Direct, Additive‐Free N ‐Formamide Synthesis From Secondary Amines and Oxygen With Visible‐Light‐Responsive TiO 2 Catalysts

This study presents an environmentally friendly method to synthesize N ‐formamides from secondary amines and atmospheric oxygen as all reactants under mild conditions using visible light at room temperature. The reaction is efficiently catalyzed by metal ion‐modified titanium dioxide (M n+ /TiO 2 ), a stable and nontoxic, visible‐light‐responsive photocatalyst. The M n+ /TiO 2 gave higher yields of product compared to those of conventional photocatalysts, and Cu 2+ /TiO 2 exhibited the greatest reaction efficiency for the conversion of dibutylamine among the M n+ /TiO 2 (M n+ ; Mg 2+ , Cu 2+ , Al 3+ , Mn 2+ , Ni 2+ , Zn 2+ , Cr 3+ , Fe 2+ , Rh 3+ ). Meanwhile, inexpensive Mg 2+ ‐modified TiO 2 provided a broader and more consistent substrate scope. Mechanistic studies indicated that the secondary amine is oxidized by M n+ /TiO 2 and subsequently converted to an aldehyde, which reacts with the amine to form an enamine as an intermediate. Density functional theory calculations elucidated that the enamine was oxidized to a radical cation by M n+ /TiO 2 , which reacts with O 2 to give an oxygenated cationic intermediate. The intermediate is then reduced by M n+ /TiO 2 to form a dioxetane, which disproportionates to the N ‐formamide. Deuterium isotope studies revealed a unique H/D exchange and showed the deuterium incorporation of the N ‐formamide, which can be controlled by the deuterium reagents.