An iron(III)-monoamidate complex catalyst for selective hydroxylation of alkane C-H bonds with hydrogen peroxide

Article abstract of DOI:10.1002/anie.201108933

Selective oxidation: The success of the title reaction (see scheme) is caused by the strong electron donation from the amidate moiety of the dpaq ligand to the iron center (dpaq=2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8- yl-acetamidate). This process facilitates the O-O bond heterolysis of the intermediate Fe^IIIOOH species to generate a selective oxidant without forming highly reactive hydroxyl radicals. Copyright

Full text of DOI:10.1002/anie.201108933

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DOI: 10.1002/anie.201108933
Hydroxylation
An Iron(III)–Monoamidate Complex Catalyst for Selective Hydroxyl-
À
ation of Alkane C H Bonds with Hydrogen Peroxide**
Yutaka Hitomi,* Kengo Arakawa, Takuzo Funabiki, and Masahito Kodera
Development of catalysts for the selective hydroxylation of
[1–4]
À
inert C H bonds is an important research objective.
Significant progress toward this goal has been made in
recent decades by using biologically inspired iron catalysts
and hydrogen peroxide.[5–11] H2O2 is the most ideal oxidant
from an atom-economy viewpoint, however, it is generally
difficult to use as an oxidant for iron-catalyzed selective
oxidations because it easily decomposes to hydroxyl radicals,
thus resulting in an undesirable oxidation.[12,13] Despite this
difficulty, recent studies showed that mononuclear iron
complexes with two cis-oriented labile coordination sites
can afford considerably high selectivity in alkane hydroxyl-
ation with H2O2.[14–19] It has been reported that acetic acid is
required to increase the selectivity to synthetically useful
levels in these hydroxylations.[15,19] The specific role of acetic
acid has not yet been clarified, but it is believed that it
À
facilitates the heterolytic scission of the O O bond of
a FeIIIOOH species by acting as an acid catalyst.[20,21] Very
II
À
recently, Costas and co-workers reported that Fe
Me,MePytacn (Me,MePytacn = 1-(2’-pyridylmethyl)-4,7-dimethyl-
1,4,7-triazacyclononane, Scheme 1) also catalyzes selective
À
hydroxylation of alkane C H bonds when H2O2 is used
Scheme 1. Structures of the supporting ligands.
without an acid additive,[17] and also observed the formation
À
of an iron(V) oxo-hydroxo species as an oxidant by mass
spectroscopy.[22]
Herein, we report an FeIII dpaq complex (1; dpaq = 2-
À
[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate,
Scheme 1) that can catalyze the selective hydroxylation of
À
inert C H bonds by using H2O2 as the oxidant. The crystal
structure of 1 (Figure 1) shows a slightly distorted octahedral
iron(III) center, which contains a pentadentate dpaq ligand,
and an aqua ligand in trans position to the nitrogen atom of
the amidate moiety of dpaq. In our strategy, the aqua ligand is
replaced by H2O2 to form an FeIIIOOH species, in which
electron donation from the amidate ligand to the iron center
promotes the generation of a formally FeVoxo species through
À
the heterolytic scission of the O O bond (Scheme 2), since
Figure 1. Ortep (50% probability) diagram of the cation of 1. The
counter anion and the hydrogen atoms, with the exception of H1 and
H2, are omitted for clarity. Selected bond lengths [ꢀ]: Fe1–N1 1.976(3),
Fe1–N2 1.975(3), Fe1–N3 1.987(3), Fe1–N4 1.857(3), Fe1–N5
1.980(3), Fe1–O1 1.963(3).
[*] Prof. Y. Hitomi, K. Arakawa, Dr. T. Funabiki, Prof. M. Kodera
Department of Molecular Chemistry and Biochemistry
Faculty of Science and Engineering, Doshisha University
Kyotanabe, Kyoto 610-0321 (Japan)
the anionic amidate ligand should stabilize the high-valent
FeVoxo species.
