Hydrido-Cobalt Complexes for the Chemo- and Regioselective 1,2-Silylative Dearomatization of N-Heteroarenes

Abstract

We describe an efficient regio-and chemoselective dearomatization of N-heteroarenes using hydrido-cobalt catalysts. Reactions were performed under mild conditions on a wide range of N-heteroarenes leading exclusively to the silyl-1,2-dihydroheteroarene. Various quinolines and pyridines bearing electron-donating and electron-withdrawing substituents are compatible with this methodology. DFT calculations, NMR spectroscopic studies, and X-ray diffraction analysis underlined the importance of a second silane for the final step of the reaction.

Cobalt‐Catalyzed Bis‐Borylation of Quinolines: The Importance of the Cobalt Triplet State

“…[17] This result proves that the first hydroboration is not sensitive to the steric hindrance, contrary to our previously published hydrosilylation. [12] Thus, quinaldines 1 s, 1 t, 1 u, 1 v, 1 w were selectively converted into the desired products without formation of side-products and borylated tetrahydroquinolines 5 s-5 w were isolated in yield range of a 46-67 %. First and foremost, only the trans-diastereoisomer was obtained, which was further confirmed by NMR analysis together with the obtention of X-ray structures for 5 t and 5 u.…”

“…The second step would then be the Markovnikov‐hydroboration of the resulting double bond. To confirm this hypothesis, we first checked if the initial elementary step was similar to the one involved in the hydrosilylation of quinolines [12,18h] namely an oxidative addition in the hydride donor. An experiment between Co‐1 and 10 equivalents of pinacolborane HBPin in a J Young NMR tube was conducted and analysed by 1 H and 31 P NMR spectroscopy: only a loss of triphenylphosphine was observed [23] with no evidence of oxidative addition as previously underlined by Chirik [16b] .…”

“…This field has attracted a great deal of interest these past few years and while selective methodologies based on hydrosilanes and hydroboranes as mild reducing agents are now well developed, [10] the use of earth‐abundant metals such as cobalt is still limited. The same year, Geetharani [11] and our group [12] reported respectively the 1,2‐hydroboration and 1,2‐hydrosilylation of N ‐heteroarenes (see Scheme 1a) [13] …”

“…This field has attracted a great deal of interest these past few years and while selective methodologies based on hydrosilanes and hydroboranes as mild reducing agents are now well developed, [10] the use of earth-abundant metals such as cobalt is still limited. The same year, Geetharani [11] and our group [12] reported respectively the 1,2-hydroboration and 1,2-hydrosilylation of N-heteroarenes (see Scheme 1a). [13] On the other hand, despite huge progress, the site-selective introduction of substituents onto quinoline rings to synthesize substituted THQs remains a difficult task that has been barely addressed yet.…”

Regioselective 1,2-Hydrosilylation of N-Heterocycles Catalyzed by a Ruthenium Olefin Metathesis Catalyst

“…While this manuscript was being assembled, Petit and co-workers reported a hydrido-cobalt-complexes-catalyzed hydrosilylation reaction requiring high temperature and long reaction time . With the exception of these previous studies, simple synthetic methods are needed, especially considering the importance of the 1,2-dihydroquinoline derivatives …”