om400733u_si_001.pdf (1.35 MB)
Efficient Lewis Acid Promoted Alkene Hydrogenations Using Dinitrosyl Rhenium(−I) Hydride Catalysts
journal contribution
posted on 2013-12-09, 00:00 authored by Yanfeng Jiang, Wenjing Huang, Helmut W. Schmalle, Olivier Blacque, Thomas Fox, Heinz BerkeHighly
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 NOLA 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 ONO–LA bonds. LA detachment from the ONO 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 NOLA 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.