Iron(II)-Catalyzed Biomimetic Aerobic Oxidation of Alcohols

Article abstract of DOI:10.1002/anie.202000054

We report the first Fe^II-catalyzed biomimetic aerobic oxidation of alcohols. The principle of this oxidation, which involves several electron-transfer steps, is reminiscent of biological oxidation in the respiratory chain. The electron transfer from the alcohol to molecular oxygen occurs with the aid of three coupled catalytic redox systems, leading to a low-energy pathway. An iron transfer-hydrogenation complex was utilized as a substrate-selective dehydrogenation catalyst, along with an electron-rich quinone and an oxygen-activating Co(salen)-type complex as electron-transfer mediators. Various primary and secondary alcohols were oxidized in air to the corresponding aldehydes or ketones with this method in good to excellent yields.

Full text of DOI:10.1002/anie.202000054

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Title: Iron(II)-Catalyzed Aerobic Biomimetic Oxidation of Alcohols
Authors: Jan-E. Bäckvall, Kim Schlipköter, and Arnar Guðmundsson
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10.1002/anie.202000054
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Iron(II)-Catalyzed Aerobic Biomimetic Oxidation of Alcohols
Arnar Guðmundsson, Kim Elisabeth Schlipköter+ and Jan-E. Bäckvall*
Abstract: Herein we report on the first Fe(II)-catalyzed aerobic
biomimetic oxidation of alcohols. The principle of this oxidation,
which involves several electron transfer steps, is reminiscent of
biological oxidation in the respiratory chain. The electron transfer
from the alcohol to molecular oxygen occurs with the aid of three
coupled catalytic redox systems, leading to a low-energy pathway.
An iron transfer hydrogenation complex was utilized as a substrate-
selective dehydrogenation catalyst along with an electron-rich
quinone and an oxygen-activating Co(salen)-type complex as
electron transfer mediators. Various primary and secondary alcohols
were oxidized in air to their corresponding aldehydes or ketones with
this method in good to excellent yields.
Iron catalysis has gained significant attention in the past few
years and has found applications in many different
transformations including cross-coupling reactions and transfer
hydrogenation reactions.[4-6] (Cyclopentadienone)iron tricarbonyl
complexes, first synthesized by Reppe and Vetter in 1953,[7]
constitute a prominent class of catalysts for hydrogen transfer
reactions, and the first catalytic reaction with these types of
complexes was reported by the group of Casey in 2007.[8] One
of these complexes is I, which can be activated in situ to
generate dicarbonyl intermediate I′, which can in turn be
reduced to iron hydride II (Scheme 2). The activation of I can be
done in different ways, one of which being oxidative
decarbonylation by trimethylamine N-oxide (TMANO).[9] Iron
hydride II was first isolated by the group of Knölker.[10,11,12]
Complex I and related iron tricarbonyl complexes have found
extensive use in transfer hydrogenation reactions,[8,13] and our
group has applied I in both the dynamic kinetic resolution of sec-
alcohols[14] and in the cycloisomerization of -functionalized
allenes.[15]
Oxidations constitute a fundamental class of reactions in
organic chemistry and many important chemical transformations
involve oxidation steps. Although a large number of oxidation
reactions have been developed, there is an increasing demand
for more selective, mild, efficient, and scalable oxidation meth-
ods.[1] Of particular interest are methods inspired by biological
pathways,[2] in which oxidants such as molecular oxygen (O2) or
hydrogen peroxide (H2O2) are employed. These oxidants are
ideal to use in industrial oxidations as they are inexpensive and
environmentally friendly. However, the direct oxidation of an
organic substrate by H2O2 or O2 is challenging owing to the large
energy barrier and low selectivity associated with such reac-
tions. The use of a substrate-selective redox catalyst (SSRC) in
combination with direct re-oxidation of the reduced form of the
SSRC (i.e. SSRCred) by H2O2 or O2 is also often associated with
a high energy barrier. Nature’s elegant way of circumventing this
problem is through the orchestration of a variety of enzymes and
co-enzymes that can act as electron transfer mediators (ETMs),
which lower the overall barrier for electron transfer from SSRCred
to H2O2 or O2 (Scheme 1). These ETMs are part of what is called
the electron transport chain (ETC), where O2 is typically used as
the terminal oxidant.[3] This bypasses the high kinetic barrier
associated with direct oxidation by O2 and leads to a lower
overall energy barrier via stepwise electron transfer.
