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Reoxidant

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In chemistry, a reoxidant is a reagent that regenerates a catalyst by oxidation. In some cases they are used stoichiometrically, in other cases only small amounts are required.

Applications

OsO4-catalyzed dihydroxylations

Reoxidants are commonly used in reactions catalyzed by osmium tetroxide, which is a primary oxidant converting alkenes to glycols. The spent catalyst is an osmium(VI) complex, which reacts with a reoxidant to regenerate Os(VIII). Typical reoxidants for this application include pyridine-N-oxide, ferricyanide/water, and N-methylmorpholine N-oxide.

Vanadium(III)-based alkene polymerizations

As catalysts for the polymerization of dienes, vanadium complexes are activated with alkylaluminium chlorides, e.g. diethylaluminium chloride. The organoaluminium reagent installs alkyl groups on the V(III) precatalyst. During catalysis or during catalyst activation, some vanadium(III) is reduced to inactive vanadium(II) derivatives. To correct for this reduction, reoxidants such as methyl trichloroacetate are added. The alkyl chloride functions as a source of a chlorine radical, which adds to the inactive V(II) species. In some cases, the reoxidants are called rejuvenators.

Oxidations with TEMPO

(2,2,6,6-Tetramethylpiperidin-1-yl)oxyl, commonly known as TEMPO, is an expensive but effective oxidant for converting alcohols to carbonyls. With iodine as the reoxidant, TEMPO-H is oxidized back to TEMPO, which then functions catalytically:

oxidation: RCH2OH + 2 TEMPO → RCHO + 2 TEMPO-H
reoxidation: 2 TEMPO-H + I2 → 2 TEMPO + 2 HI

References

  1. Timothy J. Donohoe; Katherine M. P. Wheelhouse (née Gosby); Peter J. Lindsay-Scott; Paul A. Glossop; Ian A. Nash; Jeremy S. Parker (2008). "Pyridine-N-Oxide as a Mild Reoxidant Which Transforms Osmium-Catalyzed Oxidative Cyclization". Angew. Chem. Int. Ed. 47 (15): 2872–2875. doi:10.1002/anie.200705425. PMID 18324650.
  2. Kotohiro Nomura; Shu Zhang (2011). "Design of Vanadium Complex Catalysts for Precise Olefin Polymerization". Chem. Rev. 111 (3): 2342–2362. doi:10.1021/cr100207h. PMID 21033737.
  3. R. A. Miller; R. S. Hoerrner (2003). "Iodine as a Chemoselective Reoxidant of TEMPO: Application to the Oxidation of Alcohols to Aldehydes and Ketones". Org. Lett. 5 (3): 285–287. doi:10.1021/ol0272444. PMID 12556173.
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