E-mail: yhitomi@mail.doshisha.ac.jp
We first performed the cyclohexane oxidation in aceto-
nitrile at room temperature. H2O2 was added to a reaction
vessel under air over a period of 30 minutes by using a syringe
pump. The reaction gave cyclohexanol (A) and cyclohexa-
none (K) with an A/K ratio of 11.3:1. Formation of hydroxyl
radicals should decrease the A/K ratio to about 1:1 through
[**] This work was supported by Grant-in-Aid for Scientific Research on
Priority Areas (Nos. 19028033 and 20037039, “Chemistry of
Concerto Catalysis”) from the Ministry of Education, Culture,
Sports, Science, and Technology (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108933.
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Table 1: Oxidation of cyclohexane with CmOOH catalyzed by iron
complexes.[a]
Entry
Catalyst
Alcohol[b]
Ketone[b]
A/K[c]
CmOH/PhCOMe
1
2
3
4
1
10
10
4.8
3.9
1.0
6.2
4.3
3.5
10
17.8:1 (9.2:1)[e]
1.7:1 (2.0:1)[e]
1.8:1 (1.6:1)[e]
1:81.3 (1:99)[e]
1[d]
2
1.6
1.1
1.1
3
[a] Catalyst/CmOOH/cyclohexane=1:20:1000 in CH3CN at RT. CmOOH
was added over a period of 30 min by using a syringe pump. [b] Turnover
number (mol of product/mol of iron). [c] A=cyclohexanol, K=cyclo-
hexanone. [d] Catalyst/2,4,6-collidine/CmOOH/cyclohexane=
1:1:20:1000. [e] Values in parentheses are the ratio of CmOH:PhCOMe
obtained after the reaction of an iron complex with equimolar amounts
of CmOOH for 35 min without cyclohexane (catalyst/CmOOH=1:1, no
cyclohexane). In the absence of iron complexes, 0.9% of CmOH and 4%
of PhCOMe were produced under the current conditions.
Scheme 2. Formation of the FeVoxo species from complex 1.
(CSI-MS). The positive-ion CSI-MS spectrum of the reaction
mixture of 1 with 50 equivalents of H2O2 in CH3CN at 273 K
showed an ion signal at a mass-to-charge (m/z) ratio of 471.1,
the mass and isotope distribution pattern of which were
consistent with the chemical formula of [FeIII(dpaq)(OOH)]+
(calculated m/z = 471.1; Figure S2). When H218O2 was used,
the position of the ion signal changed to 475.1 (shift of four
mass units). These results unequivocally indicate the forma-
tion of a H2O2 adduct of [Fe(dpaq)] by mixing 1 and H2O2 in
CH3CN. More importantly, the CSI-MS analysis of the
solution also showed a weak ion signal at m/z ratio of 553.0.
The mass and isotope distribution pattern were consistent
with the chemical formula of {[FeV(dpaq)(O)](ClO4)}+ (cal-
culated m/z = 553.1), the signal position of which shifted to
555.1 (shift of two mass units) when H218O2 was used
(Figure S2). These results suggest the formation of an
FeVoxo species by mixing 1 and H2O2 in CH3CN. It has been
reported that FeIIIOOH species can be easily prepared by
mixing mononuclear nonheme FeII complexes with H2O2,[28]
but FeVoxo species have been observed only in a few
cases.[22,29,30]
a Russell-type termination mechanism, which involves long-
lived carbon centered radicals and molecular dioxygen.[23] The
A/K ratio was not changed under an N2 atmosphere (A/K =
11.4:1). Furthermore, the distribution of the products over
time showed that the A/K ratio was initially 36:1 and then
gradually decreased to 11:1 (see Figure S1 in the Supporting
Information). These results indicate the formation of cyclo-
hexanone by further oxidation of cyclohexanol and not
through the Russell-type termination mechanism involving
hydroxyl radicals. Thus, these results suggest the generation of
À
metal-based oxidants through O O bond heterolysis of an
FeIIIOOH species. To validate this observation, we conducted
catalytic oxidation of cyclohexane using other oxidants,
including m-chloroperoxybenzoic acid (mCPBA), methyl 2-
iodosylbenzoate (IBX ester),[24] tert-butyl hydroperoxide, and
cumene hydroperoxide (CmOOH, Cm = C(CH3)2C6H5). The
obtained A/K ratios were nearly identical, regardless of the
oxidant that was used (Table S1). This result suggests the
generation of a common metal-based oxidant, probably
a formally iron(V)oxo species, because mCPBA and IBX
ester would be expected to transfer an oxo group to the
iron(III) center of 1.
