Non-Heme Iron Catalysts with a Rigid Bis-Isoindoline Backbone and Their Use in Selective Aliphatic C?H Oxidation

Article abstract of DOI:10.1002/adsc.201700239

Iron complexes derived from a bis-isoindoline-bis-pyridine ligand platform based on the BPBP ligand (BPBP=N,N′-bis(2-picolyl)-2,2′-bis-pyrrolidine) have been synthesized and applied in selective aliphatic C?H oxidation with hydrogen peroxide under mild conditions. The introduction of benzene moieties on the bis-pyrrolidine backbone leads to an increased preference of tertiary over secondary C?H bond oxidation (3°/2° ratio up to 33). On the other hand, substituting the meta-position of the pyridines with bulky silyl groups affords enhanced secondary C?H oxidation selectivity and generally leads to higher product yields and mass balances. (Figure presented.).

Full text of DOI:10.1002/adsc.201700239

Title: Non-Heme Iron Catalysts with a Rigid Bis-Isoindoline Backbone
and Their Use in Selective Aliphatic C−H Oxidation
Authors: Jianming Chen, Martin Lutz, Michela Milan, Miquel Costas,
Matthias Otte, and Bert Klein Gebbink
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201700239
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10.1002/adsc.201700239
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Non-Heme Iron Catalysts with a Rigid Bis-Isoindoline
Backbone and Their Use in Selective Aliphatic C−H Oxidation
Jianming Chen,a Martin Lutz,b Michela Milan,c Miquel Costas,c Matthias Otte,a and
Robertus J. M. Klein Gebbinka*
a
Organic Chemistry & Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99,
3584 CG Utrecht, The Netherlands
Email: r.j.m.kleingebbink@uu.nl
Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Utrecht University, Padualaan 8, 3584
b
CH Utrecht, The Netherlands
Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona, Campus
c
Montilivi, E-17071 Girona, Catalonia, Spain
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
tetradentate N4 ligands have been developed.[5]
Abstract. Iron complexes derived from a bis-isoindoline-
bis-pyridine ligand platform based on the BPBP ligand
(BPBP = N,N′-bis(2-picolyl)-2,2′-bis-pyrrolidine) have
been synthesized and applied in selective aliphatic C–H
oxidation with hydrogen peroxide under mild conditions.
The introduction of benzene moieties on the bis-
pyrrolidine backbone leads to an increased preference of
tertiary over secondary C–H bond oxidation (3°/2° ratio
up to 33). On the other hand, substituting the meta-
position of the pyridines with bulky silyl groups affords
enhanced secondary C–H oxidation selectivity and
generally leads to higher product yields and mass
balances.
Amongst them, iron complexes featuring bis-
alkylamine-bis-pyridine (N2Py2) ligands have been
proven to be the most effective.[6] However,
modifications of this ligand platform often ended up
with lower efficiency.[6] These studies were mainly
done under large excess of substrate and had limited
interest from a synthetic perspective. However, in
2007,[7] White and co-workers described catalytic
oxidations under substrate limiting conditions. In this
work, a ligand modification strategy was introduced
that rigidifies the bis-alkylamine backbone by
incorporating the alkylamines into pyrrolidines,
forming the BPBP ligand (BPBP = N,N′-bis(2-
picolyl)-2,2′-bis-pyrrolidine; Figure 1, left), which
Keywords: bioinspired catalysis; C–H oxidation;
hydrogen peroxide; iron; product selectivity
translated into improved product selectivities.[7]
A
similar strategy was adopted by Rybak-Akimova and
co-workers, where they rigidified the alkylamine
moiety into 6-membered piperidine rings.[8] Many
efforts have been spent on the modification of the
pyridine moieties as well.[9] We have recently
reported that the introduction of bulky TIPS (tris-
(isopropyl)silyl) groups at the meta-position of the
pyridine rings will lead to increased secondary C–H
oxidation and improved mass balance.[10] Through
this modification, site-selective methylene oxidation
of steroidal substrates has also been achieved.
