Lewis acid-activated oxidation of alcohols by permanganate

Article abstract of DOI:10.1039/c1cc12024g

The oxidation of alcohols by KMnO₄ is greatly accelerated by various Lewis acids. Notably the rate is increased by 4 orders of magnitude in the presence of Ca²⁺. The mechanisms of the oxidation of CH ₃OH and PhCH(OH)CH₃ by MnO₄^- and BF₃·MnO₄^- have also been studied computationally by the DFT method.

Full text of DOI:10.1039/c1cc12024g

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COMMUNICATION
Lewis acid-activated oxidation of alcohols by permanganatew
Hongxia Du,ab Po-Kam Lo,b Zongmin Hu,abc Haojun Liang,*ac Kai-Chung Lau,*b
Yi-Ning Wang,b William W. Y. Lamb and Tai-Chu Lau*bc
Received 9th April 2011, Accepted 4th May 2011
DOI: 10.1039/c1cc12024g
The oxidation of alcohols by KMnO4 is greatly accelerated by
various Lewis acids. Notably the rate is increased by 4 orders of
magnitude in the presence of Ca2+. The mechanisms of
À
the oxidation of CH3OH and PhCH(OH)CH3 by MnO4 and
BF3ÁMnO4À have also been studied computationally by the DFT
method.
The design of highly active metal oxo species that can mimic
the reactivity of the iron oxo active intermediates in enzymes
such as cytochrome P450 and methane monooxygenase
continues to be a challenge for chemists.1–3 Our strategy to
Fig. 1 Kinetic trace at 526 nm for the BF3 (4.0 Â 10À3 M) activated
generate highly oxidizing species is to make use of Lewis acids
to activate metal oxo species that are otherwise relatively
stable and unreactive.4–8 For example, we have reported
that the oxidation of alkanes by KMnO4 in acetonitrile is
accelerated by over seven orders of magnitude in Àthe presence
of BF3.4 In the absence of substrates BF3/MnO4 undergoes
an intramolecular OÁ Á ÁO coupling reaction to generate O2.9
We report here that the oxidation of alcohols by KMnO4 in
CH3CN is also greatly accelerated by various Lewis acids,
including BF3, Sc(CF3SO3)3, Zn(CF3SO3)2, Ca(CF3SO3)2 and
Ba(CF3SO3)2.
oxidation of CH3OH (1.0 Â 10À2 M) by KMnO4 (1.0 Â 10À4 M) in
CH3CN at 298 K. The inset shows the plot of ln(At À Af) vs. t.
attribute this step to the formation of a BF3/MnO4À adduct, as
shown in eqn (1); while the second step is the oxidation of
CH3OH by the adduct. In the presence of excess BF3 and
À
CH3OH, the decay of MnO4 at 526 nm (the second step)
follows clean pseudo-first-order kinetics for over three
half-lives (Fig. 1). The pseudo-first-order rate constant,
k
obs, increases linearly with [BF3] (Fig. S2, ESIw). kobs also
increases with [CH3OH], but reaches saturation at high
[CH3OH]; and the plot of 1/kobs vs. 1/[CH3OH] is linear
(Fig. 2). At 298 K, KCH OH and kKBF are found to be
A solution of KMnO4 in CH3CN reacts very slowly with
CH3OH at room temperature. The half-life (t1/2) for the
reaction of 1 Â 10À4 M of KMnO4 with 0.1 M CH3OH in
CH3CN at 298 K is B110 h, and we estimate that k2, the
3
3
(7.06 Æ 0.31) Â 102 MÀ1 and (2.67 Æ 0.07) Â 102 MÀ1 sÀ1
respectively. These results are consistent with the reaction
second-order rate constant, is B1 Â 10À5 MÀ1
s
À1. On the
other hand, in the presence of BF3, the oxidation of CH3OH
by KMnO4 in CH3CN occurs within seconds. There is a very
rapid decrease in absorbance at around 550 nm, followed by a
much slower decay; as monitored by UV/Vis stopped-flow
spectrophotometry (Fig. S1, ESIw). The initial decay is too
fast to be followed by stopped-flow spectrophotometry, we
a CAS Key Laboratory of Soft Matter Chemistry,
University of Science and Technology of China, Hefei,
Anhui 230026, P. R. China. E-mail: hjliang@ustc.edu.cn
b Department of Biology and Chemistry and Institute of Molecular
Functional Materials, City University of Hong Kong,
Tat Chee Avenue, Kowloon Tong, Hong Kong, P. R. China.
E-mail: kaichung@cityu.edu.hk, bhtclau@cityu.edu.hk
c Advanced Laboratory of Environmental Research and
Technology(ALERT), Joint Advanced Research Center, USTC-City U,
Suzhou, Jiangsu 215124, P. R. China. Fax: +86 852 3442 0522
w Electronic supplementary information (ESI) available: Experimental
details, kinetic data, potential energy diagrams. See DOI: 10.1039/
c1cc12024g
Fig. 2 Plot of kobs vs. [CH3OH]. [BF3] = 4.0 Â 10À3 M, [KMnO4] =
1.0 Â 10À4 M, T = 298 K. The inset shows the corresponding plot
of 1/kobs vs. 1/CH3OH (slope = (1.27 Æ 0.01) Â 10À3, y-intercept =
(9.43 Æ 0.02) Â 10À1, r = 0.998).
c
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Chem. Commun., 2011, 47, 7143–7145 7143

