Solvent- and Additive-Free Dehydrogenation of N-Heterocycles with Oxygen Catalyzed by Polyoxovanadate-Based...

N-heteroarenes are a family of organics with significant chemical and pharmaceutical applications. They are generally prepared by the catalytic oxidative dehydrogenation (ODH) of partially saturated N-heterocycles. In this work, we prepare and demonstrate the catalytic ODH applications of two polyoxovanadate-based metal-organic frameworks of the general formula {[M^II(bibp)_1.5][V^V₂O₆]}·H₂O (M = Ni 1, Co 2; bibp = 4,4'-bis(imidazol-1-ylmethyl)biphenyl). They are based on nonprecious metals, need no additives or organic solvents typically required for catalytic ODH, and utilize molecular O₂ as the oxidant, thus possessing all the traits desirable for practical catalysis. Catalyst 1 shows tolerance for a range of substrates with different electronic and steric features, including 2,3-dihydro-1H-indole and tetrahydroquinolines substituted with various functional groups. Mechanistic studies supported primarily by evidence from electron paramagnetic resonance and X-ray photoelectron spectra suggest that the V^V sites in 1 are catalytically responsible, first enabling the formation of the substrate-based radical species by a single electron transfer event while being converted into its mixed-valence form, followed by the production of the superoxide radical anion (O₂^•-) upon contact with O₂. The reaction mixture containing O₂^•- and the initially formed substrate-based radical then undergoes a series of steps, including the hydrogen abstraction and formation of the hydroperoxyl radical, the production and tautomerization of the partially dehydrogenated intermediate, and finally a repeating cycle of the aforementioned steps, to achieve the high-yield conversion of substrates to the corresponding N-heteroarenes.