Oxidation of aliphatic C–H groups is of particular
interest in organic synthesis due to the large
abundance of aliphatic moieties in natural products
and petrochemical platform molecules, as well as the
added value of their corresponding hydroxyl and
carbonyl compounds.[1] To date, a vast number of
iron-containing enzymes, such as methane
monooxygenase (MMO) and α-ketoglutarate-(α-KG)-
dependent dioxygenases, have been revealed to
perform biological C–H oxidations through the
activation of dioxygen in a selective manner.[2]
Inspired by these enzymes, the past decades have
witnessed the development of a variety of biomimetic
non-heme iron catalysts.[3] In 1997,[4] a synthetic Fe
coordination complex was firstly demonstrated to be
capable of stereospecific alkane hydroxylation by
Que and co-workers, where the tetradentate N-donor
ligand TPA (tris(2-pyridylmethyl)amine) was
employed and H2O2 was used as terminal oxidant.
Since then, several Fe complexes based on
R
N
N
N
N
N
N
OSO2CF3
OSO2CF3
OSO2CF3
OSO2CF3
Fe
Fe
N
N
R
S, S-[Fe(RBPBI)(OTf)2]
2: R = H
S, S-[Fe(BPBP)(OTf)2]
1
3: R = TIPS
1
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10.1002/adsc.201700239
Advanced Synthesis & Catalysis
Figure 1. Complexes 1-3 studied in this work.
(2.1390(4)-2.192(3) Å) are shorter than the Fe-
N(amine) distances of 2.199(3)-2.2249(14) Å. This
difference is more distinct in 2 and 3 compared to 1.
Overall, these long Fe-N distances are consistent with
high-spin Fe(II) complexes.[13b,16] The main
differences in the crystal structures are found for the
orientation of the triflate ligands. Slightly smaller O-
Fe-O angles in 3 (100.17(6) and 100.56(6)°) are
observed compared to 2 (102.49(15)°), which might
be due to the larger steric demand of the ligand in 3;
the smallest O-Fe-O angle is found in 1. A
quaternion fit of 1 and 2 clearly shows that the BPBP
and BPBI ligands provide a very similar steric
environment around the iron center (see Figure S4 in
Supporting Information). Interestingly, 1H NMR
spectra of complex 2 and 3 show a single
paramagnetic species (see Supporting Information),
indicating that these complexes have a high-spin state
in solution and that the C2 symmetric structure of the
complexes is retained in solution. Cyclic voltammetry
measurements showed that 2 and 3 have very similar
Fe(II)/Fe(III) potentials (E1/2 = 0.82, 0.81 V,
respectively; see Figure S5 in Supporting
Information). These values are somewhat more
positive than that of 1 (E1/2 = 0.70 V), which is likely
due to the electron-withdrawing properties of the
benzene rings in the BPBI ligand.
With the goal of achieving enhanced C–H oxidation
selectivities, herein we report iron complex S, S-
[Fe(BPBI)(OTf)2] (2) (BPBI = (N,N′-bis(2-picolyl)-
2,2′-bis-isoindoline) and its TIPS substituted
analogue S, S-[Fe(TIPSBPBI)(OTf)2] 3 (Figure 1),
which bear a further rigidified bis-isoindoline
backbone compared to the bis-pyrrolidine in the
BPBP ligand. We have found that the BPBI complex
2 gives rise to higher tertiary over secondary C–H
oxidation selectivities than the parent complex S, S-
[Fe(BPBP)(OTf)2] (1). On the other hand, 3 exhibits
enhanced secondary C–H oxidation ability and
increased product yields.
With 2 and 3 in hand we investigated their ability
to catalyse the oxidation of aliphatic C–H bonds. To
do so, a set of substrates consisting of cyclohexane
(4), cis-1,2-dimethylcyclohexane (7), trans-1,2-
dimethylcyclohexane (11), trans-decalin (15),
adamantane (19), L-(−)-menthyl acetate (23) and
methyl hexanoate (26) has been studied. For
Figure 2. Molecular structures of 2 (left) and 3 (right) in
the crystal, drawn at the 50% probability level.[11] Only the
coordinated O atoms of the triflate groups are shown.
Hydrogen atoms, solvent molecules and the second
independent molecule of 3 are omitted for clarity.
comparison purpose, complex
1
bearing the
structurally related BPBP ligand was also tested.
