Ruthenium‐Mediated [2 + 2 + 2] Cycloaddition

Ruthenium-Catalyzed Cycloadditions to Form Five-, Six-, and Seven-Membered Rings

“…Metal-catalyzed cycloadditions represent an expansive, atom-efficient class of transformations that have found wide-ranging utility in chemical synthesis. − The vast majority of metal-catalyzed cycloadditions exploit cobalt, − nickel, − and rhodium; − however, in the past two to three decades, significant progress on ruthenium-catalyzed cycloadditions has been made, − including fundamentally new cycloaddition processes that are unknown for other metals . While utilized less frequently as catalysts for cycloaddition, ruthenium offers several advantages.…”

“…Ruthenium can adopt a wide range of oxidation levels and engages in diverse modes of reactivity, including C–C bond activation, metallacycle formation via oxidative coupling, carbene/vinylidene/allenylidene formation, alcohol-mediated hydrogen transfer, and much more. Although metal-catalyzed cycloadditions have been reviewed − and related ruthenium-catalyzed cycloadditions have appeared intermittently in more broadly focused monographs, − ruthenium-catalyzed cycloadditions have not been exhaustively catalogued. Here, we provide a summary of ruthenium-catalyzed cycloadditions to form five-, six-, and seven-membered rings (Figure ).…”

Ruthenium-Catalyzed [2 + 2 + 2] Cycloaddition Reaction Forming 2-Aminopyridine Derivatives from α,ω-Diynes and Cyanamides

“…Alternatively, transition-metal-catalyzed [2 + 2 + 2] cycloaddition reactions are powerful methods to prepare heteroaromatic compounds . In particular, [2 + 2 + 2] cycloadditions of alkynes and nitriles have been extensively explored to access pyridine derivatives. − However, as far as 2-aminopyridines are concerned, only a few examples of [2 + 2 + 2] cycloaddition of α,ω-diynes and cyanamides based on Co, Rh, Ni, Fe, and Ir catalysis have been reported (Scheme ).…”

“…The transition-metal-catalyzed cycloaddition of diversely substituted unsymmetrical diynes with nitriles for the regioselective synthesis of pyridines has been carefully studied. − However, little attention has been paid to halogen-substituted diynes . The first example of halopyridines, prepared from haloalkynes and nitriles, was recently reported by Kotora with a low regioselectivity .…”

Ruthenium‐Catalyzed [2+2+2] Cycloaddition of 1,6‐Enynes and Unactivated Alkynes: Access to Ring‐Fused Cyclohexadienes

“…Recent developments by the groups of Shibata and Evans based on the use of cationic rhodium catalysts were dedicated to the enantioselective synthesis of chiral bicyclohexadienes containing one quaternary stereocenter generated at the ring-fused position. [13g, 15] Despite the ability of ruthenium complexes to catalyze a myriad of [2+2+2] cycloadditions of 1,6-diynes with alkynes [16] or electron-deficient carbon-heteroatom multiplebond compounds, [17] to the best of our knowledge, the intermolecular cross-carbocyclization of enynes with alkynes, leading to type II compounds, has not yet been described. Herein, we have achieved the intermolecular carbocyclization of enynes with unactivated alkynes by using a neutral ruthenium catalyst to provide ready access to cyclohexa-1,3-diene frameworks.…”