a facile 6-endo-trig annulation because of its distinctive eta(4) coordinated structure. The recognition of the metallallylcarbenoid mechanism should be helpful to understand the mechanism, reactivity, and selectivity of ruthenium catalysis.