The chiral (S,S)-bis-isoindoline backbone of the BPBI
ligand was prepared according to a previously
reported procedure.[12] It was readily converted into
target complexes 2 and 3 by alkylation with 2-picolyl
chloride derivatives and subsequent complexation
with Fe(OTf)2·2CH3CN (see Supporting Information
for experimental details). Figure 2 shows the X-ray
crystal structures of complexes 2 and 3. The absolute
structure of the two complexes was proven by the
Flack parameter (see Supporting Information) and
similarly to 1, they also feature a cis-α coordination
topology, which has been shown a crucial factor for
good catalytic efficiency in previous studies.[13] In
complex 3, the installation of bulky TIPS groups on
the two pyridines leads to a crowded envelope-like
configuration around the cis labile positions of the
iron center[10] (Figure 2, right, also see Figure S3),
where the plausible reactive Fe-oxo species that is
responsible for site selective C–H oxidation is
generated.[14] In this sense, the bulky nature may
result in modulated regioselectivity. Only slight
differences are seen between the three complexes for
the bond lengths and angles of the Fe coordination
environment. Selected bond lengths and angles for
(R,R)-1[15] (S,S)-2 and (S,S)-3 are listed in Table S1.
As expected, the Fe-N(pyridine) distances
Table 1. Cyclohexane Oxidation by 1, 2 and 3.a)
Fe-cat. (1.0 mol%)
O
OH
+
H2O2 (1.2 equiv.)
AcOH, CH3CN
Method A
5
4
6
Substrate cat.
5, 6
(%)b)
Conv.
(%)b)
K/Ac)
Mass
balance
(%)
16.7, 2.4
8.8, 5.9
27, 6.8
45
42
64
7.0
1.5
4.0
42
35
53
4
4
4
1
2
3
a)
Reaction conditions (method A): Fe-cat. : H2O2
:
substrate ：AcOH = 1 : 120 : 100 : 50, 0 °C, oxidant added
by syringe pump over 6 min, and reaction mixture stirred
b)
c)
for additional 10 min.
Ketone/alcohol ratio = 5/6.
Determined by GC analysis.
Using 1.0 mol% catalyst loading, cyclohexane (4)
is oxidized to cyclohexanone (5) and cyclohexanol
(6) in the presence of 1.2 equiv. H2O2 and 0.5 equiv.
o
acetic acid in CH3CN at 0 C. For all three catalysts,
5 is formed as the major product (5 is formed through
oxidation of initially generated 6 due to the substrate
2
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10.1002/adsc.201700239
Advanced Synthesis & Catalysis
limiting reaction conditions). Reaction with 1 gives a
higher ketone to alcohol (K/A) ratio compared to 2
(7.0 over 1.5, Table 1). With 3 a medium K/A ratio
(4.0) was observed, albeit with the highest substrate
conversion and mass balance amongst the three
complexes (64% and 53%, respectively).
AcOH = 1 : 150 : 100 : 50, 0 °C, oxidant added by syringe
pump over 30 min, and reaction mixture stirred for
b)
additional 1.5 h. Determined by GC analysis. c) 3°/2° =
f)
8/(9 + 10). d) 3°/2° = 12/(13 + 14) e) 3°/2° = 16/(17 + 18)
3°/2° = 3 x 20/(21 + 22).
The ability of these complexes to discriminate
between tertiary and secondary C–H bonds is
illustrated by the 3°/2° ratios in the oxidation of 7, 11,
15 and 19, which have multiple secondary and
tertiary C–H sites (Table 2). Oxidation of 7 catalyzed
by 1 generates 3° oxidation product 8 as the major
product with 2° oxidation products 9 and 10 as minor
products, affording a 3°/2° ratio of 7.0. The
corresponding 3°/2° ratio was found to increase to 8.8
when changing the catalyst to 2, along with the
decrease in conversion from 61% to 32% (Table 2).
The oxidation of tertiary over secondary C–H bonds
is less preferential when performing this reaction with
3 (with a value of 4.6, Table 2). This observation is
consistent with our previous study, which showed
that the introduction of bulky TIPS groups can lead to
an enhanced 2°/3° C–H oxidation ratio, because
oxidation of the sterically more congested tertiary C-
H bond is disfavored.[10] Similarly, a slightly higher
3°/2° ratio (1.4) is found for 2 in the oxidation of 11
than for 1 (1.0). Much more secondary oxidized
products are formed in the reaction catalyzed by 3,
giving a 3°/2° ratio of 0.3.