Scheme 2. Activation of iron tricarbonyl complex (I) and Shvo′s catalyst (III)
The cyclopentadienone(iron) and cyclopentadienyl(iron) cata-
lysts (I, I’, and II) are related to the Shvo catalyst III and its
monomers IV and V (Scheme 2). In fact, I’ and II are
isoelectronic to IV and V, respectively. Intermediates I’ and IV
are proposed to be the active dehydrogenation catalysts in the
racemization of alcohols.[14,16] We therefore envisioned that
cyclopentadienone(iron) complexes A could be used as efficient
dehydrogenation catalysts for alcohols and that the
cyclopentadienyl(iron) hydride B formed could be recycled to A
via oxidation by a benzoquinone (Scheme 3). Reoxidation of the
hydroquinone formed could occur by the use of a combined
system consisting of O2 and an oxygen-activating metal
macrocycle (e.g. VI) leading to a biomimetic aerobic oxidation of
the alcohol. The biomimetic approach depicted in Scheme 3 was
previously successfully applied by our group using the Shvo
catalyst III.[17] Related biomimetic oxidations via similar electron
transfer chains have also been reported by our group using
palladium[18] or osmium[19] as substrate-selective redox
catalysts.[20]
Scheme 1. Principle for oxidation with O2 or H2O2 via the use of ETMs (ETM =
electron transfer mediator; SSRC = substrate selective redox catalyst)
[*]
A. Guðmundsson, K. E. Schlipköter+, Prof. Dr. J.-E. Bäckvall*
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University
Stockholm SE-10691 (Sweden)
E-mail: jeb@organ.su.se
[+]
Current address: Institute of Technical Biocatalysis
Hamburg University of Technology TUHH
Hamburg 21071 (Germany)
Supporting information for this article is given via a link at the end of
the document.
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5[d]
6[d]
THF
VIc
VIc
Air
Air
8
8
8
9
8
2-Me
THF
10
7
Anisole
Anisole
VIc
VIc
Air
Air
40
3
44
3
47
3
8[e]
[a] General reaction conditions: The reaction was conducted at 100 °C with 0.5
mmol of 1b, 0.6 mmol of DMBQ, 0.24 mmol of VI, 0.05 mmol of I, 0.05 mmol
of TMANO, and 3 mL of solvent. [b] Yields were determined by GC analysis.
[c] 40 mol% of VI. [d] Run at 70 °C. [e] Blank reaction without I.
Scheme 3. The biomimetic oxidation approach using an iron catalyst as SSRC
Other groups have reported on the use of Fe(III) porphyrin
complexes in biomimetic oxidation of alcohols.[21] However, to
the best of our knowledge there are no examples on the use of
Fe(II) complexes in biomimetic aerobic oxidation of alcohols.
Our initial attempts began by investigating various 1,4-
benzoquinones) as stoichiometric oxidants in the iron-catalyzed
oxidation of 1-phenylethanol (1a) using I as catalyst. Of the
quinones tested, 2,6-dimethoxy-1,4-benzoquinone (DMBQ) was
found to be the best, giving acetophenone (2a) in a yield of 9%
after 60 min (see Supplementary Information Table S1).
We next studied the use of cyclopentadienone(iron) catalyst
VII previously reported by the group of Funk to be more active
than I in both the oxidation of alcohols and in the reduction of
ketones and aldehydes (Figure 1).[13a] The change of catalyst
from I to VII led to a significant improvement of the oxidation of
1b by DMBQ (1.2 equiv) under air, most likely due to the
increased stability of complex VII in the presence of air (Table 2,
entry 1).
