LETTER
▌1371
letter
Development of Manganese(VI) Oxidising Agents Soluble in Organic Solvents
Development of Manganese(VI) Oxidising Agents
Rhys Ellis, Kee-Han Lee, Matthew Ainsworth, Alexander Kerr, Eddy M. E. Viseux*
School of Life Sciences, University of Sussex, Falmer, Brighton, BN1 9AG, UK
E-mail: e.m.e.viseux@sussex.ac.uk
Received: 01.03.2012; Accepted after revision: 15.03.2012
th
The author would like to dedicate this paper to Prof. Philip J. Parsons on the occasion of his 60 birthday.
trimethylammonium iodide: bistetrabutylammonium
Abstract: Two manganate(VI) reagents have been prepared from
manganate (bTBAM, Scheme 1) and the mixed salt tetra-
permanganate salts that show excellent oxidising properties in com-
butylammonium phenyltrimethylammonium manganate.
mon organic solvents.
Of note those salts were found to be unstable in acidic me-
Key words: manganite, oxidising agent, allylic alcohol, ketone,
amine
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dium, just like the other manganate oxides.
Manganese(VI) oxides are typically green in colour, due
to a combination of the relatively large molar extinction
coefficient ε, arising from the ligand-to-metal charge
transfer, and to the absorption within the visible region of
the spectrum. They are usually characterised by an ab-
sorption maxima at a wavelength of 606 nm with an ε val-
ue of 1200 ± 50 dm mol cm . Permanganate, on the
other hand, has an absorption maximum at a wavelength
of 525 nm.
Manganese exhibits many oxidation states from +I to
+VII, +II being the most stable ion. Mn(II) ions are re-
quired as a trace mineral for all living organisms as they
function as a cofactor for a large number of enzymes.
There are few manganese(VI) oxides in the literature that
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are useful to synthetic chemists. Barium and potassium
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variants have been extensively studied and used, but
show their limitations due to their lack of solubility in
common organic solvents. Mn(VI) oxides, just like man-
ganese dioxide, are excellent oxidising agents and are par-
i
Bu4NMnO2
(Bu4N)2MnO2
ticularly effective at oxidising primary and secondary Scheme 1 Reagents and conditions: (i) Bu NOH, H O, sonication,
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allylic and benzylic alcohols to their corresponding alde-
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hydes, ketones, or even carboxylic acids, and at oxidising
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amines, silyl ethers, and thiols.
Though the salts are soluble in a variety of organic sol-
vents, including DMSO, DMF, MeCN, acetone, THF,
EtOAc, 1,4-dioxane, pyridine, CHCl , and CH Cl , the
A wide range of conditions have been used including the
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use of microwaves, solvent-free reactions, or in con-
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best solvents for the oxidation were found to be THF and
acetonitrile. The compound is also moderately soluble in
benzene to produce ‘green benzene’ by analogy with the
well-known ‘purple benzene’ reported by Herriott and
junction with other metals or semimetals like lithium,
copper, arsenic, tellurium, thallium, ruthenium
III), and lead. Despite their interesting chemoselectiv-
ity, a large excess of the reagents is usually required, from
–50 equivalents, which renders them particularly expen-
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3j
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(
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Picker. Table 1 exemplifies the use of bTBAM as a mild
oxidising agent, with reactions occurring at room temper-
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sive on larger scale with significant waste in manganese
element.
ature. Though MnO is usually successful at oxidising al-
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lylic alcohols, yields are usually lower. BaMnO and
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To avoid this shortfall, we synthesised two complexes that
are soluble in both organic and aqueous solvents based on
the corresponding ammonium salts. Because alkaline so-
lution of potassium iodide is usually used to reduce potas-
K MnO can also perform such oxidations illustrated in
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Table 1, but require a large excess of the oxidising agent.
In conclusion we have shown that ammonium salts of
manganate oxides allow the oxidation of a variety of func-
tional groups under mild conditions in an easy and widely
applicable procedure.
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sium permanganate to potassium manganate, our initial
attempts involved the use of ammonium iodides as a re-
ducing agent for potassium permanganate, but the stabili-
ty of the green complex was short-lived, with degradation
to brown residues, presumably insoluble Mn(IV). Best re-
sults were obtained with trialkylammonium hydroxides
and tetrabutylammonium permanganate (Bu N) MnO .
Preparation of (Bu N) MnO (bTBAM)
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Tetrabutylammonium permanganate (1.00 g, 2.77 mmol) was add-
ed to a flask containing TBAOH solution (40 wt%, 5 mL, 7.70
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Two reagents were synthesised using tetrabutylammoni-
mmol) and distilled H O (45 mL). The mixture was then placed into
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um hydroxide, tetrabutylammonium iodide, and phenyl- a sonicator bath and sonicated for 10 min at 60 °C. The mixture was
subsequently poured through a sinter funnel to remove any undis-
solved permanganate salts. The green solution was subsequently
lyophilised to give a green solid which can be kept in a desiccator
under reduced pressure, or in a sealed flask under argon. H NMR
SYNLETT 2012, 23, 1371–1373
Advanced online publication: 14.05.2012
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DOI: 10.1055/s-0031-1290691; Art ID: ST-2012-D0187-L
Georg Thieme Verlag Stuttgart · New York
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