Manganese-Catalyzed C(sp²)–H Borylation of Furan and Thiophene Derivatives

Abstract

Aryl boronic esters are bench-stable, platform building-blocks that can be accessed through metal-catalyzed aryl C­(sp2)–H borylation reactions. C­(sp2)–H bond functionalization reactions using rare- and precious-metal catalysts are well established, and while examples utilizing Earth-abundant alternatives have emerged, manganese catalysis remains lacking. The manganese-catalyzed C–H borylation of furan and thiophene derivatives is reported alongside an in situ activation method providing facile access to the active manganese hydride species. Mechanistic investigations showed that blue light irradiation directly affected catalysis by action at the metal center, that C­(sp2)–H bond borylation occurs through a C–H metallation pathway, and that the reversible coordination of pinacolborane to the catalyst gave a manganese borohydride complex, which was as an off-cycle resting state.

Iron-catalysed alkene and heteroarene H/D exchange by reversible protonation of iron-hydride intermediates

“…Pre-catalyst activation with alkoxide salts has been shown to generate catalytically active metal-hydride species when combined with commercial silane or borane reagents such as pinacolborane (HBpin). 11 This removes the need to generate and handle the highly air- and moisture sensitive iron dihydride. To assess the catalytic feasibility of this strategy in situ generated [dmpe 2 FeH 2 ] 2 (from [dmpe 2 FeCl 2 ] 1 + NaO t Bu + HBpin) was used in combination with 2-methylfuran 3a, blue light irradiation (450 nm) and a variety of deuterium sources.…”

Iron‐Catalysed C(sp2)‐H Borylation with Expanded Functional Group Tolerance†

“…[ 1‐7 ] Currently this process is dominated by rhodium and iridium‐based catalysts, [ 8‐16 ] although a handful of Earth‐abundant metal examples have been reported. [ 17‐23 ] Iron has high natural abundance, is inexpensive, and has low toxicity, so offers an ideal choice as a metal for catalysis. [ 24‐26 ]…”

“…[ 18,34 ] This complex and the analogous manganese species have shown good activity for C(sp 2 )‐H borylation under blue light irradiation, but these systems were limited to the borylation of heteroarenes. [ 17 ] Additionally, free alcohols and amines would be tolerated by use of HBpin as a traceless protecting group. [ 14‐15,35 ]…”

Visible-light-induced 3d transition metal-catalysis: A focus on C–H bond functionalization

“…Very recently, Thomas and co-workers described a visible light induced manganese-catalyzed C-H borylation of furan and thiophene derivatives (Figure 2c). The photoactive Mn(I) hydride species, dmpe2MnH3, generated in situ from the dibromide precursor, dmpe2MnBr2 (dmpe = 1,2-bis(dimethylphosphino)ethane), in the presence of pinacolborane (HBPin) and sodium tert-butoxyde (NaOtbu), is prone to hydrogen loss that enables the catalytic C-H borylation process under visible light irradiation [37].…”