In
chemistry, a reoxidant is a
reagent that regenerates a
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
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
Osmium tetroxide (also osmium(VIII) oxide) is the chemical compound with the formula OsO4. The compound is noteworthy for its many uses, despite its toxicity and the rarity of osmium. It also has a number of unusual properties, one being that the ...
, which is a primary oxidant converting alkenes to
glycol
A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified.
The most common industrial diol is e ...
s. 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
Pyridine-''N''-oxide is the heterocyclic compound with the formula C5H5NO. This colourless, hygroscopic solid is the product of the oxidation of pyridine. It was originally prepared using peroxyacids as the oxidising agent. The compound is used ...
, ferricyanide/water, and
N-methylmorpholine N-oxide
''N''-Methylmorpholine ''N''-oxide (more correctly 4-methylmorpholine 4-oxide), NMO or NMMO is an organic compound. This heterocyclic amine oxide and morpholine derivative is used in organic chemistry as a co-oxidant and sacrificial catalyst in ...
.
Vanadium(III)-based alkene polymerizations
As catalysts for the polymerization of dienes, vanadium complexes are activated with alkylaluminium chlorides, e.g.
diethylaluminium chloride
Diethylaluminium chloride, abbreviated DEAC, is an organoaluminium compound. Although usually given the chemical formula (C2H5)2AlCl, it exists as a dimer, C2H5)2AlClsub>2 It is a precursor to Ziegler-Natta catalysts employed for the production ...
. 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
In musical terminology, tempo ( Italian, 'time'; plural ''tempos'', or ''tempi'' from the Italian plural) is the speed or pace of a given piece. In classical music, tempo is typically indicated with an instruction at the start of a piece (ofte ...
, 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:
[{{cite journal, title=Iodine as a Chemoselective Reoxidant of TEMPO: Application to the Oxidation of Alcohols to Aldehydes and Ketones, authors=R. A. Miller, R. S. Hoerrner, journal=Org. Lett., year=2003, volume=5, pages=285–287, doi=10.1021/ol0272444]
:''oxidation:'' RCH
2OH + 2 TEMPO → RCHO + 2 TEMPO-H
:''reoxidation:'' 2 TEMPO-H + I
2 → 2 TEMPO + 2 HI
References
Reaction mechanisms
Catalysis
Oxidizing agents