Bench-Stable Dinuclear Mn(I) Catalysts in E-selective Alkyne Semihydrogenation: A Mechanistic Investigation

Dinuclear complexes [Mn2(CO)8(µ-H)(µ-PR2)] (R = Ph, 1; R = iPr, 2) were synthesized and the role of phosphido substitu-ent in E-selective alkyne semi-hydrogenation (E-SASH) catalysis was examined. Using diphenylacetylene (dpa) as the sub-strate, 2 has improved efficiency over 1, achieving 75% conversion to stilbene (E/Z ≈ 8-15 depending on conditions) with remaining dpa converting into hexaphenylbenzene and hydrophosphinated alkene (hpa) byproducts. Investigation of the mechanism revealed rich coordination chemistry and insights into the active catalyst speciation. Having isolated each species individually, the different complexes 3-8 were independently subjected to catalytic conditions: only 2, 3, and 5 furnished product with similar conversions and E-selectivity and these complexes have similar dinuclear structures. Control reactions with Mn2(CO)10, HMn(CO)5, and [Mn(CO)4(µ-H)]3 resulted in no to very low conversions and these did not exhibit E-selectivity. When (cyclopropylethynyl)benzene was used instead of dpa, the inserted species 10 was obtained with cyclopro-pane ring intact, and treatment of 10 with H2 furnished the corresponding trans-(2-cyclopropylvinyl)benzene. In addition to other tests for radicals, these latter two results indicate a non-radical mechanism for E-SASH which is highly unusual for MnH catalysts. Finally, the reaction between 3 and H2 furnishes exclusively trans-stilbene. Combined with the fact that 2 and 3 are virtually inactive towards cis to trans alkene isomerization, a mechanism and origin of selectivity is proposed, one where dinuclear complexes play a critical role.