Bench‐Stable Dinuclear Mn(I) Catalysts in E‐Selective Alkyne Semihydrogenation: A Mechanistic Investigation. Issue 53 (27th July 2022)

• Record Type: Journal Article
• Authors: Abhyankar, Preshit C.; MacMillan, Samantha N.; Lacy, David C.
• Is Part Of: Chemistry. Volume 28:Issue 53(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-07-27
• Subjects: diastereoselectivity -- hydrogenation -- kinetics -- manganese -- transition metals -- reaction mechanisms; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.202201766
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: Dinuclear manganese hydride complexes of the form [Mn2 (CO)8 ( μ ‐H)( μ ‐PR2 )] (R=Ph, 1 ; R= i Pr, 2 ) were used in E ‐selective alkyne semi‐hydrogenation ( E ‐SASH) catalysis. Catalyst speciation studies revealed rich coordination chemistry and the complexes thus formed were isolated and in turn tested as catalysts; the results underscore the importance of dinuclearity in engendering the observed E ‐selectivity and provide insights into the nature of the active catalyst. The insertion product obtained from treating 2 with (cyclopropylethynyl)benzene contains a cis ‐alkenyl bridging ligand with the cyclopropyl ring being intact. Treatment of this complex with H2 affords exclusively trans ‐(2‐cyclopropylvinyl)benzene. These results, in addition to other control experiments, indicate a non‐radical mechanism for E ‐SASH, which is highly unusual for Mn−H catalysts. The catalytically active species are virtually inactive towards cis to trans alkene isomerization indicating that the E ‐selective process is intrinsic and dinuclear complexes play a critical role. A reaction mechanism is proposed accounting for the observed reactivity which is fully consistent with a kinetic analysis of the rate limiting step and is further supported by DFT computations. Abstract : Bench‐stable dinuclear Mn hydrides (single‐step & gram‐scale from commercially available materials) have utility in alkyne semihydrogenation where the trans ‐alkene is the kinetic product. This selectivity mechanism contrasts typical tandem hydrogenation then isomerization systems.