The reaction of 1 with H2O2 affords FeIIIOOH and
V
À
a proton, and the O O bond heterolysis gives an Fe oxo
species and a water molecule (Scheme 2). In the last step, the
distal oxygen atom of the FeIIIOOH species should be
protonated to afford the water molecule. In order to examine
When CmOOH was used as the oxidant for cyclohexane
oxidation, cumene alcohol (CmOH) was obtained as a major
product with trace amounts of acetophenone (PhCOMe;
CmOH/PhCOMe ꢀ 18:1). Similar CmOH/PhCOMe ratios
were obtained without cyclohexane (Table 1). CmOOH has
been frequently used as a mechanistic probe to clarify the
À
whether the protonation is essential for the O O bond
heterolysis, we conducted cyclohexane oxidation by H2O2 in
the presence of an equimolar mixture of 1 and 2,4,6-collidine,
which is a strong base with a pKa value of 14.7 in CH3CN.[31,32]
As a result, the A/K ratio reduced to 1.3:1. Furthermore, the
reaction of 1 with CmOOH in the presence of 2,4,6-collidine
gave significant amounts of PhCOMe, and the CmOH/
PhCOMe ratio reduced from 9.2 to 2.0 (Table 1, entries 1
and 2). These results suggest that the amidate coordination of
À
À
cleavage manner of the O O bond, because the O O
À
heterolysis of M OOCm (M = metal) affords CmOH,
À
whereas O O homolysis gives PhCOMe through b scission
of a cumyloxyl radical.[25–27] The fact that CmOH was obtained
as the major product in the cyclohexane oxidation with
III
À
À
À
CmOOH strongly suggests that the O O bond of Fe OOCm
Fe dpaq itself is not enough to accomplish the O O bond
À
is predominantly cleaved in a heterolytic manner to give
heterolysis. In other words, the proton facilitates O O bond
a formally FeVoxo species.
heterolysis in collaboration with the amidate ligation. In this
sense, it can be noted that FeIIIdpaq performs heme-enzyme-
type chemistry based on the “push–pull” concept.[33]
The above-mentioned results suggest that the reaction of
1 with H2O2 gives the corresponding FeVoxo species through
III
À
O O bond heterolysis of an Fe OOH species, which was
Interestingly, it has been reported that the structurally
À
further supported by cold-spray-ionization mass spectroscopy
related monoamidate iron(III) complex FeIII PaPy3 (2;
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PaPy3 = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-
2-carboxamidate, Scheme 1) has poor selectivity in alkane
oxidation with H2O2.[34] To determine the cause of this
difference in selectivity, we conducted the same reaction
using 2 instead of 1. The A/K ratio in cyclohexane oxidation
with H2O2 reduced to 1.1 (Table S2, entry 3). Cyclohexane
oxidation with CmOOH gave products derived from
CmOOH in a CmOH/PhCOMe ratio of 1.8, and cyclohexanol
mep = N,N’-dimethyl-N,N’-bis(2-pyridylmethyl)-ethane-1,2-
II
À
diamine) and Fe (S,S)-PDP (5; (S,S)-PDP = 2-[{(S)-2[((S)-
1-pyridin-2-ylmethyl)pyrrolidin-2-yl]pyrrolidin-1-yl}-methyl]-
pyridine, Scheme 1). Acetic acid increased RC selectivity
from 56% (in the absence of acetic acid) to 92% and from
60% to 90% with 4 and 5, respectively.[15] However, in the
case of 1, use of acetic acid as an additive did not improve RC
selectivity (Table 2, entry 2).
and cyclohexanone in an A/K ratio of 1.1 (Table 1, entry 3). In
II
À
addition, when Fe N4Py (3; N4Py = N,N-bis(2-pyridyl-
Table 2: Regioselective hydroxylation of cis-4-methylcyclohexyl-1-piva-
late.
methyl)-N-bis(2-pyridyl)methylamine, Scheme 1), which
lacks an amidate ligand, was used as a control, the A/K
ratio became about 1:1 and PhCOMe was a major product
derived from CmOOH (Table 1, entry 4). This result agrees
[35]
À
with the previous report that 3 promotes O O homolysis.
