A computational study on ruthenium-catalyzed [4 + 1] annulation via C–H activation: the origin of selectivity and the role of the internal oxidizing group
We report a density functional theory study on ruthenium-catalyzed [4 + 1] annulation of benzamides with propargyl alcohols via C–H activation. The plausible catalytic cycles involve N–H deprotonation, C–H activation, alkyne insertion, β-H elimination, enol–keto tautomerism, oxidative addition, C–N reductive elimination, protonation, and catalyst recovery. Importantly, the preference of the [4 + 1] annulation over the [4 + 2] annulation is discussed in detail. Alkynes bearing a β-H group and a hydroxyl group are necessary for β-H elimination and enol–keto tautomerism, which lead to the formation of a [4 + 1] product. In contrast, alkynes without a β-H group or a hydroxyl group could only lead to [4 + 2] annulation. Additionally, this article addresses the key role of the internal oxidizing group, which is responsible for converting RuII into RuIV and thus favoring the following C–N reductive elimination. Hence, the studied mechanism of [4 + 1] annulation is a novel RuII/IV/II pathway rather than a RuII/0/II pathway.