C(sp2)–H Activation with Bis(silylene)pyridine Cobalt(III) Complexes: Catalytic Hydrogen Isotope Exchange of Sterically Hindered C–H Bonds

The bis(silylene)pyridine cobalt(III) dihydride boryl, trans-[ptolSiNSi]Co(H)2BPin (ptolSiNSi = 2,6-[EtNSi(NtBu)2CAr]2 C5H3N, where ptol = 4-MeC6H4, and Pin = pinacolato) has been used as a precatalyst for the hydrogen isotope exchange (HIE) of arenes and heteroarenes using benzene-d6 as the deuterium source. Use of D2 as the source of the isotope produced modest levels of deuterium incorporation, and stoichiometric studies established modification of the pincer ligand through irreversible addition of H2 across the silylene leading to catalyst deactivation. High levels of deuterium incorporation were observed with benzene-d6 as the isotope source and enabled low (0.5–5 mol %) loadings of the cobalt precursor. The resulting high activity for C–H activation resulted in deuterium incorporation at sterically encumbered sites previously inaccessible with first-row metal HIE catalysts. The cobalt-catalyzed method was also compatible with aryl halides, demonstrating a kinetic preference for chemoselective C(sp2)–H activation over C(sp2)–X (X = Cl, Br) bonds. Monitoring the catalytic reaction by NMR spectroscopy established cobalt(III) resting states at both low and high conversions of the substrate, and the overall performance was inhibited by the addition of HBPin. Studies on precatalyst activation with cis-[ptolSiNSi]Co(Bf)2H and cis-[ptolSiNSi]Co(H)2Bf (where Bf = 2-benzofuranyl) support the intermediacy of bis(hydride)aryl cobalt intermediates as opposed to bis(aryl)hydride cobalt complexes in the catalytic HIE method. Mechanistic insights resulted in an improved protocol using [ptolSiNSi]Co(H)3·NaBHEt3 as the precatalyst, ultimately translating onto higher levels of isotopic incorporation.