Entry Catalyst AcOH
Yield [%][c] Conv. [%][d] RC [%][e] Ref.
Thus, it is evident that the pentadentate monoamidate
supporting ligands dpaq and PaPy3 tend to facilitate the
heterolysis of the O O bond, but the neutral pentadentate
1
2
3
4
5
6
1[a]
1[a]
4[b]
4[b]
5[b]
5[b]
–
38
40
25
12
41
15
42
94
90
56
92
60
90
this work
this work
[15]
[15]
[15]
0.5 equiv 23
–
À
7
supporting ligand N4Py does not facilitate it. Furthermore,
the dpaq ligand promotes the heterolysis more efficiently
than the PaPy3 ligand. The measured FeII/FeIII redox potential
of 1 is lowered by 0.24 V compared to that of 2, thus indicating
that the dpaq ligand is a better electron donor than the PaPy3
ligand (Figure S3). Density functional calculations also sup-
port this conclusion (Figure S4). The difference in electron
donation abilities of 1 and 2 may in part explain the observed
difference between 1 and 2, although further studies will need
to clarify the mechanism that better electron donor ligands
0.5 equiv 26
14
0.5 equiv 38
–
[15]
[a] Catalyst/H2O2/substrate=1:100:120 in CH3CN at RT. The oxidant
was added over a period of 30 min by using a syringe pump. [b] Catalyst/
H2O2/substrate=1:20:24 in CH3CN at RT. [c] (mol of trans-OH-S1)/(mol
of S1)ꢁ100. [d] Conversion efficiency of S1. [e] {trans-OH-S1/(trans-OH-
S1+cis-OH-S1)}ꢁ100.
Chen and White also reported that 5 catalyzes the
regioselective hydroxylation of 1-substituted derivatives of
3,7-dimethyloctane (S2), in which two tertiary carbon atoms
that are electronically different owing to the substituted
group X (X = H, OAc, and Br) are present.[15] Subsequently,
Costas and co-workers reported an improvement in turnover
numbers for the same reaction using a sterically crowded
version of an iron complex, Fe (S,S,R)-mcpp (6; (S,S,R)-
mcpp = N,N’-dimethyl-N,N’-bis[(R)-[4,5]-pineno-2-
pyridylmethyl][(1S,2S)-1,2-cyclohexanediamine],
À
induce the heterolysis of the O O bond.
Determination of the intermolecular kinetic isotope effect
(KIE) for cyclohexane and [D12]-cyclohexane is a useful
method for evaluating the capability of oxidants to discrim-
À
inate between alkane C H bonds with different strengths.
The KIE determined by using a mixture of cyclohexane and
[D12]-cyclohexane with various molar ratios (from 1:1 to 1:6)
was 3.9 (Figure S5). Adamantane has also been used to
evaluate the discrimination ability of oxidants; in this case, it
is used to examine the ability to discriminate between
II
À
Scheme 1).[19] Note that both 5 and 6 were used in the
presence of acetic acid and the reactions were repeated two or
three times to improve the conversion efficiency (Table 3,
entries 4–9). Surprisingly, 1 gave products for three deriva-
tives of S2 with a moderate conversion efficiency and better
selectivity, even after a single reaction. In the case of X = Br,
À
secondary and tertiary C H bonds with bond dissociation
energies of 100.2 and 96.2 kcalmolÀ1, respectively.[36] The
oxidation of adamantane with H2O2 in the presence of 1 gave
products in a 38/28 ratio of 18.8.[37] These high selectivities
(KIE and 38/28 values for 1) are comparable to or even higher
than those reported for iron complexes with two cis-oriented
labile coordination sites (Tables S2 and S3). These results
demonstrate high selectivity of the oxidant derived from the
reaction of 1 with H2O2.