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scheme shown in eqn (1)–(6). If CF3CO2H is replaced by BF3,
the rate is slower by 4103 under the same conditions.
Table 2 Representative rate data for the oxidation of alcohols by
BF3/KMnO4
Substrate
KROHa/MÀ1
kKBF a/MÀ1 sÀ1
3
Methanol
Ethanol
(7.06 Æ 0.31) Â 102 (2.67 Æ 0.07) Â 102
(1.50 Æ 0.04) Â 103 (6.07 Æ 0.09) Â 102
(3.88 Æ 0.16) Â 102 (7.60 Æ 0.12) Â 102
(4.90 Æ 0.24) Â 102 (2.08 Æ 0.51) Â 103
(1.72 Æ 0.14) Â 102 (6.80 Æ 0.21) Â 102
(5.58 Æ 0.25) Â 102 (1.69 Æ 0.27) Â 103
(3.23 Æ 0.61) Â 101 (6.00 Æ 0.44) Â 103
(4.28 Æ 0.05) Â 101 b
2-Propanol
Cyclohexanol
2-Heptanol
1-Heptanol
Benzyl alcohol
1-Phenylethanol
4-Chlorobenzyl alcohol (9.98 Æ 0.10) Â 101 (2.28 Æ 0.08) Â 103
Cyclobutanol
(4.52 Æ 0.09) Â 102 (6.22 Æ 0.04) Â 102
a
b
KMnO4, 1.0 Â 10À4 M; BF3, 4.0 Â 10À3 M; 298 K, CH3CN. This
rate constant should be equal to kKBF KROH
.
3
a,b
The effects of other Lewis acids on the oxidation of CH3OH
by KMnO4 have also been investigated at [Lewis acid] =
1 Â 10À3 M. The rate law is: Rate = k2[MnO4À][CH3OH] for
these Lewis acids, and no saturation kinetics are observed. The
observed saturation kinetics in the case of BF3 is probably due
to stabilization of the intermediate by RO–HÁ Á ÁF–B hydrogen
bonding, which is supported by DFT calculations described
below. From Table 1 it can be seen that these Lewis acids
accelerate the oxidation of CH3OH by KMnO4 by 3–7Àorders
of magnitude. The ability of Ca2+ to activate MnO4 by 4
orders of magnitude is of particular interest, because Ca2+
plays a key role in the CaMn4Ox active site in the oxygen
evolving complex (OEC) in Photosystem II (PSII).10–12
Table 3 Product yields for the oxidation of alcohols by KMnO4/BF3
Alcohol
Product
mmol (%yield)
Cyclohexanol
Benzyl alcohol
2-Heptanol
Cyclohexanone
Benzaldehyde
2-Heptanone
0.020 (100)
0.020 (100)
0.016 (80)
0.015 (75)
Cyclobutanol
Cyclobutanone
a
KMnO4, 0.008 mmol; BF3, 0.12 mmol; alcohol, 0.8 mmol. T = 23 1C.
Time = 15 min. All the reactions were carried out in CH3CN (2 ml)
b
under argon with chlorobenzene as internal standard. Yields are
calculated based on KMnO4 acting as a 5-electron oxidant.
abstraction mechanism.13 A deuterium isotope effect of
The BF3-activated oxidation of various other alcohols by
KMnO4 in CH3CN has also been studied (Table 2). Rate
saturation with respect to [ROH] is observed for all substrates,
except 1-phenylethanol, where the rate increases linearly with
[1-phenylethanol] from 0.001–0.01 M. Notably the rate
constants are within a very narrow range, despite their large
differences in C–H bond dissociation energies (BDFE) and
hydride affinities (HA) (Table S1, ESIw). Table 3 shows
product yields for selected alcohols. In the oxidation of
cyclohexanol and benzyl alcohol, 2.5 mol of the carbonyl
product is formed per mol of KMnO4, suggesting that it
functions as a 5-electron oxidant. The absence of broad
absorptions in the range 400–600 nm due to colloidal MnO2
and the inability of the final solution to oxidize IÀ to I2 are
consistent with the product being Mn(II) rather than MnO2.
On the other hand, in the absence of BF3, brown precipitate of
MnO2 was observed at the end of the reaction. In the
oxidation of cyclobutanol by BF3/MnO4À, cyclobutanone is
1.5 Æ 0.2 was obtained for the oxidation of both methanol
(kCH OH/kCD OD) and benzyl alcohol (kC H CH OH/kC D CD OH
)
3
3
6
5
2
6
5
2
by BF3/MnO4À. These are much smaller than the corres-
ponding values for other metal oxo species. For example, for
Ru(bpy)2(py)(O)2+, the deuterium isotope effects are 9 and
50 for CH3OH/CD3OH and C6H5CH2OH/C6H5CD2OH,
respectively.14 However small deuterium isotope effects of
1.84 and 2.58 have been reported for known hydride transfer
from (CH3)2CHOH to Ph3C+ and HCO2À, respectively.15
In order to gain more insight into the mechanism of alcohol
oxidation by Lewis acid/KMnO4, DFT calculations have been
