Chemistry of heterocyclic N-oxides and related compounds. 11. Reaction of pyridine N-oxide with metal ketyls and benzophenone dianions

Complexation Facilitated Reduction of AromaticN-Oxides by Aqueous FeII–Tiron Complex: Reaction Kinetics and Mechanisms

“…Based on the analysis of reaction products, deoxygenation is considered as the main process. It has been reported that the reduction of organic compounds by one-electron reductants, such as Fe II , often yields transient free radicals. , The reduction of N -oxides in organic solvents also leads to the formation of radical N -oxide anions as the intermediate, − followed by deoxygenation. , On the basis of the reported information and our findings, reaction schemes for the reduction of aromatic N -oxides are proposed below.…”

“…Indeed, formation of bonds between the metal ion and the O in Noxides decreased the NO double bond character as well as electron density on the ring, 61 and facilitated the deoxygenation. 48,62 In our case, we hypothesize that the strength of the N−O bond in the N-oxides is weakened in the anion radicals, 34 due to the extensive delocalization between the aromatic ring and the side chain and the decreased contribution of the resonance structures E−G as shown in Scheme 1. The weak complexation of Fe II species with the N-oxide O atom might also facilitate the cleavage of the N−O bond.…”

“…The reduction of CDX is initiated with the formation of a 5-membered ring by FeL 2 6‑ complexation on the side chain; meanwhile three resonance structures exist (Va, Vb, and Vc in Scheme c) . The formed inner-sphere complex facilitated one electron transfer from FeL 2 6‑ to CDX to form a radical with three similar resonance structures (VIa, VIb, and VIc) . Similar to 5 and 9 , transfer of another electron leads to deoxygenation from the N 1 or N 4 position to yield two CDX mono- N -oxides.…”

“…Weak complexation of Fe II species (not necessarily FeL 2 6‑ ) with the oxygen atom on the N -oxides might facilitate the reduction of the N -oxides the same way as protonation does. Indeed, formation of bonds between the metal ion and the O in N -oxides decreased the NO double bond character as well as electron density on the ring, and facilitated the deoxygenation. , …”

“…36 The formed inner-sphere complex facilitated one electron transfer from FeL 2 6to CDX to form a radical with three similar resonance structures (VIa, VIb, and VIc). 48 Similar to 5 and 9, transfer of another electron leads to deoxygenation from the N 1 or N 4 position to yield two CDX mono-N-oxides. The mono-Noxides will be reduced in a similar way leading to the formation of the final stable product DCDX.…”