E-Selective Manganese-Catalyzed Semihydrogenation of Alkynes with H₂ Directly Employed or In Situ-Generated

Abstract

Selective semihydrogenation of alkynes with the Mn(I) alkyl catalyst fac -[Mn(dippe)(CO) 3 (CH 2 CH 2 CH 3 )] (dippe = 1,2-bis(di- iso -propylphosphino)ethane) as a precatalyst is described. The required hydrogen gas is either directly employed or in situ-generated upon alcoholysis of KBH 4 with methanol. A series of aryl-aryl, aryl-alkyl, alkyl-alkyl, and terminal alkynes was readily hydrogenated to yield E- alkenes in good to excellent isolated yields. The reaction proceeds at 60 °C for directly employed hydrogen or at 60–90 °C with in situ-generated hydrogen and catalyst loadings of 0.5–2 mol %. The implemented protocol tolerates a variety of electron-donating and electron-withdrawing functional groups, including halides, phenols, nitriles, unprotected amines, and heterocycles. The reaction can be upscaled to the gram scale. Mechanistic investigations, including deuterium-labeling studies and density functional theory (DFT) calculations, were undertaken to provide a reasonable reaction mechanism, showing that initially formed Z- isomer undergoes fast isomerization to afford the thermodynamically more stable E- isomer.

Bench‐Stable Dinuclear Mn(I) Catalysts in E‐Selective Alkyne Semihydrogenation: A Mechanistic Investigation**

“…The remaining question is how does this system preferentially form trans ‐stilbene over cis ‐stilbene? Most E ‐selective catalysts produce the kinetic cis ‐product first and then the thermodynamic trans ‐product in a separate isomerization catalytic cycle [21,23,25,26,39,40] . Exceptions to this are the mononuclear ruthenium catalysts studied by Fürstner and coworkers and some other dinuclear systems [18,41–43] .…”

“…Most E-selective catalysts produce the kinetic cis-product first and then the thermodynamic trans-product in a separate isomerization catalytic cycle. [21,23,25,26,39,40] Exceptions to this are the mononuclear ruthenium catalysts studied by Fürstner and coworkers and some other dinuclear systems. [18,[41][42][43] These exceptional cases operate via so-called "intrinsic trans-selective" processes.…”

“…The typical mechanism for E-selective alkyne semihydrogenation is Z!E isomerization, which is common for mononuclear systems and recently shown with Mn. [21] To our knowledge, a mechanism invoking a species like gem-13 has never been proposed for a manganese-based catalytic system. Our studies demonstrating negligible Z!E isomerization indicates something exceptional about 2 and 3 and the gem-intermediate is one possibility.…”

“…Significant work remains to uncover the nature of Mncatalyzed hydrogenation reaction mechanisms, and dinuclear catalysts have not been explored in detail. Furthermore, the importance of E-SASH to prepare biologically active compounds [16][17][18][19] using first row metals [20][21][22][23][24][25][26] holds promise for an applications-based outcome as a longer-term goal in sustainable metal catalysis. [27] As shown by us previously, bench-stable complexes such as 1 can be prepared in a single step from commercially available starting materials, namely HPR 2 and Mn 2 (CO) 10 (Scheme 1 and TOC graphic).…”

Manganese Alkyl Carbonyl Complexes: From Iconic Stoichiometric Textbook Reactions to Catalytic Applications

“…Gratifyingly, Mn4 showed high reactivity and selectivity in the semihydrogenation of disubstituted alkynes. 33 In fact, remarkable E -selectivity was achieved with a catalyst loading of merely 1 mol % at 60 °C under 30 bar hydrogen pressure ( Scheme 12 ). Furthermore, we envisioned semihydrogenation with in situ generated hydrogen gas and thus without the need of costly high-pressure setups, e.g., autoclaves.…”