Similar catalytic outcomes were found in the
oxidations of 15 and 19 in the presence of 1-3. The
tertiary C–H oxidation products are more
preferentially formed in the reactions with 2 than in
the cases of 1 and 3, with a 3°/2° ratio of 0.35 in the
case of 15 and 33 in the case of 19 (Table 2). Notably,
the latter ratio is amongst the highest reported 3°/2°
selectivities for non-heme Fe catalyzed adamantane
oxidation.[3a,17] The lowest 3°/2° ratios (0.07 and 16,
respectively) were observed for complex 3, further
supporting the enhanced ability of 3 to catalyze
secondary C–H oxidations. For the reactivities of the
reactions in Table 2, 3 stands out amongst these three
complexes with highest conversions.
Table 2. Oxidation of substrates 7, 11, 15 and 19.a)
O
OH
Fe-cat. (1.0 mol%)
O
+
+
H2O2 (1.2 equiv.)
AcOH, CH3CN
Method A
10
14
8
9
7
O
Fe-cat. (1.0 mol%)
O
OH
+
+
+
H2O2 (1.5 equiv.)
AcOH, CH3CN
Method B
12
13
11
O
H
H
H
H
H
OH
Fe-cat. (1.0 mol%)
O
+
H2O2 (1.5 equiv.)
AcOH, CH3CN
Method B
H
17
H
16
15
18
OH
OH
O
Fe-cat. (1.0 mol%)
+
+
H2O2 (1.2 equiv.)
AcOH, CH3CN
Method A
21
19
20
22
Alkane cat.
8, 9, 10
Conv. 3°/2°c)
(%)b)
Mass
balance
(%)b)
(%)
39, 2.6, 3.0
22, 1.2, 1.3
37, 4.1, 3.9
12, 13, 14
(%)b)
61
32
65
7.0
8.8
4.6
73
77
69
7
7
7
1
2
3
Alkane cat.
3°/2°d)
Conv.
(%)b)
Mass
balance
(%)
83
71
11, 3.7, 7.6
6.2, 1.6, 2.9
8.9, 7.5, 19
16, 17, 18
(%)b)
27
15
47
1.0
1.4
11
11
11
1
2
3
0.3
75
Alkane cat.
3°/2°e)
Conv.
(%)b)
Mass
balance
The oxidations of unfunctionalized aliphatic
alkanes 4, 7, 11, 15 and 19 give different outcomes,
responding to the structural changes in complexes 1-3.
The change in ligand backbone from bis-pyrrolidine
in 1 to more rigid bis-isoindoline in 2 gives rise to a
higher alcohol selectivity (Table 1) and improved
3°/2° ratios (Table 2). On the contrary, enhanced
selectivities for methylene are achieved when 3 is
applied. These observations stimulated our interest in
the oxidation of functionalized aliphatic substrates.
(%)
62
60
53
36
60
0.2
0.35
15
15
15
1
2
3
5.5, 10.7, 17
5.6, 6.7, 9.2
2.2, 11.8, 21
20, 21, 22
(%)b)
0.07
59
Alkane cat.
3°/2°f)
Mass
balance
(%)
42
Conv.
(%)b
14, 0.3, 1.6
24, 0.5, 1.7
24, 1.1, 3.3
38
40
53
21
33
16
19
19
19
1
2
3
66
54
a)
For substrate 7 and 19, method A is used. For substrate
11 and 15, method B is used: Fe-cat. : H2O2 : substrate :
3
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10.1002/adsc.201700239
Advanced Synthesis & Catalysis
Table 3. Oxidation of substrate 23 and 26 by 1, 2 and 3.a)
in drug discovery[18] and their various physical and
biological properties based on the different oxidation
patterns.[19]
HO
Fe-cat. (1.0 mol%)
OAc
OAc
OAc
H2O2 (1.2 equiv.)
AcOH, CH3CN
Method C
O
O
O
O
C12
+
Fe-cat. (3.0 mol%)
C12
OH
24
+
25
C14
H2O2 (2.0 equiv.)
AcOH, CH3CN
Method D
23
C6
C7
AcO
AcO
AcO
C6
29
Fe-cat. (1.0 mol%)
O
O
O
O
H2O2 (1.2 equiv.)
