Ruthenium-Catalyzed Geminal Hydroborative Cyclization of Enynes

Disclosed here is the first geminal (gem-) hydroborative cyclization of enynes. Different from known hydroborative cyclizations, this process adds hydrogen and boron to the same position, leading to a new reaction mode. With [Cp*RuCl](4) as catalyst, a range of gem-hydroborated bicyclic products bearing a cyclopropane unit could be rapidly assembled from simple enyne substrates. Control experiments and density functional theory (DFT) calculations provided important insights into the reaction mechanism. Notably, two major competing pathways may operate with substrate-dependence. 1,6-Enynes favor initial oxidative cyclometalation to form a ruthenacyclopentene intermediate prior to engaging hydroborane, while other enynes (e.g., 1,7-enynes) that lack strong propensity toward cyclization prefer initial alkyne gem-(H,B)-addition to form an alpha-boryl ruthenium carbene followed by intramolecular olefin cyclopropanation. This process also represents the first ruthenium-catalyzed enyne hydroborative cyclization.

Automated summary

The first geminal hydroborative cyclization of enynes is disclosed in this study. Unlike known hydroborative cyclizations, this reaction adds hydrogen and boron to the same position, leading to a new reaction mode. By using a specific catalyst, a variety of gem-hydroborated bicyclic products with a cyclopropane unit can be rapidly formed from simple enyne substrates. Control experiments and calculations provide important insights into the reaction mechanism. Notably, two competing pathways may operate, depending on the substrate.