We next studied the oxidation using DMBQ as the
stoichiometric oxidant. As the oxidation of the alcohol can be
done at a lower oxidation potential if the alcohol is more
electron-rich, the substrate was changed to 1-(p-
methoxyphenyl)ethanol (1b). The catalyst loading of I was also
increased to 10 mol%. Under these reaction conditions a yield of
26% of 2b was obtained after 60 min (Table 1, entry 1). With the
ultimate goal of developing an aerobic oxidation reaction, it was
of interest to determine the sensitivity of the system towards O2.
We therefore studied the oxidation with DMBQ (1.2 equiv) under
air as well as under pure O2 with and without cobalt complex VI
(for the structure of VI see Scheme 3). In both cases (O2 or air),
the expected oxidative deactivation of the catalyst was observed
(entries 2-3). Next, several solvents were screened (entries 4-7)
in the presence of air, with anisole giving the best result (entry
7). To demonstrate the necessity of iron catalyst I for the
reaction, a blank reaction without I was performed, which as
expected gave negligible yield of ketone product 2b (entry 8).
Figure 1. Iron tricarbonyl complex VII (DMPh = 3,5-dimethylphenyl)
At this point, we set out to make the reaction catalytic with
respect to VI, VII and DMBQ. When the amounts of DMBQ and
cobalt complex VI were reduced to the amounts used in our
previous work on ruthenium-catalyzed oxidation of alcohols,[17a]
the conversion dropped significantly (Table 2, entry 2). Doubling
the amounts of either VII or DMBQ led to an increase in yield to
74-76 % (entries 3-4). However, when both VII and DMBQ were
increased, only a marginal further increase was observed (entry
5). Varying the amount of VI was found not to affect the reaction
rate significantly (cf. entries 3 and 6). Reducing the temperature
to 80 °C significantly lowered the yield of ketone 2b (entry 7),
and reducing the O2 amount to 2 % did not significantly affect
the yield (entry 8). To examine the effect of water on the
reaction, 2 equiv of water were added to the reaction mixture,
which led to a decrease in yield from 75 to 64 %. (cf. entries 6
and 9). The addition of 4Å molecular sieves to the reaction
mixture did not lead to any observed improvement and in fact
proved slightly detrimental (entry 10). Increasing the
concentration twofold (from 0.17 M to 0.33 M) led to essentially
full conversion after 1 h (entry 11). The addition of 1 equiv of
K2CO3 was found not to affect the yield, but led to a more robust
and easily reproducible procedure (entry 12).
Table 1. Optimization of reaction conditions using DMBQ as oxidant.
Entry[a]
Solvent
Addi-
tive
Atmo-
sphere
GC yield
10 min
(%)[b]
GC yield
30 min
(%)[b]
GC yield
60 min
(%)[b]
1
2
3
4
Toluene
Toluene
Toluene
CPME
-
Ar
O2
Air
Air
15
7
21
7
26
8
-
VIc
VIc
9
12
11
14
11
11
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10.1002/anie.202000054
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of aldehyde 2m. The ability of iron catalyst VII to promote
oxidation of primary alcohols is in contrast to our previous work
on the corresponding biomimetic aerobic ruthenium-catalyzed
oxidation of alcohols using the Shvo catalyst. In our previous
work, aldehydes could not be obtained from primary alcohols
due to disproportionation of the aldehydes caused by the Shvo
catalyst.[17a]
Table 2. Optimization of the amounts of VI, VII and DMBQ.