AcOH, CH3CN
Method C
+
O
O
O
O
C12
C12
Fe-cat. (3.0 mol%)
O
O
+
MeO
26
MeO
C14
C7
MeO
27
H2O2 (2.0 equiv.)
AcOH, CH3CN
Method D
C6
28
AcO
AcO
AcO
C6
30
O
Substrate cat.
24, 25
Conv. 24/25
Mass
(%)b)
(%)b)
balance
Scheme 1. Catalytic Oxidation of cis-acetylandrosterone
acetate (29) and trans-acetylandrosterone acetate (30).
(%)
28, 5.2
19, 3.6
30, 5.5
40
30
40
5.4
5.3
5.5
83
75
89
23
23
23
1
2
3
Oxidation of 29 and 30 was performed with 3 mol%
Fe catalyst, 200 mol% H2O2 and 150 mol% AcOH at
Substrate cat.
27/28
Mass
balance
(%)
93
27, 28
(%)b)
Conv.
(%)b
o
0 C. Poor yields and mass balances were obtained
when 2 was employed in both reactions and no
statistical site selectivities could be estimated in these
cases (Table 4). Yields and mass balances increased
responding to the steric bulk of catalyst 3. For both
substrates, the C6 oxidized ketone is preferentially
formed (58% and 72% selectivity, respectively), with
C7 and C12 oxidized ketones as minor products. Of
note is that these observations support the importance
of bulky silyl moieties on the ligand for achieving
site-selective oxidations of acetylandrosterone
derivatives.[10] In previous studies, 1 gave poor C6
over C12 selectivity,[20] while 49–82% selectivity for
C6 oxidation was obtained in the cases of the
corresponding TIPS-substituted 1.[10]
12.6,7.8
9.7, 5.7
13.3, 9.0
22
16
23
1.6
1.7
1.5
26
26
26
1
2
3
96
97
a)
Reaction conditions (method C): Fe-cat. : H2O2
:
substrate : AcOH = 1 : 120 : 100 : 50, 0 °C, oxidant added
by syringe pump over 0.5 h, and reaction mixture stirred
for additional 2 h. b) Determined by NMR analysis.
To probe the steric effect on the oxidation site
selectivity, the oxidation of L-(−)-menthyl acetate
(23), containing two tertiary C–H bonds (both at a γ-
position to the acetoxy group) with different steric
environments, by complexes 1-3 was examined
(Table 3). The lower steric hindrance hydroxylation
product 24 was preferentially formed in the oxidation
of 23 for all catalysts tested, indicating that these
reactions in the presence of 1-3 are sensitive to steric
factors. No difference in the preference of formation
of 24 were observed, i.e., in all the cases, 24/25 ratios
around 5.4 were found (Table 3). In terms of
conversion and mass balance, 3 outperformed the
other catalysts.
Methyl hexanoate (26), containing an electron-
withdrawing ester-group, was tested in order to look
into electronic influences on site selectivity. It was
found that the change in catalyst has nearly no effect
on the ratios of δ-oxidized product (27) to γ-oxidized
product (28) in the oxidation of 26 (all are around 1.6,
Table 3). Notably, excellent mass balances were
observed in all cases (93%–97%), with 3 again
showing the highest product formation.
Table 4. Oxidation of steroidal substrates by 2 and 3.a)
Steroid cat. Conv.
(%)b)
C6/C7/C14
(%)b)
Norm. yield
C6/C7/C12
(%)
31
31
4/3/2
11/5/3
29
29
2
3
58/26/18
Norm. yield
C6/C7/C12/C14
(%)
Steroid cat.
C6/C7/C12/C14
Conv.
(%)b)
(%)b)
23
38
5/1/1/5
23/4/4/1
30
30
2
3
72/13/13/2
a)
Reaction conditions (method D): Fe-cat. : H2O2
:
substrate : AcOH = 3 : 200 : 100 : 150, 0 °C, oxidant added
by syringe pump over 30 min, and reaction mixture stirred
for additional 10 min. b) Determined by GC analysis.