À
1 hydoxylated the C H bond at the 7 position 15 times more
selective than that at the 3 position, which is slightly better
than the reported selectivities for 5 and 6 (Table 3, entries 3, 6,
and 9). Thus, although 1 does not have two cis-oriented labile
coordination sites as other iron catalysts for selective alkane
hydroxylation do, it exhibits high regioselectivity for hydrox-
À
Another criterion for selective hydroxylation of C H
À
bonds with respect to tertiary C H bonds concerns stereose-
À
lectivity; that is, whether hydroxylation occurs with inversion
or retention of configuration. We chose cis-4-methylcyclo-
hexyl-1-pivalate (S1) as a substrate to evaluate the capability
ylation of alkane C H bonds. More interestingly, the deriv-
ative with X = H was slightly more favorably hydroxylated at
the 7 position than at the 3 position, although both 5 and 6 did
not show any preference for the hydroxylation site (Table 3,
entries 1, 4, and 7). This preference might be caused by
greater steric hindrance around the iron-oxo group of 1 than
those of 5 and 6, as can be estimated from the crystal
structures of their parent complexes.[15,19]
À
of 1 to hydroxylate tertiary C H bonds with retention of
configuration (RC). The FeIII dpaq complex 1 catalyzed the
À
hydroxylation of S1 at the 4 position with high RC selectivity
(94%), in which RC selectivity is the fraction of trans
conformers of 4-hydroxylated products. Chen and White
reported a profitable effect of acetic acid on RC selectivity in
This study illustrates the uniqueness of the pentadentate
II
the oxidation of S1 with H2O2 catalyzed by Fe mep (4;
monoamidate FeIII complex, FeIII dpaq, in the catalysis of the
À
À
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Table 3: Regioselective hydroxylation of 1-substituted 3,7-dimethyl-
octane.
Entry Catalyst X=
Repetition Yield [%] 7-OH
/3-OH
Ref.
1
2
3
4
5
6
7
8
9
1[a]
5[b]
6[c]
H
ꢁ1
39
53
38
48
43
39
35
49
51
3:2
9:1
15:1
1:1
5:1
9:1
1:1
5:1
7:1
this work
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[15]
[15]
[15]
[19]
[19]
[19]
OAc ꢁ1
Br
H
OAc ꢁ3
Br
H
ꢁ1
ꢁ3
ꢁ3
ꢁ2
OAc ꢁ2
Br ꢁ2
[a] Catalyst/H2O2/substrate=1:100:120 in CH3CN at RT. The oxidant
was added over a period of 30 min by using a syringe pump.
[b] 3ꢁ(Catalyst/H2O2/substrate/AcOH=1:24:20:10) in CH3CN at RT.
[c] 2ꢁ(Catalyst/H2O2/substrate/AcOH=1:120:100:50) in CH3CN at RT.
À
hydroxylation of inert alkane C H bonds by H2O2 with a high
level of selectivity and without an acid additive. To the best of
our knowledge, FeIII dpaq is the first example of mononu-
À
clear nonheme FeIII complex catalysts for selective hydroxyl-
À
ation of C H bonds with H2O2. In our opinion, as a selective
oxidation catalyst with H2O2 FeIII dpaq is superior to the
À
reported FeII complexes with two cis-oriented labile coordi-
nation sites for the following reasons. 1) FeIII dpaq can afford
À
the corresponding FeVoxo species through the formation of
FeIIIOOH. On the other hands, FeII complexes must be
oxidized first to the corresponding FeIII species by H2O2
(0.5 equiv) prior to their entry into a catalytic cycle compris-
ing FeIII and FeVoxo species (Scheme 3). The oxidation step of
FeII to FeIII should generate some oxidants. 2) In terms of
selective oxidations, a sole oxidant is desirable to be
Scheme 3. Catalytic cycles of hydroxylation with a) FeIII–dpaq, and
b) an FeII complex with two cis-oriented labile sites.
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À
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Keywords: homogeneous catalyst · hydrogen peroxide ·
hydroxylation · iron · oxidation
.
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[37] The 38/28 ratio is derived from the amount of 1-adamantanol
divided by the amounts of 2-adamantanol and 2-adamantanone
and multiplied by 3 to correct for the higher number of
À
secondary C H bonds.
[38] CCDC867530 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
[28] M. Costas, M. P. Mehn, M. P. Jensen, L. Que, Jr., Chem. Rev.
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