Efficient Lewis Acid Promoted Alkene Hydrogenations Using Dinitrosyl Rhenium(−I) Hydride Catalysts

Highly efficient alkene hydrogenations were developed using NO-functionalized hydrido dinitrosyl rhenium catalysts of the type [ReH­(PR₃)₂(NO)­(NO­(LA))]­[Z] (2, LA = B­(C₆F₅)₃; 3, LA = [Et]⁺, Z = [B­(C₆F₅)₄]⁻; 4, LA = [SiEt₃]⁺, Z = [HB­(C₆F₅)₃]⁻; R = iPr a, Cy b). Lewis acid attachment to the NO ligand was found to facilitate bending at the N_OLA atom and concomitantly to open up a vacant site at the rhenium center. According to DFT calculations, the ability to bend follows the order 4 > 3 > 2, which did not match with the order of increasing hydrogenation activities: 3 > 4 > 2. The main factor spoiling catalytic performance was catalyst deactivation by detachment of the LA group occurring during the catalytic reaction course, which was found to go along with the decrease in order of DFT-calculated strengths of the O_NO–LA bonds. LA detachment from the O_NO atom could at least partly be prevented by LA addition as cocatalysts, which led to an extraordinary boost of the hydrogenation activities. For instance the “1/hydrosilane/B­(C₆F₅)₃” (1:5:5) system exhibited the highest performance, with TOFs up to 1.2 × 10⁵ h^–1 (1-hexene, 1-octene, cyclooctene, cyclohexene). The cocatalyst [Et₃O]­[B­(C₆F₅)₄] showed the smallest effect, presumably due to the strong Lewis acidic character of the reagent causing side-reactions before reacting with 1a,b. The catalytic reaction course crucially involves not only reversible bending at the N_OLA atom but also loss of a PR₃ ligand, forming 16e or 14e monohydride reactive intermediates, which drive an Osborn-type hydrogenation cycle with olefin before H₂ addition.