Next, we turned to investigate the catalytic
performance of 2 and 3 on elaborated steroidal
substrates 29 and 30 (Scheme 1). These structurally
complex compounds are ideal substrates to
investigate site-selective C–H oxidation due to the
multiple secondary and tertiary C–H groups. Steroids
are of particular interest because of their importance
In conclusion, we have described the use of
bioinspired iron complexes based on an N2Py2 ligand
platform bearing a bis-isoindoline backbone in
selective aliphatic C–H oxidations. Compared to
parent complex 1, complex 2 shows preference for
tertiary over secondary C–H bond oxidation for
alkane substrates. The catalytic performance of 3
4
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10.1002/adsc.201700239
Advanced Synthesis & Catalysis
provides further experimental evidence that bulky
TIPS groups on meta-positions of the pyridine
moieties enhance secondary C–H bond oxidations
and site-selective oxidations of acetylandrosterone
derivatives.[10] We believe that the incorporation of a
bis-isoindoline backbone provides an additional
means of modification of the popular BPBP ligand in
oxidation catalysis. Further modification on the
aromatic rings of the chiral BPBI backbone may offer
a versatile and alternative strategy for ligand design
in N2Py2-based iron complexes. We are currently
exploring such ligand modifications in catalytic C–H
bond oxidations and enantioselective epoxidation
reactions.
Coord. Chem. Rev. 2012, 256, 1418–1434.
[4] C. Kim, K. Chen, J. Kim, L. Que, Jr., J. Am. Chem. Soc.
1997, 119, 5964–5965.
[5] a) T. A. van den Berg, J. W. de Boer, W. R. Browne,
G. Roelfes, B. L. Feringa, Chem. Commun. 2004,
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X. Ribas, J. M. Luis, L. Que, Jr., M. Costas, J. Am
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J. P. Britovsek, N. Rabadia, A. J. P. White, Inorg.
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Experimental Section
General Catalytic Conditions
[6] G. Olivo, O. Cussó, M. Costas, Chem. Asian J. 2016,
11, 3148–3158.
A 20 mL vial was charged with: substrate (0.36 mmol, 1
equiv.), catalyst (3.6 μmol, 1 mol%), CH3CN (1.5 mL). A
0.5 M CH3CO2H solution in CH3CN was added (0.36 mL,
0.18 mmol, 50 mol%). The vial was cooled on an ice bath
with stirring. Subsequently, a 1.0 M H2O2 solution in
CH3CN (120 mol% ore 150 mol%) was delivered by
syringe pump over 6-30 min. After the oxidant addition,
the resulting mixture was stirred at 0 °C for another 10
min-2 h. At this point, a 1.0 M internal standard solution in
CH3CN (0.36 mL, 0.36 mmol) was added. The solution
was diluted with Et2O to precipitate the iron complex,
passed through a cotton wool filter to remove the catalyst.
Subsequently, a sample was submitted to GC or NMR
analysis. Reported analysis data represent the outcome of
at least two independent catalysis experiments.
[7] M. S. Chen, M. C. White, Science 2007, 318, 783–787.
[8] E. A. Mikhalyova, O. V. Makhlynets, T. D. Palluccio,
A. S. Filatov, E. V. Rybak-Akimova, Chem. Commun.
2012, 48, 687–689.
[9] a) L. Gómez, I. Garcia-Bosch, A. Company, J. Benet-
Buchholz, A. Polo, X. Sala, X. Ribas, M. Costas,
Angew. Chem. 2009, 121, 5830–5833; Angew. Chem.
Int. Ed. 2009, 48, 5720–5723; b) I. Prat, L. Gómez, M.
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Acknowledgements
J. C. acknowledges the China Scholarship Council (CSC) for a
doctoral scholarship. M. O. thanks the sustainability theme of
Utrecht University for funding. The X-ray diffractometer has
been financed by the Netherlands Organization for Scientific
Research (NWO).
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J. M. Klein Gebbink, M. Costas, Angew. Chem. 2016,
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COMMUNICATION
Non-Heme Iron Catalysts with a Rigid Bis-
Isoindoline Backbone and Their Use in Selective
Aliphatic C−H Oxidation
Adv. Synth. Catal. Year, Volume, Page – Page
Jianming Chen, Martin Lutz, Michela Milan,
Miquel Costas, Matthias Otte, and Robertus J. M.
Klein Gebbinka*
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