Entry[a]
Amount
of VII
(mol%)
Amount of
VI (mol%)
Amount
of DMBQ
(mol%)
GC yield
10 min
(%)[b]
GC yield
60 min
(%)[b]
1
2
10
10
10
20
20
10
10
10
10
10
10
10
40
2
2
2
4
4
4
4
4
4
4
4
120
20
40
20
40
40
40
40
40
40
40
40
81
50
67
65
70
68
18
59
58
67
81
80
81
54
3
74
4
76
5
80
6
75
7[c]
8[d]
9[e]
10[f]
11[g]
12[g], [h]
41
69
64
71
>95
>95
[a] General reaction conditions: The reaction was conducted under air at
100 °C with 0.5 mmol of 1b, and 3 mL of anisole. [b] Yields were determined
by GC analysis. [c] 80 °C. [d] 2 % O2 atmosphere. [e] Addition of 2 equiv of
water. [f] Addition of 4Å molecular sieves. [g] Higher concentration (0.33 M of
1b, 1.5 mL of anisole). [h] Addition of 1 equiv of K2CO3.
With the optimized reaction conditions in hand, we turned our
attention to the substrate scope, starting with substitutions on
the aromatic ring. Benzylic sec-alcohols with neutral or electron-
donating groups on the aromatic ring performed very well and
gave the corresponding ketones in high yields (Scheme 4, 2a-
2c). Electron-withdrawing groups, however, gave slightly lower
yields in the range of 60 - 80 % and required a higher catalyst
loading (2d-2f). These results can be explained by the fact that
electron-deficient benzylic alcohols are not as easily oxidized as
their electron-rich counterparts. With the lower reaction rate of
the electron-deficient alcohols, competing deactivation of the
catalyst by O2 becomes more severe and, as a result, a lower
yield is obtained. Interestingly, the nitrile-substituted ketone 2f
could be isolated in a yield of 60 % even though the nitrile group
typically acts as a strong coordinating ligand to iron. The use of
1-phenylpropanol (1g) afforded ketone 2g in an isolated yield of
70 %. Alkyl-substituted alcohols could also be oxidized and 2i
was isolated in 80 % yield.
Scheme 4. Substrate scope. General reaction conditions: The reaction was
conducted under air at 100 °C with 0.5 mmol of 1, 0.05 mmol of VII, 0.05 mmol
of TMANO, 0.2 mmol DMBQ, 0.02 mmol of VI, 0.5 mmol of K2CO3, and 1.5 mL
of anisole. [a] Isolated yields. [b] NMR yield determined by using 1,3,5-
trimethoxybenzene as internal standard. [c] 0.1 mmol (20 mol%) of VII used.
In conclusion, we have developed the first example of an
iron(II)-catalyzed biomimetic aerobic oxidation of alcohols,
where electron transfer mediators are used to lower the energy
barrier for the electron transfer from substrate to molecular
oxygen. The electron transfer system used is reminiscent of the
respiratory chain. Through the use of this biologically inspired
method various aldehydes and ketones could be efficiently
prepared from their corresponding primary or secondary
alcohols in good to excellent yields.
Acknowledgements
Primary alcohols also worked well and could be oxidized to
their corresponding aldehydes 2j-2m. With benzylic primary
alcohols good to excellent yields of aldehydes 2j-2l were
obtained. However, cyclohexylmethanol gave a moderate yield
Financial support from the Swedish Research Council (2016-
03897), the Olle Engkvist Foundation, and the Knut and Alice
Wallenberg Foundation (KAW 2016.0072) is gratefully
acknowledged.
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Keywords: aerobic oxidation • iron • biomimetic oxidation •
homogeneous catalysis • electron transfer
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Entry for the Table of Contents (Please choose one layout)
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Arnar Guðmundsson, Kim Schlipköter,
Jan-E. Bäckvall*
Page No. – Page No.
Iron(II)-Catalyzed Aerobic Biomimetic
Oxidation of Alcohols
An iron(II)-catalyzed aerobic biomimetic oxidation of alcohols has been developed.
The principle is based on the biological oxidation of alcohols in the respiratory
chain. The electron transfer from the alcohol to molecular oxygen occurs via four
coupled catalytic redox reactions, leading to a low-energy pathway. An iron com-
plex was utilized as a substrate-selective catalyst along with a quinone and an
oxygen-activating Co complex as electron transfer mediators in catalytic amounts.
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