Ruthenium‐Catalyzed Geminal Hydroborative Cyclization of Enynes

• Published in: Angewandte Chemie International Edition Vol. 61; no. 31; pp. e202204319 - n/a
• Main Authors: Tan, Yun‐Xuan, Li, Shijia, Song, Lijuan, Zhang, Xinhao, Wu, Yun‐Dong, Sun, Jianwei
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01.08.2022
• Edition: International ed. in English
• Subjects: Alkynes; Boron; Catalysts; Cyclization; Cyclopropanation; Cyclopropane; Density functional theory; Geminal Addition; Hydroboration; Reaction mechanisms; Ruthenium; Ruthenium Catalysis; Substrates; Cyclopropanation; Hydroboration; Geminal Addition; Cyclization; Ruthenium Catalysis
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202204319

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 α‐boryl ruthenium carbene followed by intramolecular olefin cyclopropanation. This process also represents the first ruthenium‐catalyzed enyne hydroborative cyclization. Different from the previously known hydroborative cyclizations that add hydrogen and boron to opposite sides of an enyne, the proper choice of a ruthenium catalyst alters this propensity to a new addition mode, geminal hydroborative cyclopropanation. Two possible mechanisms are operative, which are substrate‐dependent based on DFT studies.