Manganese Dioxide Oxidation Reactions in Ionic Liquids
127
ionicliquid.Asnotedpreviously, manganesedioxidehasbeen
traditionally used as an oxidant under heterogeneous reac-
tion conditions due to its poor solubility in most solvents.
However, the solubilization of manganese(iv) using various
strategies has a long history, which was discussed by Pastor
and Pastor in a review on the oxidant’s role in analytical
chemistry.[19] In the mid-1970s, Jáky and co-workers pos-
tulated that soluble manganese(iv) species may have been
present as intermediates during the oxidation of unsaturated
carboxylic acids with acidic permanganate.[20–23] Subse-
quently, this group prepared manganese(iv) solutions by
shaking freshly precipitated manganese dioxide with aque-
ous phosphoric acid (3 M) and investigated the kinetics of
the oxidation of a range of organic compounds, including
α,β-unsaturated alcohols, formaldehyde, formic acid, oxalic
acid, and other bifunctional substrates.[24–27] Additionally,
soluble manganese(iv) has recently been evaluated as a
chemiluminescence reagent[28] and this experience led us to
consider the use of phosphoric acid to sequester the man-
ganese dioxide after product extraction to afford a simple
and effective ionic liquid regeneration process.
afterremovingresidualsolventundervacuum.Theoxidations
were then repeated in the recycled ionic liquids to afford the
corresponding aldehydes with no decrease in the yield.
The best percent recovery we obtained in our ionic liq-
uid recycling experiments described above were through use
of phosphoric acid solutions. Recoveries reached between
78% and 81% (see Experimental section for details) for this
method of recycling whereas use of the oxalic acid solution
recycling method afforded only a 60% to 70% recovery of
ionic liquids used. These levels of recovery are not yet satis-
factory in the industrial context and we are conducting further
studies to increase recovery and re-use of ionic liquids used in
these reactions. The slight solubility of [bmIm][PF6] in water
used in these methods is the likely source of ionic liquid loss.
Experimental
1H and 13C NMR spectra were recorded on a Bruker 250 MHz spectro-
meter at the Atlantic Regional Magnetic Resonance Centre (ARMRC);
unless otherwise stated, the solvent was CDCl3 for all NMR spectra.
Gas chromatography was performed using a Varian 3800 GC-FID unit.
Mass spectra were recorded in conjunction with the GC using a Varian
2000 GC/MS electron impact system with an ion trap. All solvents were
used as obtained. Diethyl ether and tetrahydrofuran were dried and dis-
tilled from a potassium/benzophenone kettle immediately before use.
γ-MnO2 was dried in an oven and heated to 110◦C under vacuum before
use. Flash chromatography was performed using silica gel 60, 230–400
mesh. Thin-layer chromatography was performed using Kieselgel 60
F254 on aluminium-backed plates. 1H and 13C NMR of aldehyde
products and thebaine were in agreement with the literature.
There were three factors which needed to be considered
when choosing the molarity of the phosphoric acid used in
the extraction process: (a) the stability of the ionic liquid
to the concentrated acid solutions,[29] (b) the solubility of
the ionic liquid in the aqueous acid solution,[30] and (c) the
density of the acid solution. Treatment of [bmIm][PF6] with
14.5 M phosphoric acid led to the appearance of degradation
products as measured by 19F NMR. These degradation prod-
ucts were considerably less in more dilute phosphoric acid
solutions (trend 14.5 M ꢀ 10 M ≈ 5 M ≈ 1 M). The presence
of [bmIm][PF6] in the acid layer was also assayed by 19F
NMR. Concentrated acid solutions contained considerable
less ionic liquid than the dilute acid solutions (trend
14.5 M ≈ 10 M ꢀ 5 M > 1 M). During the extraction of the
MnO2 from the ionic liquid considerable oxidant settles to
the bottom of the flask. It was preferential to have the acid
solution as the lower phase to facilitate uptake of the oxidant.
Manganese Dioxide Oxidations
Method A
5.0 mL of [bmIm][BF4] were heated under vacuum in an oven-dried
flask at 60◦C for 2 h. γ-MnO2 (0.87 g, 10.0 mmol, 5 eq.) was added to
the ionic liquid under a nitrogen stream, heated for 1 h, then allowed to
cool to room temperature. THF (15 mL) and benzyl alcohol (0.21 mL,
2.0 mmol) were added and the flask placed in a sonicating bath or stirred
for 2 days, after which time a further MnO2 (0.87 g, 10.0 mmol, 5 eq.)
were added to the flask under a nitrogen stream.After 4 days in total, the
upperTHF layer was decanted. The ionic liquid was extracted with ethyl
acetate (6 × 15 mL)then washedwith water(50 mL), dried over MgSO4,
filtered, and concentrated to a clear oil. Flash column chromatography
wasperformedusing5 : 1hexanes/ethylacetateasaneluent.Theproduct
was isolated as a clear oil (0.159 g, 1.50 mmol, 75%).
The density of [bmIm][PF6] is 1.37 g mL−1 [31]
phoric acid is 1.69 g mL−1. We were pleased to find that
10 M phosphoric acid has a suitable density of 1.47 g mL−1
,
14.5 M phos-
.
Ionic Liquid Regeneration
Method A
Based on the studies above, 10 M phosphoric acid was chosen
as the acid solution of choice.
Oxalic acids solutions also proved to be effective for the
washing/recycling of the [bmIm][PF6] ionic liquid used in
MnO2 oxidation reactions. Thus, after performing oxidations
of benzyl alcohol and geraniol in biphasic [bmIm][PF6]/Et2O
systems and working-up the reactions, ∼20 mL of a 1.0 M
oxalic acid solution was stirred with the [bmIm][PF6]/MnO2
suspension overnight. The result was a clear upper aqueous
layer containing a white precipitate and a tan lower ionic
liquid layer. The aqueous layer was decanted, leaving some
of the white precipitate in the ionic liquid. This was dis-
solved in acetone and filtered. The acetone was evaporated
and the cloudy tan oil was washed with water several times
until the washings were pH-neutral, then once with diethyl
ether, after which the ionic liquid became clear. The ionic liq-
uid was clean according to 1H and 13C NMR measurements
After the geranial had been isolated, H3PO4 (10 M, 10 mL) was added to
the MnO2/[bmIm][PF6] phase. After standing overnight, the MnO2 was
suspended in the lower acid layer and the top layer was clear colourless
[bmIm][PF6]. The ionic liquid was decanted and the acid layer was
extracted with dichloromethane : hexane (8/2, 3 × 25 mL). The com-
bined ionic liquid extracts were washed with saturated NaHCO3 until
the aqueous extracts were basic, then washed with water (25 mL). Sol-
vents were removed in vacuo at room temperature and clear colourless
[bmIm][PF6] was recovered in 78–81% yield.
Method B
After the geranial had been isolated oxalic acid (1.0 M, 25 mL) was
added to the MnO2/[bmIm][PF6] phase, which formed a reddish upper
layerandbubbleswereproduced.After2 h, theupperlayerwasclearwith
a white precipitate.The lower layer was a clear tan oil.The aqueous layer
was decanted, and the ionic liquid dissolved in acetone. The precipitate
was filtered from the organic phase and the filtrate evaporated. The tan