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

Abstract

Dinuclear manganese hydride complexes of the form [Mn2(CO)8(μ‐H)(μ‐PR2)] (R=Ph, 1; R=iPr, 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.

Synthesis, Characterization, and Reactivity of a (PPP) Pincer-Ligated Manganese Carbonyl Complex: Polarity Reversal Imparted by the Electrophilic Nature of a Planar Mn-P(NR2)2 Fragment

“…Far less attention has been given toward reactivity across manganese–phosphorus bonds, particularly when the phosphorus atom is incorporated into an N-heterocycle, for these hydrogenation/dehydrogenation processes. − Gudat and co-workers demonstrated the catalytic dehydrogenation of ammonia borane across a Mn–P bond using a manganese carbonyl complex featuring a monodentate N-heterocyclic phosphenium (NHP + ) ligand (Chart ). In this monodentate ( Dipp NHP)­Mn­(CO) 4 (Dipp = 2,6-diisopropylphenyl) system, an electrophilic phosphenium ligand provides a Lewis acidic reaction center to promote this transformation and is the first example of a terminal N-heterocyclic phosphenium complex applied in catalysis.…”

Recent advances in the selective semi-hydrogenation of alkyne to (E)-olefins

“…However, subsequent hydrogen addition followed by isomerization resulted in thermodynamically stable ( E )-alkene (Scheme 21). 30…”