performed on the oxidation of CH3OH and PhCH(OH)CH3
by KMnO4 and BF3ÁMnO4À. The reaction barriers (DGo298) in
gas phase and in acetonitrile are summarized in Table 4. The
reaction barriers are substantially lowered in the presence of
BF3, consistent with the observed accelerating effects.
The singlet and triplet potential energy surfaces (PESs) for
the oxidation of CH3OH by [BF3ÁMnO4]À is shown in
Scheme 1. CH3OH and [BF3ÁMnO4]À first form an intermediate
formed exclusively, suggesting
a
two-electron hydride
Table 1 Rate constants at 298 K for the oxidation of CH3OH by
KMnO4 in CH3CN in the presence of various Lewis acidsa
Table 4 Reaction barriers (DGa298, in kcal molÀ1) for the oxidation
of methanol and 1-phenylethanol by MnO4À and [BF3ÁMnO4]À in gas
phase and in acetonitrile
Lewis acid
k2/MÀ1 sÀ1
Rate accelerationc
None
B1 Â 10À5
(8.6 Æ 0.2) Â 10À2
(1.12 Æ 0.03) Â 10À1
(9.71 Æ 0.18) Â 10À1
(1.35 Æ 0.02) Â101
(7.6 Æ 0.4) Â 102
8.6 Â 103
1.1 Â 104
9.7 Â 104
1.3 Â 106
7.6 Â 107
DGa
Gas phase
PCM (acetonitrile)
Ba(CF3SO3)2
Ca(CF3SO3)2
Zn(CF3SO3)2
Sc(CF3SO3)3
298
MethaÀnol oxidation
MnO4
20.3
15.1
26.1
16.3
À
b
BF3ÁMnO4
BF3
1-Phenylethanol oxidation
a
b
KMnO4, 1.0 Â 10À4 M; Lewis acid, 1.0 Â 10À3 M. BF3, 4.0 Â 10À3 M.
À
MnO4
BF3ÁMnO4
16.7
20.4
24.4
16.7
c
Ratio of k2(Lewis acid/MnO4À)/k2(MnO4À).
À
c
7144 Chem. Commun., 2011, 47, 7143–7145
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Scheme 1 Potential energy surface for the oxidation of CH3OH by [BF3ÁMnO4]À at the B3LYP level using LanL2DZ basis set for transition metal
(Mn) and 6-311++G(d,p) basis set for nonmetal atoms. Relative 298 K Gibbs free energies in gas phase and in acetonitrile (in parentheses) are
given in kcal molÀ1
.
(11M-INT1), [(MnO3)(OBF3)(CH3OH)]À, followed by hydride
transfer to form (31M-INT2), [MnO2(OH)(OBF3CH2OH)]À.
The hydride transfer step occurs with a reaction barrier of
The work described in this communication was supported by
Hong Kong University Grants Committee Area of Excellence
Scheme (AoE/P-03-08) and General Research Fund (CityU
101709). H. J. Liang also acknowledges financial support from
the National Natural Science Foundation of China (Nos.
20874094, 20934004) and NBRPC (No.2010CB934500).
1
DGo298 = 16.3 kcal molÀ1 in CH3CN via 1M-TS1 and forms
31M-INT2 by singlet-triplet PES crossing. This hydride transfer
is followed by combination of the resulting carbocation with
3
Mn–OBF3 to form a very stable intermediate 1M-INT2 and
the DGo298 of the hydride abstraction is À56.1 kcal molÀ1
relative to 11M-INT1 (in CH3CN). The intermediate
31M-INT3, formed by internal rotation of hydroxyl groups in
31M-INT2, undergoes proton transfer from the hydroxyl group
of the alcohol to Mn–OH to form 31M-INT4. INT4 then
decomposes to CH2O, H2O and 3MnO2(OBF3)À, the latter
Mn(V) species presumably undergoes disproportionation to
give Mn(VII) and Mn(IV). The mechanism of 1-phenylethanol
oxidation by BF3ÁMnO4À is similar to that of methanol, except
because of the bulkiness of PhCH(OH)CH3, the BF3 is required
to bind to a free manganese oxo group that is not directly
involved in bonding interactions with the CH3OH molecule
during the redox process (ESIw). Apparently this leads to a
smaller effect of BF3 on the reaction barrier (Table 4), resulting
in CH3OH and PhCH(OH)CH3 being oxidized at similar rates,
despite their large differences in BDFE and HA.
Notes and references
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In conclusion, we have reported a number of novel effects of
Lewis acids on the oxidation of alcohols by MnO4À. First, rate
accelerations of 3–7 orders of magnitude are observed.
À
Second, MnO4 usually functions as a 3-electron oxidant in
organic solvents, but in the presence of Lewis acids it may
function as a 5-electron oxidant, apparently Lewis acids can
also activate the intermediate MnO2 to carry out further
oxidation.
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 7143–7145 7145