Ruthenium‐Catalyzed Geminal Hydroborative Cyclization of Enynes

Abstract

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 substratedependence. 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.

Ru-Catalyzed Hydroboration of Ynones Leads to a Nontraditional Mode of Reactivity

“…Similar to the typical hydrofunctionalization of alkynes catalyzed by cationic CpRu- and Cp*Ru-based complexes, ,, the reaction starts by ligand exchange with the alkyne substrate and hydroborane to form an 18-electron complex INT2 (Figure B). Starting from this complex, there are several possible reaction modes, including 1,2-hydroboration of the triple bond or the carbonyl group, as known in the literature (Scheme ).…”

“…Ruthenium-based complexes have been known as versatile catalysts for 1,2-hydroboration of alkynes, providing access to the valuable vinyl boranes . Recently, we have reported a new geminal (1,1-) addition mode in Ru-catalyzed alkyne hydroboration, in which hydrogen and boron were initially added to the same terminal of the alkyne triple bond . In continuation of our interests in the study of alkyne hydrofunctionalization reactions, we were curious about the behavior of ynones when subjected to these powerful ruthenium-catalyzed hydroboration conditions.…”

Light‐Driven Alkyne gem‐Hydrogenation: An Intramolecular Approach to Hoveyda–Grubbs Catalysts

“…shift 1 to the corresponding (E)-alkene as the final product; gem-hydrogenation 2 is hence innately linked to alkyne trans-hydrogenation, [1,7] which itself is a rather unorthodox but synthetically enabling transformation. [5,6,8,9] Importantly, however, it is also possible to outperform this downstream pathway and, in doing so, harness genuine carbene reactivity: a set of previously unknown transformations, including hydrogenative cyclopropanation, [10] hydrogenative metathesis, [10,11] hydrogenative rearrangement and cycloisomerization reactions, [12,13] hydrogenative heterocycle syntheses, [14] and hydrogenative CÀ H insertions [15] bear witness of this notion.…”

“…However, this potential gain is partly offset by the fact that an iodine-containing alkyne such as 2b is used, which itself is rather expensive. 6 Therefore, we envisaged to explore yet another approach aimed at converting cheaper substrates into the targeted complexes in an entropically favorable intramolecular setup.…”

Gold-Catalyzed Cyclization/Hydroboration of 1,6-Enynes: Synthesis of Bicyclo[3.1.0]hexane Boranes

“…Though gold-catalyzed intramolecular alkyne hydroboration using amine boranes has been reported by Shi and co-workers, the compatibility of cationic gold with reductive Lewis base–borane adducts remains a big concern due to the easy reduction of gold­(I) to gold(0). Very recently, Sun and co-workers reported a related ruthenium-catalyzed geminal hydroborative cyclization of 1,6-enynes with pinacolboranes (HBpin), allowing the formation of bicyclo[3.1.0]­hexane borates in good yields . Unlike gold catalysis, ruthenium-catalyzed cyclization of 1,6-enynes occurs by simultaneous coordination of the alkene and the alkyne to ruthenium followed by oxidative cyclometalation to give a ruthenacyclopentene intermediate.…”